WO2024259252A1 - Excursion sensors for audio playback devices - Google Patents

Abstract

An audio transducer includes a membrane and a voice coil operably coupled to the membrane, wherein the voice coil is configured to move axially within a first excursion range. An amplifier is electrically connected to the voice coil. A sensor coil radially separated from the voice coil is arranged to at least partially overlap the voice coil as the voice coil moves within the first excursion range. In operation, the amplifier provides a first signal to the voice coil, which induces a second signal that can be read from the sensor coil. A parameter indicative of the amount of axial overlap between the voice coil and the sensor coil can be obtained by comparing the first signal and the second signal. Based on this parameter, the position of a transducer component (e.g., the membrane) can be determined.

Description

EXCURSION SENSORS FOR AUDIO PLAYBACK DEVICES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Application No. 63/508,733, filed June 16, 2023, and U.S. Patent Application No. 63/585,788, filed September 27, 2023, each of which is incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

[0002] The present disclosure is related to consumer goods and, more particularly, to methods, systems, products, features, services, and other elements directed to media playback or some aspect thereof.

BACKGROUND

[0003] Options for accessing and listening to digital audio in an out-loud setting were limited until in 2002, when SONOS, Inc. began development of a new type of playback system. Sonos then filed one of its first patent applications in 2003, entitled “Method for Synchronizing Audio Playback between Multiple Networked Devices,” and began offering its first media playback systems for sale in 2005. The Sonos Wireless Home Sound System enables people to experience music from many sources via one or more networked playback devices. Through a software control application installed on a controller (e.g., smartphone, tablet, computer, voice input device), one can play what she wants in any room having a networked playback device. Media content (e.g., songs, podcasts, video sound) can be streamed to playback devices such that each room with a playback device can play back corresponding different media content. In addition, rooms can be grouped together for synchronous playback of the same media content, and/or the same media content can be heard in all rooms synchronously.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Features, examples, and advantages of the presently disclosed technology may be better understood with regard to the following description, appended claims, and accompanying drawings, as listed below. A person skilled in the relevant art will understand that the features shown in the drawings are for purposes of illustrations, and variations, including different and/or additional features and arrangements thereof, are possible.

[0005] Figure 1A is a partial cutaway view of an environment having a media playback system configured in accordance with examples of the disclosed technology. [0006] Figure IB is a schematic diagram of the media playback system of Figure 1 A and one or more networks.

[0007] Figure 1C is a block diagram of a playback device.

[0008] Figure ID is a block diagram of a playback device.

[0009] Figure IE is a block diagram of a network microphone device.

[0010] Figure IF is a block diagram of a network microphone device.

[0011] Figure 1G is a block diagram of a playback device.

[0012] Figure 1H is a partially schematic diagram of a control device.

[0013] Figure 2A is a front isometric view of a playback device configured in accordance with examples of the disclosed technology.

[0014] Figure 2B is a front isometric view of the playback device of Figure 2A without a grille.

[0015] Figure 2C is an exploded view of the playback device of Figure 2A.

[0016] Figure 3A is a block diagram of a playback device in accordance with examples of the disclosed technology.

[0017] Figure 3B is an isometric view of a playback device in accordance with examples of the disclosed technology.

[0018] Figure 3C is a perspective cut-away view of an audio transducer as shown in Figure 3B.

[0019] Figure 3D a cross-sectional view of the audio transducer of Figure 3C.

[0020] Figure 3E is an enlarged detail cross-sectional view of a portion of the audio transducer shown in Figure 3D.

[0021] Figure 4A illustrates signal processing steps for determining voice coil position based on sensor coil readings in accordance with examples of the present technology.

[0022] Figure 4B is an example plot of sensor coil voltage at various voice coil excursion positions.

[0023] Figure 5A is a top plan view of an audio transducer in accordance with examples of the disclosed technology.

[0024] Figures 5B and 5C are side cross-sectional views of the audio transducer shown in Figure 5A in a rest configuration and operational configuration, respectively.

[0025] Figures 6A and 6B are schematic side cross-sectional views of an audio transducer with multiple sensor coils illustrating relative axial movement of components during operation in accordance with examples of the disclosed technology. [0026] Figures 7A and 7B are schematic side cross-sectional views of an audio transducer with a single sensor coil illustrating relative axial movement of components during operation in accordance with examples of the disclosed technology.

[0027] Figures 8A and 8B are side cross-sectional views illustrating relative axial movement of a voice coil surrounded by a sensor coil in accordance with examples of the disclosed technology. [0028] Figures 9A and 9B are side cross-sectional views illustrating relative axial movement of a voice coil surrounding a sensor coil in accordance with examples of the disclosed technology.

[0029] Figures 10A and 10B are side cross-sectional views illustrating relative axial movement of two sensor coils surrounding a voice coil in accordance with examples of the disclosed technology.

[0030] Figures 11A and 1 IB are side cross-sectional views illustrating relative axial movement of multiple voice coils relative to sensor coil in accordance with examples of the disclosed technology.

[0031] Figure 12 is a side cross-sectional view of another sensor coil arrangement in accordance with examples of the disclosed technology.

[0032] Figure 13 is a side cross-sectional view of a transducer and non-overlapping sensor coil in accordance with examples of the disclosed technology.

[0033] Figure 14 is a perspective view of a transducer with a planar sensor coil in accordance with examples of the disclosed technology.

[0034] Figure 15 is a perspective view of a dual -membrane transducer with a planar sensor coil in accordance with examples of the disclosed technology.

[0035] Figure 16 is a flow chart of an example method of sensing a position of one or more components of an audio transducer in accordance with examples of the disclosed technology.

[0036] The drawings are for the purpose of illustrating example examples, but those of ordinary skill in the art will understand that the technology disclosed herein is not limited to the arrangements and/or instrumentality shown in the drawings.

DETAILED DESCRIPTION

I. Overview

[0037] Audio transducers generate sound when a diaphragm supported by a surrounding frame is moved in an oscillating, pistonic fashion inward and outward relative to the frame. In many applications related to optimization of transducer performance and related signal processing, there is a need for real-time and accurate data reflecting the diaphragm position over time. For instance, sensor data indicating the position of a transducer component (e.g., voice coil, diaphragm) can be used to monitor the health of a transducer over time. Additionally or alternatively, such sensor data may be used to adjust, in real-time, operation of the transducer by using certain correction and device protection algorithms. For instance, based on real-time sensor data, the audio transducer may limit output to maintain the diaphragm within a safe excursion range. As another example, sensor data may indicate nonlinearities in transducer output, and signal processing techniques may be used to compensate for the detected nonlinearities.

[0038] Position sensors may be particularly useful in applications involving negative stiffness audio transducers. Negative stiffness audio transducers are generally inherently unstable and may require active control systems to maintain transducer components (e.g., diaphragms, voice coils) within a predetermined excursion range. Moreover, for many negative stiffness audio transducers, when no power is supplied to the voice coil, the voice coil and/or diaphragm may fall inward (due to the negative stiffness components or due to gravity) below a maximum extent of the lower excursion range, or may be urged outward (due to the negative stiffness components) beyond a maximum extent of the upper excursion range. In such scenarios, even when power is again supplied to the voice coil, it may be insufficient to move the voice coil and diaphragm from its resting position, and as such the transducer becomes inoperable. To avoid such scenarios, a liftoff or other such recovery mechanism may be employed. However, such recovery mechanisms require position sensors to determine when the diaphragm has exceeded its maximum inward excursion limit. Additionally, a position sensor that detects the voice coil position may be needed to determine the amount of “lift-off’ needed to move the voice coil back up. Additional details regarding such negative stiffness audio transducers can be found in International Patent Publication No. WO 2023/274399, published January 5, 2023, titled “Systems and Methods for Stabilizing Playback Device,” which is hereby incorporated by reference in its entirety. Any of the systems and methods described herein can be applied to any of the negative stiffness audio transducers and components described in WO 2023/274399.

[0039] Another context in which position sensors can be beneficial involves audio transducers with multiple drive units, for example multiple voice coils configured to drive one or more diaphragms or membranes. In some implementations, dual-diaphragm audio transducers include two opposing diaphragms that move inward and outward toward and away from one another along an excursion axis. A plurality of drive units (e.g., moveable voice coils surrounding stationary magnetic components) can be coupled to respective diaphragms, with some of the drive units coupled to one diaphragm and other drive units coupled to the opposing diaphragm. In such an audio transducer such, movement of the voice coils that are coupled to the same diaphragm are desirably synchronized to achieve balanced pistonic motion of the diaphragm without wobbling, tilting, or rocking. Additionally, it can be beneficial to precisely synchronize movement of voice coils coupled to opposing diaphragms, such that the diaphragms move inward and outward in a synchronized manner, which can reduce mechanical vibration of the transducer. As such, in the case of audio transducers involving multiple drive units, accurate position sensing can be utilized to drive feedback loops that ensure the stable operation of these devices while minimizing mechanical vibration, wobbling, rocking, or other undesirable behaviors. In yet another example, position sensors that detect the position of a voice coil can be used as a sensing element for a diaphragm as a microphone or sound-sending device. As incoming audio contacts the diaphragm and moves it axially, the displacement of the voice coil can be sensed, thereby generating a signal indicative of the incident audio.

[0040] Conventional approaches for determining or estimating transducer component displacements are not suitable or practical for use in negative stiffness audio transducers, multiple drive unit transducers, or other audio transducer implementations. For instance, some optical sensors (e.g., laser sensors) are capable of accurately determining diaphragm displacement, but can be very expensive, require bulky equipment, and may be fragile. Simpler optical sensors (e.g., a light source and a photodiode) may be cheaper but are prone to interference from dust or other particles within the transducer. Inductive sensors are temperature sensitive and therefore may be inaccurate as the temperature within the audio transducer changes or generally reaches higher temperatures. Sensors that measure acceleration of transducer components typically require direct attachment to a transducer component (e.g., voice coil, diaphragm, surround or other suspension component), which undesirably adds moving mass to the transducer.

[0041] Additional displacement sensors exist but are generally more suited for industrial and robotics applications (e.g., due to size constraints, high cost, low sampling rates or low sensing resolution). Examples of such sensors include linear potentiometers or linear encoders (e.g., optical, magnetic), both of which require adding moving mass to the transducer, have relatively short wear life expectancy, low resolution, and/or require complicated electronics required to accurately read their values at a high enough rate. Currently available linear variable differential transformer (LVDT) are also expensive, add moving mass to the transducer, and require complex electronics to read their displacement. Pressure sensors are generally not suited to transducer applications due to inherent air leaks of the enclosures. Time-of-flight sensors (e.g., optical LIDAR or radar based) generally do not have the resolution or sampling rate required. Magnetic sensors (e g., hall effect sensors) may also not be suited to use within an audio transducer due to the risk of magnetic interference with the other magnetic components within the device (e.g., the main transducer magnet, the magnetic field generated by the voice coil). Finally, sonar-based ultrasonic sensors generally lack the resolution or sampling rate required, and also face the difficulty of using sound-based signals to measure position while disposed within the sound-generating transducer.

[0042] Accordingly, there remains a need for improved position sensors that can reliably detect the position of one or more transducer components (e.g., the voice coil, diaphragm, etc.) at a high sampling rate and with high resolution. For instance, a sampling rate greater than about 1kHz may be beneficial, in some embodiments between about 20-40 kHz, or at or about 44 kHz. In some embodiments, a spatial resolution of at least about 0.1 mm may be beneficial for audio transducers. It may be particularly beneficial for such position sensors to operate without adding moving mass to the transducer.

[0043] Examples of the present technology relate to improved position sensors for use in audio transducers to detect excursion of a diaphragm (or to detect the position of other moving components within the transducer). As described in more detail below, a position sensor can include a variable coupling ratio transformer disposed within the motor structure of an audio transducer. The voice coil can form one side of the transformer while a separate sensor coil disposed within the transducer forms the other side of the transformer. In some examples, the sensor coil can be stationary within the transducer, while the voice coil moves axially inward and outward along an excursion axis. An input signal carried by the voice coil (e.g., a fixed voltage AC signal such as an audio signal) will induce a corresponding signal in the sensor coil indicating an amount of axial overlap (or other change in relative axial positions) between the voice coil and the sensor coil. As the voice coil moves axially relative to the sensor coil during audio playback, the amount of overlap (or other change in relative axial positions) between the voice coil and the sensor coil varies. This variable axial positions creates a variable coupling ratio between the two coils, such that the signal detected via the sensor coil (e.g., the voltage) increases or decreases as the voice coil moves axially inward or outward. Accordingly, by reading the output of the sensor coil (e.g., measuring the voltage across terminals of the sensor coil), the amount of overlap between the sensor coil and the voice coil can be determined, which indicates an axial position of the voice coil relative to the sensor coil. As the voice coil is operably coupled to the diaphragm, this sensor coil output can be used to determine the axial position of the diaphragm.

[0044] While some examples described herein may refer to functions performed by given actors such as “users,” “listeners,” and/or other entities, it should be understood that this is for purposes of explanation only. The claims should not be interpreted to require action by any such example actor unless explicitly required by the language of the claims themselves.

[0045] In the Figures, identical reference numbers identify generally similar, and/or identical, elements. To facilitate the discussion of any particular element, the most significant digit or digits of a reference number refers to the Figure in which that element is first introduced. For example, element 110a is first introduced and discussed with reference to Figure 1A. Many of the details, dimensions, angles and other features shown in the Figures are merely illustrative of particular examples of the disclosed technology. Accordingly, other examples can have other details, dimensions, angles and features without departing from the spirit or scope of the disclosure. In addition, those of ordinary skill in the art will appreciate that further examples of the various disclosed technologies can be practiced without several of the details described below.

II. Suitable Operating Environment

[0046] Figure 1A is a partial cutaway view of a media playback system 100 distributed in an environment 101 (e.g., a house). The media playback system 100 comprises one or more playback devices 110 (identified individually as playback devices 1 lOa-n), one or more network microphone devices (“NMDs”), 120 (identified individually asNMDs 120a-c), and one or more control devices 130 (identified individually as control devices 130a and 130b).

[0047] As used herein the term “playback device” can generally refer to a network device configured to receive, process, and output data of a media playback system. For example, a playback device can be a network device that receives and processes audio content. In some examples, a playback device includes one or more transducers or speakers powered by one or more amplifiers. In other examples, however, a playback device includes one of (or neither of) the speaker and the amplifier. For instance, a playback device can comprise one or more amplifiers configured to drive one or more speakers external to the playback device via a corresponding wire or cable.

[0048] Moreover, as used herein the term NMD (i.e., a “network microphone device”) can generally refer to a network device that is configured for audio detection. In some examples, an NMD is a stand-alone device configured primarily for audio detection. In other examples, an NMD is incorporated into a playback device (or vice versa).

[0049] The term “control device” can generally refer to a network device configured to perform functions relevant to facilitating user access, control, and/or configuration of the media playback system 100.

[0050] Each of the playback devices 110 is configured to receive audio signals or data from one or more media sources (e.g., one or more remote servers, one or more local devices) and play back the received audio signals or data as sound. The one or more NMDs 120 are configured to receive spoken word commands, and the one or more control devices 130 are configured to receive user input. In response to the received spoken word commands and/or user input, the media playback system 100 can play back audio via one or more of the playback devices 110. In certain examples, the playback devices 110 are configured to commence playback of media content in response to a trigger. For instance, one or more of the playback devices 110 can be configured to play back a morning playlist upon detection of an associated trigger condition (e.g., presence of a user in a kitchen, detection of a coffee machine operation). In some examples, for instance, the media playback system 100 is configured to play back audio from a first playback device (e.g., the playback device 110a) in synchrony with a second playback device (e.g., the playback device 110b). Interactions between the playback devices 110, NMDs 120, and/or control devices 130 of the media playback system 100 configured in accordance with the various examples of the disclosure are described in greater detail below.

[0051] In the illustrated example of Figure 1A, the environment 101 comprises a household having several rooms, spaces, and/or playback zones, including (clockwise from upper left) a master bathroom 101a, a master bedroom 101b, a second bedroom 101c, a family room or den 101 d, an office lOle, a living room 10 If, a dining room 101g, a kitchen lOlh, and an outdoor patio lOli. While certain examples are described below in the context of a home environment, the technologies described herein may be implemented in other types of environments. In some examples, for instance, the media playback system 100 can be implemented in one or more commercial settings (e.g., a restaurant, mall, airport, hotel, a retail or other store), one or more vehicles (e.g., a sports utility vehicle, bus, car, a ship, a boat, an airplane), multiple environments (e.g., a combination of home and vehicle environments), and/or another suitable environment where multi-zone audio may be desirable.

[0052] The media playback system 100 can comprise one or more playback zones, some of which may correspond to the rooms in the environment 101 . The media playback system 100 can be established with one or more playback zones, after which additional zones may be added, or removed to form, for example, the configuration shown in Figure 1A. Each zone may be given a name according to a different room or space such as the office lOle, master bathroom 101a, master bedroom 101b, the second bedroom 101c, kitchen lOlh, dining room 101g, living room lOlf, and/or the balcony lOli. In some examples, a single playback zone may include multiple rooms or spaces. In certain examples, a single room or space may include multiple playback zones.

[0053] In the illustrated example of Figure 1A, the master bathroom 101a, the second bedroom 101c, the office lOle, the living room 10 If, the dining room 101g, the kitchen lOlh, and the outdoor patio lOli each include one playback device 110, and the master bedroom 101b and the den lOld include a plurality of playback devices 110. In the master bedroom 101b, the playback devices 1101 and 110m may be configured, for example, to play back audio content in synchrony as individual ones of playback devices 110, as a bonded playback zone, as a consolidated playback device, and/or any combination thereof. Similarly, in the den lOld, the playback devices HOh-j can be configured, for instance, to play back audio content in synchrony as individual ones of playback devices 110, as one or more bonded playback devices, and/or as one or more consolidated playback devices. Additional details regarding bonded and consolidated playback devices are described below with respect to Figures IB and IE.

[0054] In some examples, one or more of the playback zones in the environment 101 may each be playing different audio content. For instance, a user may be grilling on the patio lOli and listening to hip hop music being played by the playback device 110c while another user is preparing food in the kitchen lOlh and listening to classical music played by the playback device 110b. In another example, a playback zone may play the same audio content in synchrony with another playback zone. For instance, the user may be in the office lOle listening to the playback device 11 Of playing back the same hip-hop music being played back by playback device 110c on the patio lOli. In some examples, the playback devices 110c and 1 lOf play back the hip hop music in synchrony such that the user perceives that the audio content is being played seamlessly (or at least substantially seamlessly) while moving between different playback zones. Additional details regarding audio playback synchronization among playback devices and/or zones can be found, for example, in U.S. Patent No. 8,234,395 entitled, “System and method for synchronizing operations among a plurality of independently clocked digital data processing devices,” which is incorporated herein by reference in its entirety. a. Suitable Media Playback System

[0055] Figure IB is a schematic diagram of the media playback system 100 and a cloud network 102. For ease of illustration, certain devices of the media playback system 100 and the cloud network 102 are omitted from Figure IB. One or more communication links 103 (referred to hereinafter as “the links 103”) communicatively couple the media playback system 100 and the cloud network 102.

[0056] The links 103 can comprise, for example, one or more wired networks, one or more wireless networks, one or more wide area networks (WAN), one or more local area networks (LAN), one or more personal area networks (PAN), one or more telecommunication networks (e g., one or more Global System for Mobiles (GSM) networks, Code Division Multiple Access (CDMA) networks, Long-Term Evolution (LTE) networks, 5G communication network networks, and/or other suitable data transmission protocol networks), etc. The cloud network 102 is configured to deliver media content (e.g., audio content, video content, photographs, social media content) to the media playback system 100 in response to a request transmitted from the media playback system 100 via the links 103. In some examples, the cloud network 102 is further configured to receive data (e.g. voice input data) from the media playback system 100 and correspondingly transmit commands and/or media content to the media playback system 100.

[0057] The cloud network 102 comprises computing devices 106 (identified separately as a first computing device 106a, a second computing device 106b, and a third computing device 106c). The computing devices 106 can comprise individual computers or servers, such as, for example, a media streaming service server storing audio and/or other media content, a voice service server, a social media server, a media playback system control server, etc. In some examples, one or more of the computing devices 106 comprise modules of a single computer or server. In certain examples, one or more of the computing devices 106 comprise one or more modules, computers, and/or servers. Moreover, while the cloud network 102 is described above in the context of a single cloud network, in some examples the cloud network 102 comprises a plurality of cloud networks comprising communicatively coupled computing devices. Furthermore, while the cloud network 102 is shown in Figure IB as having three of the computing devices 106, in some examples, the cloud network 102 comprises fewer (or more than) three computing devices 106.

[0058] The media playback system 100 is configured to receive media content from the networks

102 via the links 103. The received media content can comprise, for example, a Uniform Resource Identifier (URI) and/or a Uniform Resource Locator (URL). For instance, in some examples, the media playback system 100 can stream, download, or otherwise obtain data from a URI or a URL corresponding to the received media content. A network 104 communicatively couples the links

103 and at least a portion of the devices (e.g., one or more of the playback devices 110, NMDs 120, and/or control devices 130) of the media playback system 100. The network 104 can include, for example, a wireless network (e.g., a WiFi network, a Bluetooth, a Z-Wave network, a ZigBee, and/or other suitable wireless communication protocol network) and/or a wired network (e.g., a network comprising Ethernet, Universal Serial Bus (USB), and/or another suitable wired communication). As those of ordinary skill in the art will appreciate, as used herein, “WiFi” can refer to several different communication protocols including, for example, Institute of Electrical and Electronics Engineers (IEEE) 802.11a, 802.11b, 802.11g, 802. l ln, 802.11ac, 802.11ac, 802. Had, 802.1 laf, 802.11 ah, 802.1 lai, 802.11aj, 802.1 laq, 802.1 lax, 802.1 lay, 802.15, etc. transmitted at 2.4 Gigahertz (GHz), 5 GHz, and/or another suitable frequency.

[0059] In some examples, the network 104 comprises a dedicated communication network that the media playback system 100 uses to transmit messages between individual devices and/or to transmit media content to and from media content sources (e.g., one or more of the computing devices 106). In certain examples, the network 104 is configured to be accessible only to devices in the media playback system 100, thereby reducing interference and competition with other household devices. In other examples, however, the network 104 comprises an existing household communication network (e.g., a household WiFi network). In some examples, the links 103 and the network 104 comprise one or more of the same networks. In some examples, for example, the links 103 and the network 104 comprise a telecommunication network (e.g., an LTE network, a 5G network). Moreover, in some examples, the media playback system 100 is implemented without the network 104, and devices comprising the media playback system 100 can communicate with each other, for example, via one or more direct connections, PANs, telecommunication networks, and/or other suitable communication links.

[0060] In some examples, audio content sources may be regularly added or removed from the media playback system 100. In some examples, for instance, the media playback system 100 performs an indexing of media items when one or more media content sources are updated, added to, and/or removed from the media playback system 100. The media playback system 100 can scan identifiable media items in some or all folders and/or directories accessible to the playback devices 110, and generate or update a media content database comprising metadata (e.g., title, artist, album, track length) and other associated information (e.g., URIs, URLs) for each identifiable media item found. In some examples, for instance, the media content database is stored on one or more of the playback devices 110, network microphone devices 120, and/or control devices 130.

[0061] In the illustrated example of Figure IB, the playback devices 1101 and 110m comprise a group 107a. The playback devices 1101 and 110m can be positioned in different rooms in a household and be grouped together in the group 107a on a temporary or permanent basis based on user input received at the control device 130a and/or another control device 130 in the media playback system 100. When arranged in the group 107a, the playback devices 1101 and 110m can be configured to play back the same or similar audio content in synchrony from one or more audio content sources. In certain examples, for instance, the group 107a comprises a bonded zone in which the playback devices 1101 and 110m comprise left audio and right audio channels, respectively, of multi-channel audio content, thereby producing or enhancing a stereo effect of the audio content. In some examples, the group 107a includes additional playback devices 110. In other examples, however, the media playback system 100 omits the group 107a and/or other grouped arrangements of the playback devices 110.

[0062] The media playback system 100 includes the NMDs 120a and 120d, each comprising one or more microphones configured to receive voice utterances from a user. In the illustrated example of Figure IB, the NMD 120a is a standalone device and the NMD 120d is integrated into the playback device 1 lOn. The NMD 120a, for example, is configured to receive voice input 121 from a user 123. In some examples, the NMD 120a transmits data associated with the received voice input 121 to a voice assistant service (VAS) configured to (i) process the received voice input data and (ii) transmit a corresponding command to the media playback system 100. In some examples, for instance, the computing device 106c comprises one or more modules and/or servers of a VAS (e.g., a VAS operated by one or more of SONOS®, AMAZON®, GOOGLE® APPLE®, MICROSOFT®). The computing device 106c can receive the voice input data from the NMD 120a via the network 104 and the links 103. In response to receiving the voice input data, the computing device 106c processes the voice input data (i.e., “Play Hey Jude by The Beatles”), and determines that the processed voice input includes a command to play a song (e.g., “Hey Jude”). The computing device 106c accordingly transmits commands to the media playback system 100 to play back “Hey Jude” by the Beatles from a suitable media service (e.g., via one or more of the computing devices 106) on one or more of the playback devices 110. b. Suitable Playback Devices

[0063] Figure 1C is a block diagram of the playback device 110a comprising an input/output 111. The input/output 111 can include an analog I/O 11 la (e.g., one or more wires, cables, and/or other suitable communication links configured to carry analog signals) and/or a digital EO 111b (e g., one or more wires, cables, or other suitable communication links configured to carry digital signals). In some examples, the analog I/O 11 la is an audio line-in input connection comprising, for example, an auto-detecting 3.5mm audio line-in connection. In some examples, the digital EO 11 lb comprises a Sony /Philips Digital Interface Format (S/PDIF) communication interface and/or cable and/or a Toshiba Link (TOSLINK) cable. In some examples, the digital EO 111b comprises a High-Definition Multimedia Interface (HDMI) interface and/or cable. In some examples, the digital I/O 11 lb includes one or more wireless communication links comprising, for example, a radio frequency (RF), infrared, WiFi, Bluetooth, or another suitable communication protocol. In certain examples, the analog I/O I l la and the digital 111b comprise interfaces (e.g., ports, plugs, jacks) configured to receive connectors of cables transmitting analog and digital signals, respectively, without necessarily including cables.

[0064] The playback device 110a, for example, can receive media content (e.g., audio content comprising music and/or other sounds) from a local audio source 105 via the input/output 111 (e.g., a cable, a wire, a PAN, a Bluetooth connection, an ad hoc wired or wireless communication network, and/or another suitable communication link). The local audio source 105 can comprise, for example, a mobile device (e.g., a smartphone, a tablet, a laptop computer) or another suitable audio component (e g., a television, a desktop computer, an amplifier, a phonograph, a Blu-ray player, a memory storing digital media files). In some examples, the local audio source 105 includes local music libraries on a smartphone, a computer, a networked-attached storage (NAS), and/or another suitable device configured to store media files. In certain examples, one or more of the playback devices 110, NMDs 120, and/or control devices 130 comprise the local audio source 105. In other examples, however, the media playback system omits the local audio source 105 altogether. In some examples, the playback device 110a does not include an input/output 111 and receives all audio content via the network 104.

[0065] The playback device 110a further comprises electronics 1 12, a user interface 113 (e.g., one or more buttons, knobs, dials, touch-sensitive surfaces, displays, touchscreens), and one or more transducers 114 (referred to hereinafter as “the transducers 114”). The electronics 112 is configured to receive audio from an audio source (e.g., the local audio source 105) via the input/output 111, one or more of the computing devices 106a-c via the network 104 (Figure IB)), amplify the received audio, and output the amplified audio for playback via one or more of the transducers 114. In some examples, the playback device 110a optionally includes one or more microphones 115 (e.g., a single microphone, a plurality of microphones, a microphone array) (hereinafter referred to as “the microphones 115”). In certain examples, for example, the playback device 110a having one or more of the optional microphones 115 can operate as an NMD configured to receive voice input from a user and correspondingly perform one or more operations based on the received voice input.

[0066] In the illustrated example of Figure 1C, the electronics 112 comprise one or more processors 112a (referred to hereinafter as “the processors 112a”), memory 112b, software components 112c, a network interface 112d, one or more audio processing components 112g (referred to hereinafter as “the audio components 112g”), one or more audio amplifiers 112h (referred to hereinafter as “the amplifiers 112h”), and power 112i (e.g., one or more power supplies, power cables, power receptacles, batteries, induction coils, Power-over Ethernet (POE) interfaces, and/or other suitable sources of electric power). In some examples, the electronics 112 optionally include one or more other components 112j (e.g., one or more sensors, video displays, touchscreens, battery charging bases).

[0067] The processors 112a can comprise clock-driven computing component(s) configured to process data, and the memory 112b can comprise a computer-readable medium (e.g., a tangible, non-transitory computer-readable medium, data storage loaded with one or more of the software components 112c) configured to store instructions for performing various operations and/or functions. The processors 112a are configured to execute the instructions stored on the memory 112b to perform one or more of the operations. The operations can include, for example, causing the playback device 110a to retrieve audio data from an audio source (e.g., one or more of the computing devices 106a-c (Figure IB)), and/or another one of the playback devices 110. In some examples, the operations further include causing the playback device 110a to send audio data to another one of the playback devices 110a and/or another device (e.g., one of the NMDs 120). Certain examples include operations causing the playback device 110a to pair with another of the one or more playback devices 110 to enable a multi-channel audio environment (e.g., a stereo pair, a bonded zone).

[0068] The processors 112a can be further configured to perform operations causing the playback device 110a to synchronize playback of audio content with another of the one or more playback devices 110. As those of ordinary skill in the art will appreciate, during synchronous playback of audio content on a plurality of playback devices, a listener will preferably be unable to perceive time-delay differences between playback of the audio content by the playback device 110a and the other one or more other playback devices 110. Additional details regarding audio playback synchronization among playback devices can be found, for example, in U.S. Patent No. 8,234,395, which was incorporated by reference above.

[0069] In some examples, the memory 112b is further configured to store data associated with the playback device 110a, such as one or more zones and/or zone groups of which the playback device 110a is a member, audio sources accessible to the playback device 110a, and/or a playback queue that the playback device 110a (and/or another of the one or more playback devices) can be associated with. The stored data can comprise one or more state variables that are periodically updated and used to describe a state of the playback device 110a. The memory 112b can also include data associated with a state of one or more of the other devices (e.g., the playback devices 110, NMDs 120, control devices 130) of the media playback system 100. In some examples, for instance, the state data is shared during predetermined intervals of time (e.g., every 5 seconds, every 10 seconds, every 60 seconds) among at least a portion of the devices of the media playback system 100, so that one or more of the devices have the most recent data associated with the media playback system 100.

[0070] The network interface 112d is configured to facilitate a transmission of data between the playback device 110a and one or more other devices on a data network such as, for example, the links 103 and/or the network 104 (Figure IB). The network interface 112d is configured to transmit and receive data corresponding to media content (e.g., audio content, video content, text, photographs) and other signals (e.g., non-transitory signals) comprising digital packet data including an Internet Protocol (IP)-based source address and/or an IP -based destination address. The network interface 112d can parse the digital packet data such that the electronics 112 properly receives and processes the data destined for the playback device 110a.

[0071] In the illustrated example of Figure 1 C, the network interface 1 12d comprises one or more wireless interfaces 112e (referred to hereinafter as “the wireless interface 112e”). The wireless interface 112e (e.g., a suitable interface comprising one or more antennae) can be configured to wirelessly communicate with one or more other devices (e.g., one or more of the other playback devices 110, NMDs 120, and/or control devices 130) that are communicatively coupled to the network 104 (Figure IB) in accordance with a suitable wireless communication protocol (e.g., WiFi, Bluetooth, LTE). In some examples, the network interface 112d optionally includes a wired interface 112f (e.g., an interface or receptacle configured to receive a network cable such as an Ethernet, a USB-A, USB-C, and/or Thunderbolt cable) configured to communicate over a wired connection with other devices in accordance with a suitable wired communication protocol. In certain examples, the network interface 112d includes the wired interface 112f and excludes the wireless interface 112e. In some examples, the electronics 112 excludes the network interface 112d altogether and transmits and receives media content and/or other data via another communication path (e.g., the input/output 111).

[0072] The audio components 112g are configured to process and/or filter data comprising media content received by the electronics 112 (e.g., via the input/output 111 and/or the network interface 112d) to produce output audio signals. In some examples, the audio processing components 112g comprise, for example, one or more digital-to-analog converters (DAC), audio preprocessing components, audio enhancement components, a digital signal processors (DSPs), and/or other suitable audio processing components, modules, circuits, etc. In certain examples, one or more of the audio processing components 112g can comprise one or more subcomponents of the processors 112a. In some examples, the electronics 112 omits the audio processing components 112g. In some examples, for instance, the processors 112a execute instructions stored on the memory 112b to perform audio processing operations to produce the output audio signals.

[0073] The amplifiers 112h are configured to receive and amplify the audio output signals produced by the audio processing components 112g and/or the processors 112a. The amplifiers 112h can comprise electronic devices and/or components configured to amplify audio signals to levels sufficient for driving one or more of the transducers 114. In some examples, for instance, the amplifiers 112h include one or more switching or class-D power amplifiers. In other examples, however, the amplifiers include one or more other types of power amplifiers (e.g., linear gain power amplifiers, class-A amplifiers, class-B amplifiers, class-AB amplifiers, class-C amplifiers, class-D amplifiers, class-E amplifiers, class-F amplifiers, class-G and/or class H amplifiers, and/or another suitable type of power amplifier). In certain examples, the amplifiers 112h comprise a suitable combination of two or more of the foregoing types of power amplifiers. Moreover, in some examples, individual ones of the amplifiers 112h correspond to individual ones of the transducers 114. In other examples, however, the electronics 112 includes a single one of the amplifiers 112h configured to output amplified audio signals to a plurality of the transducers 114. In some other examples, the electronics 112 omits the amplifiers 112h.

[0074] The transducers 114 (e.g., one or more speakers and/or speaker drivers) receive the amplified audio signals from the amplifier 112h and render or output the amplified audio signals as sound (e.g., audible sound waves having a frequency between about 20 Hertz (Hz) and 20 kilohertz (kHz)). In some examples, the transducers 114 can comprise a single transducer. In other examples, however, the transducers 114 comprise a plurality of audio transducers. In some examples, the transducers 114 comprise more than one type of transducer. For example, the transducers 114 can include one or more low frequency transducers (e.g., subwoofers, woofers), mid-range frequency transducers (e.g., mid-range transducers, mid-woofers), and one or more high frequency transducers (e.g., one or more tweeters). As used herein, “low frequency” can generally refer to audible frequencies below about 500 Hz, “mid-range frequency” can generally refer to audible frequencies between about 500 Hz and about 2 kHz, and “high frequency” can generally refer to audible frequencies above 2 kHz. In certain examples, however, one or more of the transducers 114 comprise transducers that do not adhere to the foregoing frequency ranges. For example, one of the transducers 114 may comprise a mid-woofer transducer configured to output sound at frequencies between about 200 Hz and about 5 kHz.

[0075] By way of illustration, SONOS, Inc. presently offers (or has offered) for sale certain playback devices including, for example, a “SONOS ONE,” “MOVE,” “PLAYA,” “BEAM,” “PLAYBAR ,” “PLAYBASE ,” “PORT,” “BOOST,” “AMP,” and “SUB.” Other suitable playback devices may additionally or alternatively be used to implement the playback devices of example examples disclosed herein. Additionally, one of ordinary skilled in the art will appreciate that a playback device is not limited to the examples described herein or to SONOS product offerings. In some examples, for example, one or more playback devices 110 comprises wired or wireless headphones (e.g., over-the-ear headphones, on-ear headphones, in-ear earphones). In other examples, one or more of the playback devices 110 comprise a docking station and/or an interface configured to interact with a docking station for personal mobile media playback devices. In certain examples, a playback device may be integral to another device or component such as a television, a lighting fixture, or some other device for indoor or outdoor use. In some examples, a playback device omits a user interface and/or one or more transducers. For example, FIG. ID is a block diagram of a playback device I lOp comprising the input/output 111 and electronics 112 without the user interface 113 or transducers 114.

[0076] Figure IE is a block diagram of a bonded playback device 1 lOq comprising the playback device 110a (Figure 1C) sonically bonded with the playback device HOi (e.g., a subwoofer) (Figure 1A). In the illustrated example, the playback devices 110a and HOi are separate ones of the playback devices 110 housed in separate enclosures. In some examples, however, the bonded playback device HOq comprises a single enclosure housing both the playback devices 110a and HOi. The bonded playback device HOq can be configured to process and reproduce sound differently than an unbonded playback device (e.g., the playback device 110a of Figure 1C) and/or paired or bonded playback devices (e.g., the playback devices 1101 and 110m of Figure IB). In some examples, for instance, the playback device 110a is full-range playback device configured to render low frequency, mid-range frequency, and high frequency audio content, and the playback device HOi is a subwoofer configured to render low frequency audio content. In some examples, the playback device 110a, when bonded with the first playback device, is configured to render only the mid-range and high frequency components of a particular audio content, while the playback device HOi renders the low frequency component of the particular audio content. In some examples, the bonded playback device HOq includes additional playback devices and/or another bonded playback device. Additional playback device examples are described in further detail below with respect to Figures 2A-2C. c. Suitable Network Microphone Devices (NMDs)

[0077] Figure IF is a block diagram of the NMD 120a (Figures 1A and IB). The NMD 120a includes one or more voice processing components 124 (hereinafter “the voice components 124”) and several components described with respect to the playback device 110a (Figure 1C) including the processors 112a, the memory 112b, and the microphones 115. The NMD 120a optionally comprises other components also included in the playback device 110a (Figure 1C), such as the user interface 113 and/or the transducers 114. In some examples, the NMD 120a is configured as a media playback device (e.g., one or more of the playback devices 110), and further includes, for example, one or more of the audio components 112g (Figure 1 C), the amplifiers 1 12h, and/or other playback device components. In certain examples, the NMD 120a comprises an Internet of Things (loT) device such as, for example, a thermostat, alarm panel, fire and/or smoke detector, etc. In some examples, the NMD 120a comprises the microphones 115, the voice processing components 124, and only a portion of the components of the electronics 112 described above with respect to Figure IB. In some examples, for instance, the NMD 120a includes the processor 112a and the memory 112b (Figure IB), while omitting one or more other components of the electronics 112. In some examples, the NMD 120a includes additional components (e.g., one or more sensors, cameras, thermometers, barometers, hygrometers).

[0078] In some examples, an NMD can be integrated into a playback device. Figure 1G is a block diagram of a playback device HOr comprising an NMD 120d. The playback device HOr can comprise many or all of the components of the playback device 110a and further include the microphones 115 and voice processing components 124 (Figure IF). The playback device HOr optionally includes an integrated control device 130c. The control device 130c can comprise, for example, a user interface (e.g., the user interface 113 of Figure IB) configured to receive user input (e.g., touch input, voice input) without a separate control device. In other examples, however, the playback device 1 lOr receives commands from another control device (e.g., the control device 130a of Figure IB).

[0079] Referring again to Figure IF, the microphones 115 are configured to acquire, capture, and/or receive sound from an environment (e g., the environment 101 of Figure 1 A) and/or a room in which the NMD 120a is positioned. The received sound can include, for example, vocal utterances, audio played back by the NMD 120a and/or another playback device, background voices, ambient sounds, etc. The microphones 115 convert the received sound into electrical signals to produce microphone data. The voice processing components 124 receive and analyzes the microphone data to determine whether a voice input is present in the microphone data. The voice input can comprise, for example, an activation word followed by an utterance including a user request. As those of ordinary skill in the art will appreciate, an activation word is a word or other audio cue that signifying a user voice input. For instance, in querying the AMAZON® VAS, a user might speak the activation word "Alexa. " Other examples include "Ok, Google" for invoking the GOOGLE® VAS and "Hey, Siri" for invoking the APPLE® VAS.

[0080] After detecting the activation word, voice processing components 124 monitor the microphone data for an accompanying user request in the voice input. The user request may include, for example, a command to control a third-party device, such as a thermostat (e.g., NEST® thermostat), an illumination device (e.g., a PHILIPS HUE ® lighting device), or a media playback device (e.g., a Sonos® playback device). For example, a user might speak the activation word “Alexa” followed by the utterance “set the thermostat to 68 degrees” to set a temperature in a home (e.g., the environment 101 of Figure 1A). The user might speak the same activation word followed by the utterance “turn on the living room” to turn on illumination devices in a living room area of the home. The user may similarly speak an activation word followed by a request to play a particular song, an album, or a playlist of music on a playback device in the home. d. Suitable Control Devices

[0081] Figure 1H is a partially schematic diagram of the control device 130a (Figures 1A and IB). As used herein, the term “control device” can be used interchangeably with “controller” or “control system.” Among other features, the control device 130a is configured to receive user input related to the media playback system 100 and, in response, cause one or more devices in the media playback system 100 to perform an action(s) or operation(s) corresponding to the user input. In the illustrated example, the control device 130a comprises a smartphone (e.g., an iPhone™ an Android phone) on which media playback system controller application software is installed. In some examples, the control device 130a comprises, for example, a tablet (e.g., an iPad™), a computer (e.g., a laptop computer, a desktop computer), and/or another suitable device (e.g., a television, an automobile audio head unit, an loT device). In certain examples, the control device 130a comprises a dedicated controller for the media playback system 100. In other examples, as described above with respect to Figure 1G, the control device 130a is integrated into another device in the media playback system 100 (e.g., one more of the playback devices 110, NMDs 120, and/or other suitable devices configured to communicate over a network).

[0082] The control device 130a includes electronics 132, a user interface 133, one or more speakers 134, and one or more microphones 135. The electronics 132 comprise one or more processors 132a (referred to hereinafter as “the processors 132a”), a memory 132b, software components 132c, and a network interface 132d. The processor 132a can be configured to perform functions relevant to facilitating user access, control, and configuration of the media playback system 100. The memory 132b can comprise data storage that can be loaded with one or more of the software components executable by the processor 132a to perform those functions. The software components 132c can comprise applications and/or other executable software configured to facilitate control of the media playback system 100. The memory 112b can be configured to store, for example, the software components 132c, media playback system controller application software, and/or other data associated with the media playback system 100 and the user.

[0083] The network interface 132d is configured to facilitate network communications between the control device 130a and one or more other devices in the media playback system 100, and/or one or more remote devices. In some examples, the network interface 132d is configured to operate according to one or more suitable communication industry standards (e.g., infrared, radio, wired standards including IEEE 802.3, wireless standards including IEEE 802.11a, 802.11b, 802.11g, 802. l ln, 802.11ac, 802.15, 4G, LTE). The network interface 132d can be configured, for example, to transmit data to and/or receive data from the playback devices 110, the NMDs 120, other ones of the control devices 130, one of the computing devices 106 of Figure IB, devices comprising one or more other media playback systems, etc. The transmitted and/or received data can include, for example, playback device control commands, state variables, playback zone and/or zone group configurations. For instance, based on user input received at the user interface 133, the network interface 132d can transmit a playback device control command (e.g., volume control, audio playback control, audio content selection) from the control device 130 to one or more of the playback devices 110. The network interface 132d can also transmit and/or receive configuration changes such as, for example, adding/removing one or more playback devices 110 to/from a zone, adding/removing one or more zones to/from a zone group, forming a bonded or consolidated player, separating one or more playback devices from a bonded or consolidated player, among others.

[0084] The user interface 133 is configured to receive user input and can facilitate control of the media playback system 100. The user interface 133 includes media content art 133a (e.g., album art, lyrics, videos), a playback status indicator 133b (e.g., an elapsed and/or remaining time indicator), media content information region 133c, a playback control region 133d, and a zone indicator 133e. The media content information region 133c can include a display of relevant information (e.g., title, artist, album, genre, release year) about media content currently playing and/or media content in a queue or playlist. The playback control region 133d can include selectable (e.g., via touch input and/or via a cursor or another suitable selector) icons to cause one or more playback devices in a selected playback zone or zone group to perform playback actions such as, for example, play or pause, fast forward, rewind, skip to next, skip to previous, enter/exit shuffle mode, enter/exit repeat mode, enter/exit cross fade mode, etc. The playback control region 133d may also include selectable icons to modify equalization settings, playback volume, and/or other suitable playback actions. In the illustrated example, the user interface 133 comprises a display presented on a touch screen interface of a smartphone (e.g., an iPhone™ an Android phone). In some examples, however, user interfaces of varying formats, styles, and interactive sequences may alternatively be implemented on one or more network devices to provide comparable control access to a media playback system.

[0085] The one or more speakers 134 (e.g., one or more transducers) can be configured to output sound to the user of the control device 130a. In some examples, the one or more speakers comprise individual transducers configured to correspondingly output low frequencies, mid-range frequencies, and/or high frequencies. In some examples, for instance, the control device 130a is configured as a playback device (e.g., one of the playback devices 110). Similarly, in some examples the control device 130a is configured as an NMD (e.g., one of the NMDs 120), receiving voice commands and other sounds via the one or more microphones 135.

[0086] The one or more microphones 135 can comprise, for example, one or more condenser microphones, electret condenser microphones, dynamic microphones, and/or other suitable types of microphones or transducers. In some examples, two or more of the microphones 135 are arranged to capture location information of an audio source (e.g., voice, audible sound) and/or configured to facilitate filtering of background noise. Moreover, in certain examples, the control device 130a is configured to operate as a playback device and an NMD. In other examples, however, the control device 130a omits the one or more speakers 134 and/or the one or more microphones 135. For instance, the control device 130a may comprise a device (e.g., athermostat, an loT device, a network device) comprising a portion of the electronics 132 and the user interface 133 (e.g., a touch screen) without any speakers or microphones. 111. Example Audio Playback Devices

[0087] Figure 2A is a front isometric view of a playback device 210 configured in accordance with examples of the disclosed technology. Figure 2B is a front isometric view of the playback device 210 without a grille 216e. Figure 2C is an exploded view of the playback device 210. Referring to Figures 2A-2C together, the playback device 210 comprises a housing 216 that includes an upper portion 216a, a right or first side portion 216b, a lower portion 216c, a left or second side portion 216d, the grille 216e, and a rear portion 216f. A plurality of fasteners 216g (e.g., one or more screws, rivets, clips) attaches a frame 216h to the housing 216. A cavity 216j (Figure 2C) in the housing 216 is configured to receive the frame 216h and electronics 212. The frame 216h is configured to carry a plurality of transducers 214 (identified individually in Figure 2B as transducers 214a-f). The electronics 212 (e.g., the electronics 112 of Figure 1 C) is configured to receive audio content from an audio source and send electrical signals corresponding to the audio content to the transducers 214 for playback.

[0088] The transducers 214 are configured to receive the electrical signals from the electronics

112, and further configured to convert the received electrical signals into audible sound during playback. For instance, the transducers 214a-c (e.g., tweeters) can be configured to output high frequency sound (e.g., sound waves having a frequency greater than about 2 kHz). The transducers 214d-f (e.g., mid-woofers, woofers, midrange speakers) can be configured output sound at frequencies lower than the transducers 214a-c (e.g., sound waves having a frequency lower than about 2 kHz). In some examples, the playback device 210 includes a number of transducers different than those illustrated in Figures 2A-2C. For example, the playback device 210 can include fewer than six transducers (e.g., one, two, three). In other examples, however, the playback device 210 includes more than six transducers (e.g., nine, ten). Moreover, in some examples, all or a portion of the transducers 214 are configured to operate as a phased array to desirably adjust (e.g., narrow or widen) a radiation pattern of the transducers 214, thereby altering a user’s perception of the sound emitted from the playback device 210.

[0089] In the illustrated example of Figures 2A-2C, a filter 216i is axially aligned with the transducer 214b. The filter 216i can be configured to desirably attenuate a predetermined range of frequencies that the transducer 214b outputs to improve sound quality and a perceived sound stage output collectively by the transducers 214. In some examples, however, the playback device 210 omits the filter 216i. In other examples, the playback device 210 includes one or more additional filters aligned with the transducers 214b and/or at least another of the transducers 214.

IV. Example Position Sensors for Audio Playback Devices a. Overview

[0090] As noted above, it can be beneficial to reliably and accurately detect the position of one or more moving components of an audio transducer in real time. For instance, the position of the diaphragm or voice coil over time can be used to monitor the health of the audio transducer and ensure that a safe excursion range is not exceeded. In some instances, position data can be used as part of a feedback loop to control the position of the voice coil, diaphragm, or other components. Such control loops may be particularly useful in negative stiffness transducers, which are at higher risk of instability and “runaway” effects in which the diaphragm moves beyond a safe excursion range to an inoperable position (e.g., at maximum inward excursion or maximum outward excursion, it may not be possible to drive the diaphragm back to a neutral position).

[0091] Examples of the present technology provide improved position sensors for use in audio transducers to detect, estimate, infer, and/or predict excursion(s) of a diaphragm(s) and/or other moving components within the transducer. In some implementations, a position sensor can include a variable coupling ratio transformer disposed within the motor structure of an audio transducer. The voice coil can form, for instance, one side of the transformer while one or more separate sensor coils disposed within the transducer form another side of the transformer.

[0092] As those of ordinary skill in the art will appreciate, transformers are commonly used when an alternating current (AC) at a given voltage needs to be converted to a new higher or lower voltage with a corresponding lower or higher current. Such transformers couple two coils of wire to a common magnetic field, in which the primary coil is driven by the AC supply and the secondary current is attached to the load (e.g., a ferromagnetic core). As the AC supply travels through the primary coil it induces an alternating magnetic field, which is typically channeled through a ferromagnetic core. As the secondary current is also coupled to the ferromagnetic core, the alternating magnetic field induces a voltage in the secondary coil, which produces an AC output in the secondary coil.

[0093] Varying the ratio of turns of the wire between the primary coil and the secondary coil varies the voltage multiplier effect of the transformer. For instance, in an ideal lossless transformer with perfect coupling between the two coils, if the primary coil has 100 turns of wire and the secondary coil has 50 turns of wire, then the AC voltage on the primary coil will be reduced by 50% when measured across the terminals of the secondary coil (e.g., if 10 volts were applied to the primary coil, 5 volts would be produced in the secondary coil). This example describes an ideal transformer in which there are no losses to heat and the coupling between the two coils is perfect. While real transformers do suffer some heat loss and imperfect coupling, the relationship between the input voltage and the output voltage of a given transformer can be well characterized.

[0094] As noted above, a position sensor for use within an audio transducer can take the form of a transformer in which a transducer voice coil forms one side of the transformer (e.g., the primary coil) and a separate sensor coil forms the other side of the transformer (e.g., the secondary coil). The sensor coil can be radially spaced apart from the transducer coil (e.g., surrounded by the voice coil, or alternatively surrounding the voice coil) with a radial gap between them. In various examples, the sensor coil can be stationary within the audio transducer, while the voice coil moves axially inward and outward along the excursion axis.

[0095] During operation, the input signal carried by the voice coil (e.g., a fixed voltage AC signal such as an audio signal) will induce a corresponding signal in the sensor coil that can correspond to an amount of axial overlap (and/or an amount of axial separation) between the voice coil and the sensor coil. As the voice coil moves axially relative to the sensor coil during audio playback, the amount of overlap between the voice coil and the sensor coil varies. This variable overlap creates a variable coupling ratio between the two coils, such that the signal detected via the sensor coil (e.g., the voltage) is a function of the amount of overlap between the voice coil and the sensor coil. Accordingly, by reading the output of the sensor coil (e.g., measuring the voltage across terminals of the sensor coil), the amount of overlap between the sensor coil and the voice coil can be determined, which indicates an axial position of the voice coil relative to the sensor coil. As the voice coil is fixedly coupled to the diaphragm, this sensor coil output can be used to determine the axial position of the diaphragm.

[0096] As noted above, in some instances the axial position determination can be used to facilitate a feedback loop that limits transducer output to maintain the diaphragm within a safe excursion range. Additional details regarding limiting transducer output to maintain the diaphragm within a safe excursion range can be found in U.S. Patent No. 11,528,552, titled “Signal Limit Based on Prediction Model,” which is hereby incorporated by reference in its entirety. In the case of audio transducers that include multiple voice coils, the axial position determinations can also be used to adjust the relative positions of the various voice coils, such as to preserve balance and synchronization between the various voice coils.

[0097] Examples of such transducers and position sensors are described below with respect to Figures 3A-13. Figures 3A-4 illustrate example single drive unit audio transducers including position sensors. Figures 5A-7B illustrate example multiple drive unit audio transducers including position sensors. Figures 8A-12 illustrate example arrangements of voice coils and sensor coils. And Figure 12 illustrates an example method for operating an audio transducer including one or more position sensors. b. Example Audio Playback Devices with Position Sensors

[0098] Figure 3A is a block diagram of an example playback device 310. As illustrated, the playback device 310 includes an audio transducer 314 coupled with an enclosure 316. The enclosure 316 can also house electronics 312, which can be similar to electronics 112 described previously with respect to Figure 1C. As shown in Figure 3A, the playback device 310 can optionally include one or more other components 3 lOj (e.g., user interface components such as buttons or switches, etc.).

[0099] The audio transducer 314 includes a frame 316h, to which a membrane (e.g., diaphragm 320) can be coupled via a flexible surround 322. The diaphragm 320 can also be operably coupled to a voice coil 328 such that, when the voice coil 328 moves axially along an excursion axis, the diaphragm 320 likewise moves axially along the excursion axis, thereby moving air to generate sound waves. The voice coil 328 can be disposed adjacent to a magnet 326 that provides a permanent magnetic field to facilitate movement of the voice coil 328 in response to current flowing therethrough.

[0100] The transducer 314 can further include one or more suspension elements 330 that secure or stabilize movement portions of the transducer 314 relative to the frame 316h. For instance, the suspension elements 330 can take the form of an annular spider extending between the frame 316h and the voice coil. In some implementations, the suspension elements 330 can include one or more negative stiffness components (e.g., as described in more detail in International Patent Publication No. WO 2023/274399, which as noted above is incorporated by reference in its entirety herein). Finally, the transducer 314 can optionally include one or more other additional components 314j as desired. [0101] With continued reference to Figure 3 A, the playback device 310 includes a stabilizer 350, which in turn includes one or more control members 352 and one or more position sensors 354. In operation, the position sensor(s) 354 can sense a position of one or more moveable components of the transducer 314 (e.g., the diaphragm 320, the voice coil 328, etc.). Based on this detected position, the control members 352 can optionally work to reposition the moveable components to a desired location. For instance, if the position sensor 354 determines that the diaphragm is moving axially inward over time, then the control member(s) 352 can act to urge the diaphragm axially outward and back toward a neutral position. In various implementations, the control members 352 can include a drive system configured to supply a signal to the voice coil 328 that moves the diaphragm to its intended position (e.g., applying a DC offset to the audio signal provided to the voice coil 328). In another example, the control members 352 can include pumps, valves, or other mechanisms configured to adjust an internal air pressure within the enclosure 316 in a manner that promotes movement of the diaphragm back toward an intended position. In additional examples, the control members 352 can include mechanical lift-off actuators configured to physically contact an underside of the diaphragm, voice coil, or other component and to urge it axially outward or inward. In some implementations, to assist with lift-off (from an inward position) or recovery (from an outward position), current can be applied through the sensing coil in addition to the voice coil to increase the magnetic field generated by the motor structure. In this manner, supplying current through the sensing coil can boost the magnetic field otherwise generated by the voice coil alone, thereby increasing ability to move the diaphragm along the excursion range. Various other approaches are possible, and any suitable control member 352 can be used to move or urge the diaphragm 320 toward an intended position.

[0102] Figure 3B is an isometric view of the playback device 310 including the audio transducer 314, and Figure 3C is a perspective cut-away view of the audio transducer 314. As shown in Figure 3B, the diaphragm 320 is coupled to the frame 316h via the surround 322, which extends circumferentially around the diaphragm 320. As best seen in Figure 3C, the diaphragm is coupled to the voice coil 328, which includes a coil 331 wound circumferentially around a cylindrical former 333. The voice coil 328 is partially received within an annular cavity 329 defined by the magnet 326. The voice coil 328 and cavity 329 are each sized and configured so that the voice coil 328 can move axially inward and outward along the excursion axis A over an excursion range. [0103] A position sensor in the form of sensor coil 354 can also be disposed within the cavity 329. As shown, the sensor coil 354 can be radially spaced apart from the voice coil 328 such that there is a radial gap between the sensor coil 354 and the voice coil 328. During operation of the transducer 314, the voice coil 328 can move along the excursion axis A such that the radial separation between the voice coil 328 and the sensor coil 354 remains substantially constant.

[0104] The sensor coil 354 can be affixed with respect to the magnet 326 and/or the cavity 329 such that the sensor 354 remains stationary within the transducer during operation of the device while the voice coil 328 moves axially inward and outward along the excursion axis A. By virtue of this movement, the amount of axial overlap between the voice coil 328 and the sensor coil 354 can vary over time. This arrangement of the voice coil 328 and the sensor coil 354 can establish a variable coupling ratio transformer, in which an input AC signal supplied to the voice coil 328 induces an output signal in the sensor coil 354 which can be detected and used to determine the amount of axial overlap between the two coils 328, 354. As the amount of overlap increases (e.g., as the voice coil 328 moves axially inward towards a bottom of the cavity 329), the voltage of the signal in the sensor coil 354 will increase. Conversely, as the voice coil 328 moves axially outwardly from the bottom of the cavity 329 and towards a maximum outward excursion point, the amount of axial overlap between the two coils 328, 354 will decrease, and accordingly the voltage of the signal in the sensor coil 354 will decrease.

[0105] In some examples, the input signal supplied to the voice coil 328 can be the audio signal used to generate audio output via the transducer. As the audio signal passes through the voice coil 328, corresponding output signals will be induced in the sensor coil 354, which can be analyzed (e.g., based on comparison to the input audio signal) to determine a coupling ratio so that the amount of axial overlap can be derived.

[0106] In some implementations, the input signal supplied to the voice coil 328 can be a pilot signal (e g., a pilot tone at an inaudible signal at a frequency above or below the audible range), such that the pilot signal does not interfere with the audio output via the transducer. For instance, the pilot signal can be supplied even when the transducer is off, or may be supplied in combination with (e.g., superimposed upon) the audio signal used to generate audio output via the transducer. The pilot signal may have a fixed or time-varying voltage, and similarly may have an alternating current at a fixed or varying frequency. In some implementations, the output signal read from the sensor coil 354 may be filtered to remove contributions from the audio signal or otherwise filtered to facilitate processing of the signal induced in the sensor coil 354 to determine a position of the voice coil 328 and/or other components of the device. In various examples, the pilot signal can be provided to the voice coil 328 through the amplifier used to deliver the audio signal, or alternatively may be supplied by separate electronics such that the pilot signal is mixed into the incoming audio signal supplied to the voice coil 328. In some examples, the pilot tone can take the form of the switching noise created by modern filterless class D amplifiers.

[0107] Although reference is made to determining a particular position of the voice coil 328, the diaphragm 320, or other moveable components of the transducer 314, one of skill in the art will understand that in operation these components are rapidly oscillating along the excursion axis A. Accordingly, in some implementations “the position” can refer to a time-averaged position of a given component, such that an average “neutral” position for a given component can be determined. If the diaphragm is slowly drifting axially outwardly, on average its position at each point along its excursion will move axially outwardly, such that a time-averaged axial position of the diaphragm will move axially outwardly away from a standard “neutral” position. Accordingly, the time-averaged sensor data can indicate that the position of the diaphragm is moving axially outwardly, and optionally corrective action can be taken (e.g., applying a DC offset to the audio signal fed to the voice coil 328, adjusting an internal pressure of the enclosure, or other suitable corrective action).

[0108] Figure 3D is a cross-sectional view of the transducer 314 shown in Figure 3C, and Figure 3E shows an enlarged detail view of a portion of the transducer cross-section shown in Figure 3D. With reference to Figures 3D and 3E together, the magnet 326 can include an annular portion 326a and a pole piece portion 326b, with the cavity 329 defined between the radially outer surface of the pole piece portion 326b and the radially inner surface of the annular portion 326a. The voice coil 328 can be disposed circumferentially around the pole piece portion 326b such that the voice coil 328 moves axially inward and outward along the excursion axis A while the pole piece portion 326b remains stationary.

[0109] As noted above, the voice coil 328 includes a voice coil former 333 (e.g., a cylindrical structural member) around which the coil 331 is wound. The voice coil 328 can be coupled to the diaphragm 320, such as by connecting an upper portion of the voice coil former 333 to a lower portion of the diaphragm 320. A lower portion of the voice coil 328 (including a portion of the coil 331) is received within the cavity 329. The sensor coil 354 is disposed within the cavity 329 and is disposed radially outward of the voice coil 328. As shown in Figure 3E, the outer diameter DI of the pole piece portion 326b is smaller than the inner diameter D2 of the sensor coil 354. The voice coil 328 is disposed within this gap between DI and D2, such that the voice coil 328 can move axially inward and outward along the excursion axis A within the cavity 329 and without directly contacting the sensor coil 354.

[0110] In the configuration illustrated in Figure 3E, the lower portion of the voice coil 328 overlaps the upper portion of the sensor coil 354 by an axial amount LI. As noted previously, the amount of axial overlap will vary as the voice coil 328 moves along the excursion axis A. As the amount of overlap increases (e.g., as the voice coil 328 moves axially inward towards a bottom of the cavity 329), the amount of overlap LI will increase, causing the voltage in the sensor coil 354 to increase. Conversely, as the voice coil 328 moves axially outwardly from the bottom of the cavity 329 and towards a maximum outward excursion point, the amount of axial overlap LI between the two coils 328, 354 will decrease, and accordingly the voltage of the signal in the sensor coil 354 will decrease.

[0U1] The voltage across terminals of the sensor coil 354 can be read by leads 356 coupled to the sensor coil 354. In the configuration shown in Figures 3E and 3F, the leads 356 extend downward through the magnet 326, though other configurations are possible. In operation, the leads 356 can be coupled to sensor electronics (not shown) and/or other components (e.g., the processor(s) 112b and/or memory 112d of Figure 1C), which can determinate, calculate, estimate, and/or predict a position of one or more moving components of the transducer 314 based on the signal(s) obtained via the leads 356. In some implementations, the voltage detected across the sensor coil 354 can be substantially linearly related to the amount of axial overlap LI between the sensor coil 354 and the voice coil 328.

[0112] Figure 4A illustrates example signal processing steps for determining the position of a voice coil (or other transducer component) based on sensor coil readings. As depicted in graph 401, the pilot tone readout 403 from the sensor coil is limited to a narrow range corresponding to the pilot tone (e.g., about 20 kHz in the illustrated example). Below this range, the sensor readout will reflect electromagnetic coupling to baseband audio content 405 (if the voice coil is being used for active playback), and above this frequency range the sensor readout reflects class-D amplifier switching noise 407. As such, it can be beneficial to select a pilot tone that falls within the gap between these two frequency ranges, for instance between about 15-30 kHz. [0113] Analog signal conditioning can be performed on the amplitude-modulated sensor coil pilot tone. In the illustrated example, the signal is received at a sensor coil input connector 409, and then fed to a passive analog high-pass filter 411 and an analog low-pass filter 413 to remove the class-D switching noise and high voltage baseband voice coil audio. Next is an analog gain stage 415 that can match the sensor coil level with the analog-to-digital converter (ADC) 417. The output of ADC 417 is depicted in graph 419, in which a time-series data depicts the amplitude- modulated pilot tone. This signal can be processed via digital signal processing (DSP) components 421 to output a detected envelope of the amplitude-modulated pilot tone readout, which is indicative of the raw voice coil position data. This data reflects the axial position of the voice coil (e g., amount of axial overlap with or relative axial movement with respect to the sensor coil) over time. This can be used to generate a voltage-displacement curve, in which voltage detected in the sensor coil corresponds to an excursion distance over the curve. In some examples, known techniques such as optical sensing can be used to calibrate a voltage-displacement curve for a given transducer configuration. One example of such a voltage-displacement curve is shown in Figure 4B. If the resulting curve is nonlinear, for instance, it may be fitted to a linear curve, either with a polynomial function or a lookup table. The resulting data can then be used to drive control systems or other algorithms such as adaptive limiters or other nonlinear correction schemes.

[0114] Figure 4B illustrates an example plot of detected voltages across the sensor coil 354 at various voice coil positions along the excursion axis. In this plot, the negative values indicate the voice coil 328 has extended axially downward further into the cavity 329, and the positive values indicate the voice coil 328 has extended axially outward away from the cavity 329. As reflected in the plot, due to the nearly linear nature of this relationship, a given voltage detected across the sensor coil 354 can be used to determine a corresponding excursion distance of the voice coil 328. [0115] In various examples, as the voice coil 328 moves inward and outward along the excursion axis A, the coupling ratio will be reasonably linear and time invariant such that there is a substantially 1 : 1 relationship between the voice coil 328 and sensor coil 354 coupling and the voice coil 328 position. In other examples, however, the coupling ratio is not linear, and one or more mathematical formulas, lookup tables, algorithms, predictive models and/or computer models can be used to determine or estimate the actual position of the voice coil 328 based on the measured voltage from the sensor coil 354. Although various examples herein refer to measuring a voltage across the sensor coil 354, any suitable signal can be read from the sensor coil 354, including voltage, current, a combination of both, or any other suitable signal. In certain examples, additional information is used in the voice coil position determination. For instance, factoring in an ambient temperature or pressure, temperatures of the sensing coil and/or voice coil, etc. may enhance the accuracy of the position determination, prediction, and/or estimation. c. Example Multiple Drive Unit Audio Playback Devices with Position Sensors

[0116] Although several examples herein are described with respect to audio transducers having a single diaphragm and/or a single drive unit (e.g., a single voice coil), the position sensors described herein can also be used with other types of audio transducers. For instance, examples of the present technology can be usefully applied to any number of audio transducer configurations, including those involving multiple drive units (e.g., multiple voice coils or other membraneactuating components) that operate concurrently. Various examples of suitable audio transducers that can be used in the context of the present technology can be found in the following commonly owned patents and patent applications, each of which is hereby incorporated by reference in its entirety: U.S. Patent No. 10,893,367, titled “Loudspeaker Unit with Multiple Drive Units,” U.S. Patent No. 11,197,102, titled “Distributed Transducer Suspension Cones (DTSC),” International Patent Application No. PCT/US2022/077272, titled “Speaker Device,” U.S. Patent No. 11,297,415, titled “Low Profile Loudspeaker Device,” U.S. Patent Application No. 17/424,181, titled “In Line Damper Bellows Dual Opposing Driver Speaker,” U.S. Patent Application No. 17/434,013, titled “Membrane Unit for Speaker Device,” U.S. Patent Application No. 17/602,314, titled “Linear Motor Magnet Assembly and Loudspeaker Unit,” U.S. Patent No. 11,166,107, titled “Speaker Unit with a Speaker Frame and Two Opposing Sound Producing Membranes,” and U.S. Patent Application No. 18/040,218, titled “Speaker Unit,” U.S. Patent Application No. 18/309,544, titled “Speaker Transducer.”

[0117] In various examples, two or more drive units can be coupled to the same diaphragm or membrane, with a position sensor coupled to each of the drive units. Additionally or alternatively, two or more drive units can be coupled to opposing membranes of an audio transducer. In either configuration, position sensor(s) can be used to determine the position of the voice coil, membrane, or other moveable component of the audio transducer during operation of the device. In some implementations, each drive unit can be coupled to a separate sensor, and the sensor data from each sensor can be used in a feedback loop that operates separately on each drive unit. For instance, position sensing may determine that axial movement between two voice coils coupled to a membrane is not synchronized or is otherwise out of balance. In response, a feedback controller can modify or correct the position or one or both voice coils, such as by providing a DC offset, time delay, or other suitable modification to the input signal to one or both of the voice coils in a manner that restores balance and/or synchronization across the voice coils.

[0118] Figure 5A is a plan view of an example audio transducer 514 with multiple drive units 515. Figures 5B and 5C are side cross-sectional views of the audio transducer 514 taken along line 5B-C at a rest position (Figure 5B) and at a position of inward excursion (Figure 5C), respectively. The audio transducer 514 can be incorporated into and/or include any of the features described elsewhere herein with respect to playback devices 110, 210, and/or 310. In some implementations, a playback device including the audio transducer 514 can include separate amplifiers for each drive unit 515. As described in more detail below, this can allow for separate corrective action to be taken for different drive units by adjusting the drive signal provided by one amplifier without necessarily making similar adjustments to other drive signals provided to other drive units. In various examples throughout, drive units 515 are described as including voice coils 328 surrounding magnets 326. However, in some implementations the drive units 515 can include any suitable membrane-actuating members and need not be limited to voice coils specifically. Although the illustrated example shows four drive units, in operation there may be any suitable number of drive units, including 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or more.

[0119] With reference to Figures 5A-5C together, the audio transducer 514 includes two opposing diaphragms or membranes 320a, 320b (collectively “diaphragms 320”) each coupled to a frame 316h via corresponding surrounds 322a, 322b (collectively “surrounds 322”). The transducer 514 further includes four separate drive units 515a-d (collectively “drive units 515”). Two of the drive units (515a and 515b) are coupled to the first diaphragm 320a, while the other two drive units (515c and 515d) are coupled to the second diaphragm 320b. Together, the drive units 515 are configured to move the diaphragms 320 inward and outward along the excursion axis A. Each of the drive units can include a corresponding driver static part configured to remain stationary with respect to the frame 316h and a driver moving part that is coupled to one of the diaphragms 320 and is configured to move with respect to the frame 316h in response to a drive signal, thereby causing the corresponding diaphragm 320 to move and produce sound. In the illustrated example, the driver static part comprises a magnet 326 (which may include a single magnetic component or a stack of discrete magnets) and the driver moving part comprises a voice coil 328. In additional examples the particular configuration and structure of the driver static part and the driver moving part can take other forms.

[0120] In various examples, a corresponding sensor coil 354 can be coupled to each of the drive units 515 to facilitate sensing the position(s) of transducer components that move in response to operation of the drive units 515. As best seen in Figures 5 A and 5B, a first sensor coil 354a can be disposed about the magnet 326a of the first drive unit 515a, and a second sensor coil 354b can be disposed about the magnet 326b of the second drive unit 515b. Although not shown, the additional drive units 515c and 515d may include additional sensor coils 326c and 326d. In various examples, each sensor coil 354 can include a coil winding or other electrically conductive member and can be disposed adjacent to, and optionally at least partially circumferentially surrounding, the magnet 326. In some examples, the sensor coil 354 can be radially spaced apart from the magnet 326, for instance by an interstitial insulating material. The sensor coil 354 can be radially spaced between the magnet 326 and the voice coil 328 and configured to remain fixed with respect to the magnet 326 such that as the voice coil 328 moves inward and outward along a direction parallel to the excursion axis A, the relative axial positions of the voice coil 328 and the sensor coil 354 varies. For instance, in the “rest” position shown in Figure 5B, the voice coil 328a and the sensor coil 354a have a first amount of overlap LI . In the inwardly excursed position shown in Figure 5C, the voice coil 328a and the sensor coil 354a have a second amount of axial overlap L2, which is greater than LI. Although this example illustrates an increased amount of axial overlap during inward excursion, in other implementations the amount of overlap may decrease during inward excursion. [0121] Among examples of the present technology, each sensor coil 354 can separately determine an axial position of its corresponding voice coil 328 in real-time. This may advantageously allow separate corrective actions to be taken for multiple different voice coils (e.g., a corresponding DC offset, time delay, or other suitable modification to be applied to drive signals provided to respective voice coils 328). In some implementations, only some of the voice coils 328 of a multiple drive unit transducer are coupled to corresponding sensor coils 354. In such cases, data measuring the axial position of one voice coil 328 may be sufficient to determine an axial position of the diaphragm 320, and accordingly a separate sensor coil 354 for each voice coil 328 may not be required.

[0122] Although the example shown in Figures 5A-5C illustrates two opposing diaphragms 320, in some implementations multiple drive units 515 can be utilized in a single-diaphragm playback device. For instance, multiple drive units 515 can be spaced apart circumferentially around a perimeter of the diaphragm 320, thereby permitting a more compact transducer arrangement. As noted above, some or all of the drive units 515 can have corresponding sensor coils 354 configured to detect the position(s) of moveable components of the drive units 515 in real-time and optionally to facilitate corrective action.

[0123] Figures 6A and 6B are schematic side cross-sectional views of an example audio transducer 614 in which multiple drive units 615 arranged coaxially and configured to drive movement of opposing diaphragms 320a and 320b. Figure 6A illustrates the transducer 614 at rest, while in Figure 6B the opposing diaphragms 320a and 320b have moved inwardly toward one another. As illustrated, the first voice coil 328a is coupled to the first diaphragm 320a and the second voice coil 328b is coupled to the second diaphragm 320b. Each of the voice coils 328a are moveably disposed about a central magnet stack 326 which includes three discrete magnets 326a, 326b, and 326c which are coaxially aligned and spaced apart from one another along the excursion axis of the transducer 614. The particular arrangement, spacing, and orientation of the magnets 326 are exemplary and may be varied in different implementations. In the illustrated configuration, the uppermost magnet 326a and the lowermost magnet 326c each has its north pole oriented upward and south pole oriented downward, while the central magnet 326b has reversed polarity such that its north pole is oriented upward and faces the north pole of the uppermost magnet 326a, while the south pole of the central magnet 326b is oriented downward and faces the south pole of the lowermost magnet 326c. Together these magnets 326 can provide a stationary magnetic field that facilitates movement of both the first voice coil 328a and the second voice coil 328b in response to current being driven therethrough.

[0124] The transducer 614 further includes first and second sensor coils 354a and 354b. The first sensor coil 354a is disposed radially between the magnet(s) 326 and the first voice coil 328a (along at least a portion of its range of motion along the excursion axis) and the second sensor coil 354b is disposed radially between the magnet(s) 326 and the second voice coil 328b (along at least a portion of its range of motion along the excursion axis). In the illustrated example, the first and second sensor coils 354 are arranged coaxially and spaced apart from one another along the excursion axis. The sensor coils 354 can circumferentially surround at least a portion of the magnet(s) 326 and provide a radial gap between each sensor coil 354 and its respective voice coil 328, such that the voice coil 328 can freely move axially inward and outward while the sensor coil 354 remains stationary with respect to the magnet(s) 326.

[0125] In operation, as the voice coils 328 move inwardly and outwardly (thereby moving the diaphragms 320 inwardly and outwardly), the amount of axial overlap between the first voice coil 328a and the first sensor coil 354a, and the amount of axial overlap between the second voice coil 328b and the second sensor coil 354b will vary. As illustrated in Figures 6A and 6B, the overlap between the first voice coil 328a and the first sensor coil 354a varies from length LI in the position shown in Figure 6A to length L2 in the position shown in Figure 6B. Similarly, the overlap between the second voice coil 328b and the second sensor coil 354b varies from length L3 in the position shown in Figure 6A to length L4 in the position shown in Figure 6B. As described elsewhere herein, the amount of axial overlap can be determined by evaluating a signal readout from the sensor coil(s) 354, thereby enabling the real-time determination of the axial position(s) of various transducer components.

[0126] In some examples, a single sensor coil 354 can be arranged and configured to detect the axial position(s) of more than one voice coil 328. For example, as shown in Figures 7A and 7B, a single sensor coil 354 can be arranged to at least partially overlap both a first voice coil 328a and a second voice coil 328b. As the voice coils 328 move inward and outward along an excursion axis, the amount of overlap between each voice coil 328 and the sensor coil 354 will vary over time. For instance, in the configuration shown in Figure 7A, the first voice coil 328a overlaps the sensor coil 354 by a length LI, while in the second configuration shown in Figure 7B the same overlap has increased to length L2. Similarly, the second voice coil 328b overlaps the sensor coil 354 by a first amount L3 in the configuration of Figure 7A and overlaps by a second amount L4 in the configuration of Figure 7B.

[0127] In some implementations, the input signal provided to the first voice coil 328a and the second voice coil 328b can be substantially identical (e.g., the same audio or pilot tone supplied to each voice coil 328), in which case the signal readout from the sensor coil 354 correlates to the combined amount of axial overlap between each of the first voice coil 328a and the second voice coil 328b with the sensor coil 354 (i.e., the sum of length LI and L2 in Figure 7A, and the sum of length L3 and L4 in Figure 7B). The amplitude of the readout signal thereby indicates the combined amount of axial overlap, and may indicate, for instance, that a maximum excursion threshold has been exceeded or may otherwise characterize movement of the voice coils 328. [0128] In some examples, it can be useful to separately sense the position of the first voice coil 328a and the second voice coil 328b with respect to the sensor coil 354. One such approach is to provide separate and distinguishable input signals to the first voice coil 328a and to the second voice coil 328b. For instance, a pilot tone at a first frequency can be supplied to the first voice coil 328a and a pilot tone at a second frequency can be supplied to the second voice coil 328b. In operation, both of these pilot tones will induce corresponding output signals in the sensor coil 354 simultaneously. However, by filtering the output signal (or using other suitable techniques to distinguish between the two induced signals), the output corresponding to the first pilot tone can be separated from the output corresponding to the second pilot tone. In this manner, a single sensor coil 354 can be used to read separate position data for both the first voice coil 328a (e.g., by evaluating the amplitude of only the induced signal corresponding to the first pilot tone) and for the second voice coil 328b (e.g., by evaluating the amplitude of only the induced signal corresponding to the second pilot tone). Using this approach, the separate positions of each of the voice coils 328 can be sensed and compared. Optionally, corrective action can be taken based on these readings. For instance, a DC offset, time delay, or other suitable adjustment can be made to the input signal(s) to one or both of the voice coils 328 in a manner that corrects the detected position defect (e.g., synchronizing movement of the two voice coils 328). d. Example Sensor Coil Configurations

[0129] In various examples, the relative radial positions of the voice coil 328 and the sensor coil 354 can be varied. For instance, in the arrangement shown in Figures 8 A and 8B, the sensor coil 354 is disposed circumferentially around the voice coil 328 such that the sensor coil 354 surrounds the voice coil 328 while being separated from the voice coil 328 by a radial gap. During operation, the voice coil 328 moves axially, such that an amount of overlap can vary (e.g., from the overlap LI in Figure 8A to overlap L2 indicated in Figure 8B).

[0130] An alternative configuration is illustrated in Figures 9A and 9B, in which the sensor coil 354 is disposed radially within the voice coil 328 such that the voice coil 328 extends circumferentially around and surrounds the sensor coil 354. During operation, the sensor coil 354 can remain stationary while the voice coil 328 moves axially, thereby varying the amount of overlap (e.g., from the overlap L3 in Figure 9A to overlap L4 in Figure 9B). Although the relative radial positions of the sensor coil 354 and the voice coil 328 are varied between Figures 8A-8B and Figures 9A-9B, the principle of operation is the same in each instance. As the amount of overlap varies, the sensor readout from the sensor coil 354 will vary, for instance with an increasing voltage indicating a greater amount of overlap and a decreasing amount of voltage indicating a lesser amount of overlap.

[0131] In several of the examples described herein, the amount of axial overlap between the sensor coil 354 and the voice coil 328 increases as the voice coil moves axially inward, and decreases as the voice coil 328 moves axially outward. However, in some implementations the relative positions of the sensor coil 354 and the voice coil 328 can vary, such that the amount of overlap increases as the voice coil 328 moves axially outward and decreases as the voice coil 328 moves axially inward. In such instances, a higher voltage in the sensor coil 354 can indicate axially outward excursion, and conversely lower voltage in the sensor coil 354 can indicate axially inward excursion. In still other implementations, the sensor coil 354 and the voice coil 328 can be positioned such that a highest amount of overlap is in an intermediate excursion point of the voice coil 328, such that axial overlap is lower both in the axially outward excursion point and in the axially inward excursion point. In various examples, at a neutral resting position, the sensor coil 354 and the voice coil 328 can axially overlap by a predetermined amount, e.g., 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90%.

[0132] While several examples herein describe the voice coil 328 moving while the sensor coil 354 remains stationary, in some implementations the voice coil 328 may be stationary while the sensor coil 354 (optionally along with the magnet 326) moves axially. Moreover, examples described above may include a single sensor coil 354. In various examples, however, multiple sensor coils 354 may be used for each corresponding voice coil 328 to further enhance accuracy of determining the position of the voice coil 328 over its entire expected excursion range. For instance, in the example shown in Figures 10A and 10B, an upper sensor coil 354a an upper sensor coil 354a is positioned toward an upper portion of the excursion range of the voice coil 328 and a lower sensor coil 354b is positioned toward a lower portion of the excursion range of the voice coil 328. In some examples, the upper sensor coil 354a is axially spaced apart from the lower sensor coil 354b. In other examples, however, the upper sensor coil 354a is adjacent to the lower sensor coil 354b. In the example illustrated in Figures 10A and 10B, the voice coil 328 moves between a first position (Figure 10A) in which the voice coil 328 partially axially overlaps the lower sensor coil 354b to a second position (Figure 10B) in which the voice coil 328 partially axially overlaps the upper sensor coil 354a. Although two sensor coils 354a and 354b are shown, in various implementations there may be 3, 4, 5, or more sensor coils that are distributed at different axial positions along the excursion range of the voice coil. In operation, as the voice coil moves through the excursion range, the sensor readout from each sensor coil will vary based on the amount of axial overlap, the input signal at that time, etc. These sensor readouts can then be used to determine the axial position of the voice coil. In some instances, this approach can allow the sensor coils to cover the full axial range of the voice coil without requiring a single sensor coil to span the entire distance, thereby reducing the size and weight of the sensor coil components.

[0133] Figures 11A and 1 IB illustrate another configuration in which a single sensor coil 354 is used in conjunction with two voice coils 328a and 328b. This configuration may be suited for audio transducers with multiple drive units, for instance being configured to drive opposing diaphragms inward and outward along an excursion axis. As illustrated, a single sensor coil 354 can be arranged to at least partially overlap both a first voice coil 328a and a second voice coil 328b, with the amount of respective overlap varying as the voice coils 328a and 328b move inward and outward. For instance, the first voice coil 328a overlaps the sensor coil 354 by a first length LI in the configuration shown in Figure 11A, and by a second, longer length L2 in Figure 11B. Similarly, the second voice coil 328b overlaps the sensor coil 354 by a first length L3 in the configuration shown in Figure 11 A and by a second length L4 in the configuration shown in Figure 11B.

[0134] As noted previously with respect to Figures 7A and 7B, in some implementations the input signal provided to the first voice coil 328a and the second voice coil 328b can be substantially identical (e g., the same pilot tone supplied to each voice coil 328), in which case the signal readout from the sensor coil 354 correlates to the combined amount of axial overlap between each of the first voice coil 328a and the second voice coil 328b with the sensor coil 354 (i.e., the sum of length LI and L2 in Figure 11A, and the sum of length L3 and L4 in Figure 1 IB). The readout signal from the sensor coil 354 therefore indicates the combined amount of axial overlap, and may indicate, for instance, that a maximum excursion threshold has been exceeded or otherwise characterize movement of the voice coils 328.

[0135] Additionally or alternatively, separate and distinguishable input signals can be provided to the first voice coil 328a and the second voice coil 328b (e g., separate pilot tones having different frequencies, or other such distinguishable signal characteristics). In operation, both of these pilot tones will induce corresponding output signals in the sensor coil 354 simultaneously. However, by filtering the output signal (or using other suitable techniques to distinguish between the two induced signals), the output corresponding to the first pilot tone can be separated from the output corresponding to the second pilot tone. In this manner, a single sensor coil 354 can be used to read separate position data for both the first voice coil 328a (e.g., by evaluating the induced signal corresponding to the first pilot tone) and for the second voice coil 328b (e.g., by evaluating the induced signal corresponding to the second pilot tone). Using this approach, the separate positions of each of the voice coils 328 can be sensed and compared, and optional corrective action can be taken.

[0136] Figure 12 illustrates an example configuration of a sensor coil 354 in which multiple segments 1202a, 1202b, 1202c are connected in series via interconnects 1204a, 1204b. Such a configuration can allow a single sensor coil 354 to extend axially along a greater length than would otherwise be possible if the entire length of the coil was tightly wound without spatial separation between segments. This can reduce the overall weight and material requirements for a sensor coil 354, while permitting the axial position most suited for sensing movement of the voice coil(s) 328 with respect to the sensor coil 354. As illustrated in Figure 12, each of the segments 1202 can be portions of tightly wound coil which are spaced apart from one another in the axial direction, and may be electrically connected in series by interconnects 1204. The interconnects 1204 can be a portion of the coil that extends axially with less (or with no) winding around the excursion axis than in the segments 1202. In operation, a signal induced in the sensor coil 354 will correspond to the amount of combined axial overlap between one or more voice coils 328a and the various segments 1202. In some implementations, any axial overlap between a voice coil and the interconnects 1204 will have little or no effect on the sensor readout signal, as the lack of coil windings in those regions mean that little or no signal is induced there in response to a signal in the adjacent voice coil.

[0137] In yet another variation, a pilot signal can be supplied to the sensor coil 354, which will then be coupled to the voice coil 328 through the same transformer principles described above. This induced signal in the voice coil 328 can then be sensed and analyzed to determine an amount of axial overlap between the two coils. In such instances, the signal from the voice coil 328 may need to be filtered from the audio signal also present in the voice coil 328 in order to extract the signal induced in the voice coil 328 in response to the pilot signal supplied to the sensor coil 354. [0138] Among examples, a two-coil sensor assembly can be used in which neither coil also serves as the voice coil. For instance, the sensor coil 354 can supply a pilot tone or other signal as described above. However, rather than the voice coil 328, a separate corresponding sensor coil can be provided which moves along with the diaphragm and/or other moveable components of the transducer. As this separate sensor coil moves along the excursion axis, its position can be detected via the sensor coil 354.

[0139] While some examples described herein relate to a voice coil 328 and a sensor coil 354 that axially overlap over at least a portion of the excursion range of the voice coil 328, in some instances the sensor coil 354 can be axially separated from the voice coil 328 such that the voice coil 328 and the sensor coil 354 do not axially overlap at any point over the excursion range of the voice coil 328. Notwithstanding this separation, if the coils remain sufficiently close, and optionally coaxially arranged, the same principle of electromagnetic coupling allows the moving voice coil 328 to induce a voltage in the stationary sensor coil 354. By reading out the time-varying voltage induced in the sensor coil 354, the position of the voice coil 328 (and therefore other transducer components) can be determined. Accordingly, in each instance herein that refers to an amount of axial overlap between two coils, a relative axial position between the two coils (whether or not there is any axial overlap) may be substituted instead.

[0140] Figure 13 is a side cross-sectional view of a transducer 1314 with a voice coil 328 and a sensor coil 354 in a non-overlapping arrangement. As illustrated, the transducer 1314 can include a voice coil 328 that is disposed circumferentially about a central magnet stack 326 (which optionally can include one or more magnetic bodies, non-magnetic spacers, or other suitable components). The voice coil 328 is operably coupled to a diaphragm 320, which in turn is coupled to a frame (not shown) by a surround 322. In operation, movement of the voice coil 328 causes the diaphragm 320 to move axially inward and outward over an excursion range relative to the magnet stack 326 and other stationary components of the transducer 314.

[0141] As illustrated in Figure 13, the sensor coil 354 can be axially displaced from the voice coil 328 by a distance LI, such as by being placed axially beneath the voice coil 328. The separation distance LI may be large enough that even as the voice coil 328 moves inward and outward over its excursion range, at no point does the voice coil 328 axially overlap with the sensor coil 354. Nonetheless, the current in the voice coil 328 will still electromagnetically couple to the sensor coil 354 to induce a voltage therein, as described above in the case of axially overlapping coils. As the voice coil 328 moves over its excursion range, the separation distance LI will vary, thereby affecting the induced voltage within the sensor coil 354. As discussed elsewhere herein, this induced voltage can thereby indicate the axial position of the voice coil 328 and/or other components of the transducer 1314.

[0142] In various examples, the sensor coil 354 can be arranged in an annular form that circumferentially surrounds a central axis of the transducer 1314. The sensor coil 354 can be centered around the axis and can be coaxially arranged with the voice coil 328. In some examples, however, the sensor coil 354 and voice coil 328 need not be coaxially arranged.

[0143] In accordance with some implementations, the sensor coil 328 can be arranged in an at least partially planar manner, such as by including conductive paths that extend in a spiral pattern within a common plane rather than extending helically. For instance, a spiral-shaped planar coil, similar to coils used for RFID readers, wireless charging, and other applications, can be used as a sensor coil 354 in various examples.

[0144] Figure 14 illustrates an example transducer 1414 with a planar sensor coil 354. In the depicted configuration, the planar sensor coil 354 can take the form of conductive traces embedded within or otherwise carried by substrate (e.g., PCB, plastic, paper, or other suitable non-conductive substrate). The conductive traces can include spiral-shaped segments, optionally with several layers of spiral-shaped segments that are electrically linked together (e.g., by vias). Additionally or alternatively, the conductive elements can include non-spiral shaped portions, such as flat or straight segments, rectangular portions, or any other suitable shape or combination of shapes.

[0145] In the illustrated example, the sensor coil 354 is disposed on an annular substrate with a central aperture that is coaxially arranged with the voice coil 328 and the magnet stack 326. However, in alternative configurations the substrate may have no aperture, or may have other shapes (e g., rectangular, irregular, etc.). In some implementations, the substrate (e.g., a PCB) can support other functional electronic components of the transducer, such as connectors for voice coil wires, passive electronics (e.g., capacitors, resistors, etc.), active electronic components (e.g., amplifiers), or any other suitable components.

[0146] As shown in Figure 14, a pair of leads 356 can be electrically coupled to the sensor coil 354 and allow a sensor readout (e.g., voltage, current, etc.) to be obtained. This readout will vary as the voice coil 328 moves closer to and further away from the sensor coil 354 while a current (e.g., an inaudible pilot tone) passes through the voice coil 328, as discussed above. While the planar sensor coil 354 is illustrated in Figure 14 as axially offset from the voice coil 328, in other arrangements a planar sensor coil 354 can be disposed at a position that axially overlaps (e.g., circumferentially surrounds or is circumferentially surrounded by) the voice coil 328.

[0147] In some implementations, the sensor coil 354 can take the form of a wireless power transfer coil, and may alternately (or concurrently) be used to transmit or receive wireless power to or from external devices. For instance, the sensor coil 354 can be configured to serve as a wireless charging coil that can receive power from an external charger device. In some implementations, the sensor coil 354 can operate in different modes, for example a first mode in which the sensor coil 354 receives power from (or transmits power to) an external device, and a second mode in which the sensor coil 354 is used to determine the position of the voice coil 328. Additional details regarding wireless power transfer in the context of playback devices can be found in commonly owned International Patent Publication No. WO 2022/047503, titled “Wireless Power Transfer for Audio Playback Devices,” which is hereby incorporated by reference in its entirety for all purposes.

[0148] Figure 15 is a perspective view of a dual-membrane transducer 1514 with a planar sensor coil 354. As illustrated, the transducer 1514 can include a first assembly 1514a with a first voice coil 328a driving a first diaphragm 320a and a second assembly 1514b with a second voice coil 328b driving a second diaphragm 320b. These assemblies 1514a and 1514b can be arranged in a back-to-back or opposing manner such that the first and second diaphragms 320a and 320b face in opposite directions. A single sensor coil 354 disposed axially between the two voice coils 328a and 328b can be used to sense the axial positions of each of the voice coils 328a and 328b. As noted above with respect to Figures 13 and 14, even when the sensor coil 354 is axially displaced from the voice coil 328, current in the voice coil (e.g., an inaudible pilot tone) will induce a voltage in the sensor coil 354 that varies as the voice coil 328 moves axially relative to the sensor coil 354. [0149] In the arrangement shown in Figure 15, each of the voice coils 328a and 328b can cause an induced voltage in the sensor coil 354 as each voice coil 328a and 328b moves over its respective excursion range. To differentiate between the two signals, different pilot tones can be used for the two voice coils 328a and 328b. For instance, the first voice coil 328a can be supplied with a first pilot tone of a first frequency, and the second voice coil 328b can be supplied with a second pilot tone of a second, different frequency. In some implementations, each of the first and second frequencies are inaudible, but are distinct enough for the resulting voltages induced in the sensor coil 354 to be identified separately. For instance, the first frequency can be 15 kHz and the second frequency can be about 20 kHz. The resulting voltage induced in the sensor coil 354 will be a result of both pilot tones. However, the sensor readout from the sensor coil 354 can be evaluated separately (e.g., using different filters to remove one of the pilot tones) to isolate the pilot tone signals from one another. This approach allows a single sensor coil 354 to monitor the axial position of both voice coils 328a and 328b independently.

VI. Example Methods

[0150] Figure 16 is a flow chart of an example method 1600 for determining a position of playback device components. The method 1600 can be performed by any example system and device described herein, such as by playback device 310, which may include any one of the audio transducers described herein, such as audio transducers 314, 514, 614, or 714. The method 1600 begins in block 1602 with applying a first signal to a first coil of an audio transducer. The first coil can be, for instance, a voice coil of the audio transducer. The first signal can be an audio signal used to produce audio output via the voice coil. Additionally or alternatively, the first signal can be a pilot signal (e.g., a pilot tone or other signal configured to be inaudible to a user).

[0151] In block 1604, the method 1600 involves detecting a second signal in a second coil of the audio transducer. As noted previously, a second coil such as a sensor coil can be disposed radially adjacent to the first coil, such that in at least some configurations there is axial overlap between the first coil and the second coil. In such configurations, the two coils can operate like a transformer, in which a signal supplied to the first coil induces a corresponding signal in the second coil due to electromagnetic coupling between the two.

[0152] The method 1600 continues in block 1606 with determining an axial position of a transducer component based on the second signal. For instance, the signal detected in the second coil can be indicative of an amount of axial overlap between the first coil and the second coil. Additionally or alternatively, the signal detected in the second coil indicates the relative axial positions (e.g., axial separation between and/or amount of overlap between) the first coil and the second coil. Accordingly, the signal indicates the relative position of one coil with respect to the other. If at least one of the coils is fixed with respect to the transducer or playback device, then the second signal can be indicative of the absolute position of the moveable coil. In implementations in which the first coil is a voice coil, the second signal detected via the stationary sensor coil indicates an amount of axial overlap between the sensor coil and the voice coil, and hence reflects an absolute position of the voice coil along an excursion axis. Moreover, if the moveable component is fixed with respect to other components of the transducer, then the second signal can be used to determine the position of those components as well. For example, the voice coil can be fixed with respect to the diaphragm, such that the axial position of the voice coil can be used to derive the axial position of the diaphragm.

[0153] While steps 1602, 1604, and 1606 may be performed with respect to a single sensor coil and a single voice coil, the same approach can be utilized for multiple voice coils as described previously herein. Optionally, a single sensor coil can be arranged to sense a respective position for each voice coil. In some examples, a single sensor coil can be configured to sense the position(s) of two or more voice coils.

[0154] In block 1608, the method 1600 involves optionally correcting the axial position of a transducer component based on the determined position. For instance, the determined position may indicate that a transducer component (e.g., the diaphragm) is at an undesirable position. This may be a stationary position (e.g., the diaphragm is stationary at an inoperable state such as maximal excursion beyond a safe excursion range) or may be a time-averaged position (e.g., the diaphragm is tending toward an average position that is axially inward relative to a desired neutral position). In either instance, corrective action may be taken, such as supplying a corrective signal to the voice coil (e.g., a DC offset added to the incoming audio signal), adjusting the internal air pressure of the enclosure (e.g., using a pump, valves, etc.), activating a manual lift-off mechanism or other engagement structure to physically reposition a transducer component, adjusting audio output (e.g., reducing volume to avoid over-excursion), or any other suitable corrective action.

[0155] In the case of audio transducers with multiple voice coils, different corrective action may be taken for different voice coils. For instance, the position sensor data may indicate that one voice coil of a multi-voice coil transducer is at an undesirable position (e.g., axially offset from another corresponding voice coil driving the same diaphragm). Corrective action may be taken to adjust the position of only that voice coil (e.g., providing a DC offset to the signal supplied only to that voice coil).

VII. Conclusion

[0156] The above discussions relating to transducers, playback devices, controller devices, playback zone configurations, and media content sources provide only some examples of operating environments within which functions and methods described below may be implemented. Other operating environments and/or configurations of transducers, media playback systems, playback devices, and network devices not explicitly described herein may also be applicable and suitable for implementation of the functions and methods.

[0157] The description above discloses, among other things, various example systems, methods, apparatus, and articles of manufacture including, among other components, firmware and/or software executed on hardware. It is understood that such examples are merely illustrative and should not be considered as limiting. For example, it is contemplated that any or all of the firmware, hardware, and/or software examples or components can be embodied exclusively in hardware, exclusively in software, exclusively in firmware, or in any combination of hardware, software, and/or firmware. Accordingly, the examples provided are not the only ways) to implement such systems, methods, apparatus, and/or articles of manufacture.

[0158] Additionally, references herein to “example” means that a particular feature, structure, or characteristic described in connection with the example can be included in at least one example of an invention. The appearances of this phrase in various places in the specification are not necessarily all referring to the same example, nor are separate or alternative examples mutually exclusive of other examples. As such, the examples described herein, explicitly and implicitly understood by one skilled in the art, can be combined with other examples.

[0159] The specification is presented largely in terms of illustrative environments, systems, procedures, steps, logic blocks, processing, and other symbolic representations that directly or indirectly resemble the operations of data processing devices coupled to networks. These process descriptions and representations are typically used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. Numerous specific details are set forth to provide a thorough understanding of the present disclosure. However, it is understood to those skilled in the art that certain examples of the present disclosure can be practiced without certain, specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring examples of the examples. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the foregoing description of examples.

[0160] When any of the appended claims are read to cover a purely software and/or firmware implementation, at least one of the elements in at least one example is hereby expressly defined to include a tangible, non-transitory medium such as a memory, DVD, CD, Blu-ray, and so on, storing the software and/or firmware.

[0161] The disclosed technology is illustrated, for example, according to various examples described below. Various examples of examples of the disclosed technology are described as numbered examples (1, 2, 3, etc.) for convenience. These are provided as examples and do not limit the disclosed technology. It is noted that any of the dependent examples may be combined in any combination, and placed into a respective independent example. The other examples can be presented in a similar manner.

[0162] Example 1. An audio transducer, comprising: a membrane; a first coil operably coupled to the membrane, wherein the first coil is configured to move axially within a first excursion range; an amplifier electrically connected to the first coil; a second coil radially separated from the first coil, wherein the second coil is arranged to at least partially axially overlap the first coil as the first coil moves within the first excursion range; one or more processors; and memory storing instructions that, when executed by the one or more processors, cause the audio transducer to perform operations comprising: providing, via the amplifier, a first signal to the first coil; receiving, via the second coil, a second signal; obtaining, based on a comparison of the first and second signals, a first parameter indicative of an amount of axial overlap between the first coil and second coil; and determining, based on first parameter indicative of the amount of axial overlap, a position of a transducer component.

[0163] Example 2. The audio transducer of any one of the Examples herein, wherein the position of the transducer component comprises an axial position of the first coil.

[0164] Example 3. The audio transducer of any one of the Examples herein, wherein the position of the transducer component comprises an axial position of the membrane.

[0165] Example 4. The audio transducer of any one of the Examples herein, further comprising a suspension assembly that imparts a negative stiffness to movement of the first coil within the first excursion range.

[0166] Example 5. The audio transducer of any one of the Examples herein, wherein the first signal comprises an inaudible pilot tone.

[0167] Example 6. The audio transducer of any one of the Examples herein, wherein the first signal comprises an audio signal. [0168] Example 7. The audio transducer of any one of the Examples herein, wherein the first coil comprises a voice coil.

[0169] Example 8. The audio transducer of any one of the Examples herein, wherein the first coil extends circumferentially around the second coil.

[0170] Example 9. The audio transducer of any one of the Examples herein, wherein the second coil extends circumferentially around the first coil.

[0171] Example 10. The audio transducer of any one of the Examples herein, wherein the first coil is electromagnetically coupled to the second coil such that the first signal in the first coil causes the second signal to be induced in the second coil, and wherein a coupling ratio between the first signal and the second signal is indicative of an amount of axial overlap between the first coil and the second coil.

[0172] Example 11. The audio transducer of any one of the Examples herein, wherein the first parameter indicative of the amount of axial overlap comprises a voltage across the second coil, wherein a higher voltage indicates a greater degree of overlap than a lower voltage.

[0173] Example 12. The audio transducer of any one of the Examples herein, wherein the first parameter indicative of the amount of axial overlap comprises a current passing through the second coil, wherein a higher current indicates a greater degree of overlap than a lower current.

[0174] Example 13. The audio transducer of any one of the Examples herein, wherein the first parameter comprises a time-averaged value indicative of an average amount of axial overlap over a given time period.

[0175] Example 14. The audio transducer of any one of the Examples herein, wherein the operations further comprise, based on the determined position of the transducer component, correcting a position of the first coil.

[0176] Example 15. The audio transducer of any one of the Examples herein, wherein correcting the position of the first coil comprises one or more of adjusting an internal air pressure of the audio transducer; or applying a DC offset to the first signal applied to the first coil.

[0177] Example 16. The audio transducer of any one of the Examples herein, further comprising: a second membrane operably coupled to a third coil and configured to move along a second axis within a second excursion range; and a fourth coil radially separated from the third coil with respect to the second axis, wherein the operations further comprise: providing, via the amplifier, a third signal to the third coil; receiving, via the fourth coil, a fourth signal; obtaining, based on a comparison of the third and fourth signals, a second parameter indicative of an amount of axial overlap between the third coil and fourth coil; and determining, based on the second parameter indicative of the amount of axial overlap, a position of a second transducer component.

[0178] Example 17. The audio transducer of any one of the Examples herein, wherein the first signal and the second signal are identical.

[0179] Example 18. An audio transducer, comprising: a diaphragm; a first coil operably coupled to the diaphragm, wherein the first coil is configured to move axially within a first excursion range; an amplifier electrically connected to the first coil and configured to provide a first signal to the first coil; a second coil radially separated from the first coil, the second coil configured to electromagnetically couple to the first coil to induce a second signal in the second coil in response to the first signal in the first coil; and a controller configured to: detect the second signal in the second coil, the second signal indicative of an amount of axial overlap between the first coil and the second coil; and based on the second signal, adjusting a position of the first coil.

[0180] Example 19. The audio transducer of any one of the Examples herein, wherein adjusting the position of the first coil comprises adjusting an internal air pressure of the audio transducer.

[0181] Example 20. The audio transducer of any one of the Examples herein, wherein adjusting the position of the first coil comprises applying a DC offset to the first signal provided to the first coil.

[0182] Example 21. The audio transducer of any one of the Examples herein, further comprising a suspension assembly that imparts a negative stiffness to movement of the first coil within the first excursion range.

[0183] Example 22. The audio transducer of any one of the Examples herein, wherein the first signal comprises an inaudible pilot tone.

[0184] Example 23. The audio transducer of any one of the Examples herein, wherein the first signal comprises an audio signal.

[0185] Example 24. The audio transducer of any one of the Examples herein, wherein the first coil comprises a voice coil.

[0186] Example 25. The audio transducer of any one of the Examples herein, wherein the first coil extends circumferentially around the second coil.

[0187] Example 26. The audio transducer of any one of the Examples herein, wherein the second coil extends circumferentially around the first coil. [0188] Example 27. A method of operating an audio transducer, the method comprising: supplying a first signal to a first coil of the transducer, the first coil coupled to a membrane such that the first signal causes the first coil to move axially within a first excursion range; detecting a second signal in a second coil of the transducer, the second coil radially spaced apart from the first coil and at least partially axially overlapping the first coil as the first coil moves within the first excursion range; and based on the second signal, correcting an axial position of the first coil.

[0189] Example 28. The method of any one of the Examples herein, wherein adjusting the axial position of the first coil comprises adjusting an internal air pressure of the audio transducer.

[0190] Example 29. The method of any one of the Examples herein, wherein adjusting the axial position of the first coil comprises applying a DC offset to the first signal provided to the first coil. [0191] Example 30. The method of any one of the Examples herein, wherein the first signal comprises an inaudible pilot tone.

[0192] Example 31. The method of any one of the Examples herein, wherein the first signal comprises an audio signal.

[0193] Example 32. The method of any one of the Examples herein, wherein the first coil comprises a voice coil.

[0194] Example 33. The method of any one of the Examples herein, wherein the first coil extends circumferentially around the second coil.

[0195] Example 34. The method of any one of the Examples herein, wherein the second coil extends circumferentially around the first coil.

[0196] Example 35. An audio transducer, comprising: a first membrane and a second opposing membrane each configured to move axially inward and outward along an excursion axis; a first voice coil operably coupled to the first membrane and configured to move axially along a first excursion range; a second voice coil operably coupled to the second membrane and configured to move axially along a second excursion range; a first sensor coil radially separated from the first voice coil, wherein the first sensor coil is arranged to at least partially axially overlap the first voice coil as the first voice coil moves within the first excursion range; a first amplifier electrically coupled to the first voice coil; a second amplifier electrically coupled to the second voice coil; one or more processors; and memory storing instructions that, when executed by the one or more processors, cause the audio transducer to perform operations comprising: providing, via the first amplifier, a first signal to the first voice coil; receiving, via the first sensor coil, a second signal; obtaining, based on a comparison of the first and second signals, a first parameter indicative of a first amount of axial overlap between the first voice coil and first sensor coil; and determining, based on the first parameter indicative of the first amount of axial overlap, a position of a first transducer component.

[0197] Example 36. The audio transducer of any one of the preceding Examples, further comprising a second sensor coil radially separated from the second voice coil, wherein the second sensor coil is arranged to at least partially axially overlap the second voice coil as the second voice coil moves within the second excursion range, and wherein the operations further comprise: providing, via the second amplifier, a third signal to the second voice coil; receiving, via the second sensor coil, a fourth signal; obtaining, based on a comparison of the third and fourth signals, a second parameter indicative of a second amount of axial overlap between the second voice coil and the second sensor coil; and determining, based on the second parameter indicative of the amount of axial overlap, a position of a second transducer component.

[0198] Example 37. The audio transducer of any one of the preceding Examples, wherein the operations further comprise, based on the determined position of at least one of the first transducer component or the second transducer component, adjusting a position of at least one of the first voice coil or the second voice coil.

[0199] Example 38. The audio transducer of any one of the preceding Examples, wherein adjusting the position of at least one of the first voice coil or the second voice coil comprises applying a DC offset to at least one of the first signal applied to the first voice coil or the third signal applied to the second voice coil.

[0200] Example 39. The audio transducer of any one of the preceding Examples, wherein the operations further comprise applying a DC offset to the first signal applied to the first voice coil without applying a DC offset to the third signal applied to the second voice coil.

[0201] Example 40. The audio transducer of any one of the preceding Examples, wherein the first signal applied to the first voice coil and the third signal applied to the second voice coil are identical.

[0202] Example 41. The audio transducer of any one of the preceding Examples, wherein the first voice coil and the second voice coil are laterally spaced apart from one another around the perimeter of the first membrane and the second membrane. [0203] Example 42. The audio transducer of any one of the preceding Examples, further comprising: a third voice coil operably coupled to the first membrane and configured to move axially along a third excursion range; a third sensor coil radially separated from the third voice coil, wherein the third sensor coil is arranged to at least partially axially overlap the third voice coil as the third voice coil moves along the third excursion range, wherein the first and third voice coils laterally spaced apart from one another around the perimeter of the first membrane; and a third amplifier electrically coupled to the third voice coil, wherein the operations further comprise: providing, via the third amplifier, a third signal to the third voice coil; receiving, via the third sensor coil, a fourth signal; obtaining, based on a comparison of the third and fourth signals, a second parameter indicative of a second amount of axial overlap between the third voice coil and third sensor coil; and determining, based on the second parameter indicative of the second amount of axial overlap, a position of a second transducer component.

[0204] Example 43. The audio transducer of any one of the preceding Examples, wherein determining the position of the first transducer component comprises determining a position of the first voice coil, and wherein determining the position of the second transducer component comprises determining a position of the third voice coil.

[0205] Example 44. The audio transducer of any one of the preceding Examples, wherein the operations further comprise: based at least in part on the determined position of the first transducer component and the position of the second transducer component, adjusting a position of at least one of the first voice coil or the third voice coil.

[0206] Example 45. The audio transducer of any one of the preceding Examples, wherein adjusting the position of at least one of the first voice coil or the third voice coil comprises adjusting the position of the first voice coil without adjusting a position of the third voice coil.

[0207] Example 46. The audio transducer of any one of the preceding Examples, wherein the determined position of the first transducer component and the position of the second transducer component indicates a degree of tilt of the first membrane.

[0208] Example 47. The audio transducer of any one of the preceding Examples, wherein the first voice coil and the second voice coil are arranged coaxially.

[0209] Example 48. The audio transducer of any one of the preceding Examples, wherein the first sensor coil is radially separated from the second voice coil and at least partially axially overlaps the second voice coil as the second voice coil moves within the second excursion range. [0210] Example 49. The audio transducer of any one of the preceding Examples, wherein the operations further comprise: providing, via the second amplifier, a third signal to the second voice coil; receiving, via the first sensor coil, a fourth signal; obtaining, based on a comparison of the third and fourth signals, a second parameter indicative of an amount of axial overlap between the second voice coil and the first sensor coil; and determining, based on the second parameter indicative of the amount of axial overlap, a position of a transducer component.

[0211] Example 50. The audio transducer of any one of the preceding Examples, wherein the first signal comprises a pilot tone having a first frequency, and wherein the third signal comprise a second pilot tone having a second frequency, and wherein the first signal and the second signal are applied concurrently to the first voice coil and the second voice coil, respectively.

[0212] Example 51. The audio transducer of any one of the preceding Examples, wherein: receiving, via the first sensor coil, the third signal comprises obtaining a fifth signal via the first sensor coil and filtering the fifth signal to remove at least some frequencies outside of the first frequency; and receiving, via the first sensor coil, the fourth signal comprises obtaining a fifth signal via the first sensor coil and filtering the fifth signal to remove at least some frequencies outside of the second frequency.

[0213] Example 52. The audio transducer of any one of the preceding Examples, wherein the position of the transducer component comprises an axial position of the first voice coil.

[0214] Example 53. The audio transducer of any one of the preceding Examples, wherein the position of the transducer component comprises an axial position of the first membrane.

[0215] Example 54. The audio transducer of any one of the preceding Examples, wherein the first signal comprises an inaudible pilot tone.

[0216] Example 55. The audio transducer of any one of the preceding Examples, wherein the first signal comprises an audio signal.

[0217] Example 56. The audio transducer of any one of the preceding Examples, wherein the first voice coil extends circumferentially around the first sensor coil.

[0218] Example 57. The audio transducer of any one of the preceding Examples, wherein the first sensor coil extends circumferentially around the first voice coil.

[0219] Example 58. The audio transducer of any one of the preceding Examples, wherein the first voice coil is electromagnetically coupled to the first sensor coil such that the first signal in the first voice coil causes the second signal to be induced in the first sensor coil, and wherein a coupling ratio between the first signal and the second signal is indicative of an amount of axial overlap between the first voice coil and the first sensor coil.

[0220] Example 59. The audio transducer of any one of the preceding Examples, wherein the first parameter indicative of the amount of axial overlap comprises a voltage across the first sensor coil, wherein a higher voltage indicates a greater degree of overlap than a lower voltage.

[0221] Example 60. The audio transducer of any one of the preceding Examples, wherein the first parameter indicative of the amount of axial overlap comprises a current passing through the first sensor coil, wherein a higher current indicates a greater degree of overlap than a lower current. [0222] Example 61. The audio transducer of any one of the preceding Examples, wherein the first parameter comprises a time-averaged value indicative of an average amount of axial overlap over a given time period.

[0223] Example 62. An audio transducer, comprising: a first diaphragm; a first voice coil operably coupled to the first diaphragm, wherein the first voice coil is configured to move axially within a first excursion range; a second voice coil operably coupled to the first diaphragm, wherein the second voice coil is configured to move axially within a second excursion range; a first amplifier electrically connected to the first voice coil and configured to provide a first signal to the first voice coil; a second amplifier electrically connected to the second voice coil and configured to provide a second signal to the second voice coil; a first sensor coil radially separated from the first voice coil, the first sensor coil configured to electromagnetically couple to the first voice coil to induce a third signal in the first sensor coil in response to the first signal in the first voice coil; a second sensor coil radially separated from the second voice coil, the second sensor coil configured to electromagnetically couple to the second voice coil to induce a fourth signal in the second sensor coil in response to the second signal in the second voice coil; and a controller configured to: detect the third signal in the first sensor coil, the third signal indicative of a first amount of axial overlap between the first voice coil and the first sensor coil; detect the fourth signal in the second sensor coil, the fourth signal indicative of a second amount of axial overlap between the second voice coil and the second sensor coil; and based at least in part on the third signal or the fourth signal, adjusting a position of a transducer component.

[0224] Example 63. The audio transducer of any one of the preceding Examples, wherein the first voice coil and the second voice coil are laterally spaced apart from one another around a perimeter of the first diaphragm. [0225] Example 64. The audio transducer of any one of the preceding Examples, wherein the first voice coil and the second voice coil are arranged on opposing lateral sides of the first diaphragm.

[0226] Example 65. The audio transducer of any one of the preceding Examples, wherein adjusting the position of the transducer component comprises adjusting a position of the first voice coil without adjusting a position of the second voice coil.

[0227] Example 66. The audio transducer of any one of the preceding Examples, wherein adjusting the position of the first voice coil comprises applying a DC offset to the first signal provided to the first coil.

[0228] Example 67. The audio transducer of any one of the preceding Examples, wherein adjusting the position of the transducer component comprises adjusting an internal air pressure of the audio transducer.

[0229] Example 68. The audio transducer of any one of the preceding Examples, wherein each of the first signal and the second signal comprises an inaudible pilot tone.

[0230] Example 69. The audio transducer of any one of the preceding Examples, wherein each of the first signal and the second signal comprises an audio signal.

[0231] Example 70. The audio transducer of any one of the preceding Examples, wherein the first voice coil extends circumferentially around the first sensor coil.

[0232] Example 71. The audio transducer of any one of the preceding Examples, wherein the first sensor coil extends circumferentially around the first voice coil.

[0233] Example 72. A method of operating an audio transducer, the method comprising: supplying a first signal to a first voice coil of the transducer, the first voice coil coupled to a membrane of the audio transducer such that the first signal causes the first voice coil to move axially within a first excursion range; supplying a second signal to a second voice coil of the transducer, the second voice coil coupled to a membrane of the audio transducer such that the second signal causes the second voice coil to move axially within a second excursion range; detecting a third signal in a first sensor coil of the transducer, the first sensor coil arranged coaxially with and at least partially axially overlapping the first voice coil; detecting a fourth signal in a second sensor coil of the transducer, the second sensor coil arranged coaxially with and at least partially axially overlapping the second voice coil; and based on at least one of the third signal and the fourth signal, correcting an axial position of at least one of the first voice coil or the second voice coil.

[0234] Example 73. The method of any one of the preceding Examples, wherein the first voice coil and the second voice coil are coupled to the same membrane.

[0235] Example 74. The method of any one of the preceding Examples, wherein the first voice coil and the second voice coil are coupled to separate opposing membranes of the transducer.

[0236] Example 75. The method of any one of the preceding Examples, wherein correcting the axial position of at least one of the first voice coil or the second voice coil comprises correcting the axial position of the first voice coil without adjusting the axial position of the second voice coil.

[0237] Example 76. The method of any one of the preceding Examples, wherein correcting the axial position of at least one of the first voice coil or the second voice coil comprises adjusting an internal air pressure of the audio transducer.

[0238] Example 77. The method of any one of the preceding Examples, wherein correcting the axial position of at least one of the first voice coil or the second voice coil comprises applying a DC offset to the first signal provided to the first voice coil and/or applying a DC offset to the second signal provided to the second voice coil.

[0239] Example 78. The method of any one of the preceding Examples, wherein each of the first signal and the second signal comprises an inaudible pilot tone.

[0240] Example 79. The method of any one of the preceding Examples, wherein each of the first signal and the second signal comprises an audio signal.

[0241] Example 80. A playback device, comprising: a membrane; a first coil operably coupled to the membrane, wherein the first coil extends circumferentially about an axis and is configured to move along the axis within a first excursion range; an amplifier electrically connected to the first coil; a second coil extending circumferentially about the axis, wherein the relative axial position of the first coil with respect to the second coil varies as the first coil moves within the first excursion range; one or more processors; and memory storing instructions that, when executed by the one or more processors, cause the playback device to perform operations comprising: providing, via the amplifier, a first signal to the first coil; receiving, via the second coil, a second signal; obtaining, based on a comparison of the first and second signals, a first parameter indicative of the relative axial position of the first coil; and determining, based on the first parameter, a position of a transducer component.

[0242] Example 81. The playback device of any one of the preceding Examples, wherein the position of the transducer component comprises an axial position of the first coil.

[0243] Example 82. The playback device of any one of the preceding Examples, wherein the position of the transducer component comprises an axial position of the membrane.

[0244] Example 83. The playback device of any one of the preceding Examples, further comprising a suspension assembly that imparts a negative stiffness to movement of the first coil within the first excursion range.

[0245] Example 84. The playback device of any one of the preceding Examples, wherein the first signal comprises an inaudible pilot tone.

[0246] Example 85. The playback device of any one of the preceding Examples, wherein the first signal comprises an audio signal.

[0247] Example 86. The playback device of any one of the preceding Examples, wherein the first coil comprises a voice coil.

[0248] Example 87. The playback device of any one of the preceding Examples, wherein the first coil extends circumferentially around the second coil.

[0249] Example 88. The playback device of any one of the preceding Examples, wherein the second coil extends circumferentially around the first coil.

[0250] Example 89. The playback device of any one of the preceding Examples, wherein the relative axial positions indicate varying amounts of axial overlap between the first coil and the second coil.

[0251] Example 90. The playback device of any one of the preceding Examples, wherein the first coil does not axially overlap with the second coil at any point along the first excursion range. [0252] Example 91. The playback device of any one of the preceding Examples, wherein the second coil is planar.

[0253] Example 92. The playback device of any one of the preceding Examples, wherein the second coil comprises a conductive trace disposed on a substrate.

[0254] Example 93. The playback device of any one of the preceding Examples, wherein the substrate comprises a printed circuit board (PCB). [0255] Example 94. The playback device of any one of the preceding Examples, wherein the first coil is electromagnetically coupled to the second coil such that the first signal in the first coil causes the second signal to be induced in the second coil, and wherein a coupling ratio between the first signal and the second signal is indicative of the relative axial positions of the first coil and the second coil.

[0256] Example 95. The playback device of any one of the preceding Examples, wherein the first parameter indicative of the relative axial position comprises a voltage across the second coil, wherein a higher voltage indicates a greater proximity between the first coil and the second coil than a lower voltage.

[0257] Example 96. The playback device of any one of the preceding Examples, wherein the first parameter indicative of the relative axial position comprises a current passing through the second coil, wherein a higher current indicates a greater proximity between the first coil and the second coil than a lower current.

[0258] Example 97. The playback device of any one of the preceding Examples, wherein the first parameter comprises a time-averaged value indicative of an average of the relative axial position over a given time period.

[0259] Example 98. The playback device of any one of the preceding Examples, wherein the operations further comprise, based on the determined position of the transducer component, correcting a position of the first coil.

[0260] Example 99. The playback device of any one of the preceding Examples, wherein correcting the position of the first coil comprises one or more of: adjusting an internal air pressure of the audio transducer; or applying a DC offset to the first signal applied to the first coil.

[0261] Example 100. The playback device of any one of the preceding Examples, wherein the membrane is a first membrane, the playback device further comprising: a second membrane opposing the first membrane, the second membrane operably coupled to a third coil, wherein the third coil extends circumferentially about the axis and is configured to move along the axis within a second excursion range, wherein the relative axial position of the second coil and the third coil varies as the third coil moves within the second excursion range, and wherein the operations further comprise: providing, via the amplifier, a third signal to the third coil; receiving, via the second coil, a fourth signal; obtaining, based on a comparison of the third and fourth signals, a second parameter indicative of the relative axial positions of the second coil and the third coil; and determining, based on the second parameter indicative of the relative axial positions, a position of a second transducer component.

[0262] Example 101. The playback device of any one of the preceding Examples, wherein the first signal and the third signal are pilot tones of different frequencies.

[0263] Example 102. The playback device of any one of the preceding Examples, wherein the second coil is axially disposed between the first coil and the third coil.

[0264] Example 103. The playback device of any one of the preceding Examples, wherein the second coil is stationary with respect to the playback device.

[0265] Example 104. The playback device of any one of the preceding Examples, further comprising: a second membrane operably coupled to a third coil and configured to move along a second axis within a second excursion range; and a fourth coil radially separated from the third coil with respect to the second axis, wherein the operations further comprise: providing, via the amplifier, a third signal to the third coil; receiving, via the fourth coil, a fourth signal; obtaining, based on a comparison of the third and fourth signals, a second parameter indicative of relative axial positions of the third coil and fourth coil; and determining, based on the second parameter indicative of the relative axial positions, a position of a second transducer component.

[0266] Example 105. A playback device, comprising: an amplifier; an audio transducer electrically connected to the amplifier, the audio transducer comprising a diaphragm; a voice coil operably coupled to the diaphragm, wherein the voice coil is configured to move axially within a first excursion range in response to a first signal received via the amplifier; and a sensor arranged substantially coaxially with the voice coil, the sensor configured to carry a second signal generated in the sensor in response to the first signal in the voice coil; and a controller configured to: compare the first signal and the generated second signal; and determine, based on the comparison of the first signal and the second signal, the relative axial position of the voice coil.

[0267] Example 106. The playback device of any one of the preceding Examples, wherein the controller is configured to adjust, based on the determined axial position of the voice coil, a position of the voice coil.

[0268] Example 107. The audio transducer of any one of the preceding Examples, wherein adjusting the position of the first coil comprises adjusting an internal air pressure of the audio transducer. [0269] Example 108. The audio transducer of any one of the preceding Examples, wherein adjusting the position of the voice coil comprises applying a DC offset to the first signal provided to the voice coil.

[0270] Example 109. The audio transducer of any one of the preceding Examples, further comprising a suspension assembly that imparts a negative stiffness to movement of the voice coil within the first excursion range.

[0271] Example 110. The audio transducer of any one of the preceding Examples, wherein the first signal comprises an inaudible pilot tone.

[0272] Example 111. The audio transducer of any one of the preceding Examples, wherein the first signal comprises an audio signal.

[0273] Example 112. The playback device of any one of the preceding Examples, wherein the second signal is generated in the sensor via transformer coupling between the voice coil and the sensor.

[0274] Example 113. The audio transducer of any one of the preceding Examples, wherein the voice coil extends circumferentially around the sensor.

[0275] Example 114. The audio transducer of any one of the preceding Examples, wherein the sensor extends circumferentially around the voice coil.

[0276] Example 115. The audio transducer of any one of the preceding Examples, wherein the voice coil does not axially overlap the sensor over at any point over the first excursion range.

[0277] Example 116. The audio transducer of any one of the preceding Examples, wherein the sensor comprises a planar coil.

[0278] Example 117. The audio transducer of any one of the preceding Examples, wherein the sensor comprises conductive trace disposed on a substrate.

[0279] Example 118. A method of operating an audio transducer, the method comprising: supplying a first signal to a first coil of the transducer, the first coil coupled to a membrane such that the first signal causes the first coil to move axially within a first excursion range; detecting a second signal in a second coil of the transducer, the second coil arranged coaxially with the first coil and at least partially axially offset from the first coil as the first coil moves within the first excursion range; and based on the second signal, correcting an axial position of the first coil.

[0280] Example 119. The method of any one of the preceding Examples, wherein adjusting the axial position of the first coil comprises adjusting an internal air pressure of the audio transducer. [0281] Example 120. The method of any one of the preceding Examples, wherein adjusting the axial position of the first coil comprises applying a DC offset to the first signal provided to the first coil.

[0282] Example 121. The method of any one of the preceding Examples, wherein the first signal comprises an inaudible pilot tone.

[0283] Example 122. The method of any one of the preceding Examples, wherein the first signal comprises an audio signal.

[0284] Example 123. The method of any one of the preceding Examples, wherein the first coil comprises a voice coil.

[0285] Example 124. The method of any one of the preceding Examples, wherein the first coil extends circumferentially around the second coil.

[0286] Example 125. The method of any one of the preceding Examples, wherein the second coil extends circumferentially around the first coil.

[0287] Example 126. The method of any one of the preceding Examples, wherein the second coil comprises a planar coil.

[0288] Example 127. The method of any one of the preceding Examples, wherein the second coil comprises a conductive trace disposed on a substrate.

[0289] Example 128. The method of any one of the preceding Examples, wherein the first coil does not axially overlap the second coil at any point over the first excursion range.

[0290] Example 129. A method, comprising: providing an audible first signal and an inaudible second signal to a voice coil of an audio transducer, the voice coil electromagnetically coupled to one or more magnets such that the first signal causes the voice coil to move axially within a first excursion range; detecting a third signal generated in a second coil of the transducer, the second coil arranged substantially coaxially with the voice coil; and estimating, based on a comparison of the second signal and the third signal, a relative displacement of the voice coil along the first excursion range.

[0291] Example 130. The method of any one of the preceding Examples, wherein the voice coil extends circumferentially around the second coil.

[0292] Example 131. The method of any one of the preceding Examples, wherein the second coil extends circumferentially around the voice coil. [0293] Example 132. The method of any one of the preceding Examples, wherein the second coil comprises a planar coil.

[0294] Example 133. The method of any one of the preceding Examples, wherein the second coil comprises a conductive trace disposed on a substrate.

[0295] Example 134. The method of any one of the preceding Examples, wherein the voice coil does not axially overlap the second coil at any point over the first excursion range.

Claims

1. A playback device, comprising: a membrane; a first coil operably coupled to the membrane, wherein the first coil extends circumferentially about an axis and is configured to move along the axis within a first excursion range; an amplifier electrically connected to the first coil; a second coil extending circumferentially about the axis, wherein the relative axial position of the first coil with respect to the second coil varies as the first coil moves within the first excursion range; one or more processors configured for: providing, via the amplifier, a first signal to the first coil; receiving, via the second coil, a second signal; based on the first and second signals, at least one of: correcting an axial position of the first coil; and determining a position of a transducer component.

2. The playback device of claim 1, wherein the position of the transducer component corresponds to an axial position of the first coil.

3. The playback device of claim 1 or 2, wherein the position of the transducer component corresponds to an axial position of the membrane.

4. The playback device of any preceding claim, further comprising a suspension assembly that imparts a negative stiffness to movement of the first coil within the first excursion range.

5. The playback device of any preceding claim, wherein the first signal comprises at least one of: an inaudible pilot tone; and an audio signal.

6. The playback device of any preceding claim, wherein the first coil comprises a voice coil.

7. The playback device of any preceding claim, wherein the first coil extends circumferentially around the second coil.

8. The playback device of one of claims 1 to 7, wherein the second coil extends circumferentially around the first coil.

9. The playback device of any preceding claim, wherein the relative axial positions indicate varying amounts of axial overlap between the first coil and the second coil.

10. The playback device of any preceding claim, wherein the first coil does not axially overlap with the second coil at any point along the first excursion range.

11. The playback device of claim any preceding claim, wherein the second coil is planar.

12. The playback device of any preceding claim, wherein the second coil comprises a conductive trace disposed on a substrate.

13. The playback device of claim 12, wherein the substrate comprises a printed circuit board (PCB).

14. The playback device of any preceding claim, wherein the first coil is electromagnetically coupled to the second coil such that the first signal in the first coil causes the second signal to be induced in the second coil, and wherein a coupling ratio between the first signal and the second signal is indicative of the relative axial positions of the first coil and the second coil.

15. The playback device of any preceding claim, wherein a first parameter based on a comparison of the first and second signals and indicative of the relative axial position comprises a voltage across the second coil, wherein a higher voltage indicates a greater proximity between the first coil and the second coil than a lower voltage.

16. The playback device of one of claims 1 to 15, wherein a first parameter based on a comparison of the first and second signals and indicative of the relative axial position comprises a current passing through the second coil, wherein a higher current indicates a greater proximity between the first coil and the second coil than a lower current.

17. The playback device of one of claims 1 to 15, wherein a first parameter based on a comparison of the first and second signals comprises a time-averaged value indicative of an average of the relative axial position over a given time period.

18. The playback device of any preceding claim, wherein correcting the position of the first coil comprises one or more of adjusting an internal air pressure of the audio transducer; or applying a DC offset to the first signal applied to the first coil.

19. The playback device of any preceding claim, wherein the membrane is a first membrane, the playback device further comprising: a second membrane opposing the first membrane, the second membrane operably coupled to a third coil, wherein the third coil extends circumferentially about the axis and is configured to move along the axis within a second excursion range, wherein the relative axial position of the second coil and the third coil varies as the third coil moves within the second excursion range, and wherein the one or more processers are further configured for: providing, via the amplifier, a third signal to the third coil; receiving, via the second coil, a fourth signal; based on the third and fourth signals, at least one of: determining a position of a second transducer component; and correcting an axial position of a second transducer component.

20. The playback device of claim 19, wherein the first signal and the third signal are pilot tones of different frequencies.

21. The playback device of claim 19 or 20, wherein the second coil is axially disposed between the first coil and the third coil.

22. The playback device of one of claims 19 to 21, wherein the second coil is stationary with respect to the playback device.

23. The playback device of claim 1, further comprising: a second membrane operably coupled to a third coil and configured to move along a second axis within a second excursion range; and a fourth coil radially separated from the third coil with respect to the second axis, wherein the operations further comprise: providing, via the amplifier, a third signal to the third coil; receiving, via the fourth coil, a fourth signal; based on the third and fourth signals, at least one of: determining a position of a second transducer component; and correcting an axial position of the second transducer.

24. A playback device, comprising: an amplifier; an audio transducer electrically connected to the amplifier, the audio transducer comprising — a diaphragm; a voice coil operably coupled to the diaphragm, wherein the voice coil is configured to move axially within a first excursion range in response to a first signal received via the amplifier; and a sensor arranged substantially coaxially with the voice coil, the sensor configured to carry a second signal generated in the sensor in response to the first signal in the voice coil; and a controller configured to: compare the first signal and the generated second signal; and determine, based on the comparison of the first signal and the second signal, the relative axial position of the voice coil.

25. The playback device of claim 24, wherein the controller is configured to adjust, based on the determined axial position of the voice coil, a position of the voice coil.

26. The audio transducer of claim 25, wherein adjusting the position of the first coil comprises one of: adjusting an internal air pressure of the audio transducer; and applying a DC offset to the first signal provided to the voice coil.

27. The audio transducer of one of claims 24 to 26, further comprising a suspension assembly that imparts a negative stiffness to movement of the voice coil within the first excursion range.

28. The audio transducer of one of claims 24 to 27, wherein the first signal comprises at least one of: an inaudible pilot tone; and an audio signal.

29. The playback device of one of claims 24 to 28, wherein the second signal is generated in the sensor via transformer coupling between the voice coil and the sensor.

30. The audio transducer of one of claims 24 to 29, wherein the voice coil extends circumferentially around the sensor.

31. The audio transducer of one of claims 24 to 31, wherein the sensor extends circumferentially around the voice coil.

32. The audio transducer of one of claims 24 to 31, wherein the voice coil does not axially overlap the sensor over at any point over the first excursion range.

33. The audio transducer of one of claims 24 to 32, wherein the sensor comprises a planar coil.

34. The audio transducer of one of claims 24 to 33, wherein the sensor comprises conductive trace disposed on a substrate.

35. A method of operating an audio transducer, the method comprising: supplying a first signal to a first coil of the transducer, the first coil coupled to a membrane such that the first signal causes the first coil to move axially within a first excursion range; based on a second signal in a second coil of the transducer, the second coil arranged coaxially with the first coil and at least partially axially offset from the first coil as the first coil moves within the first excursion range, at least one of: adjusting an axial position of the first coil; and determining a position of the transducer.

36. The method of claim 35, wherein adjusting the axial position of the first coil comprises at least one of: adjusting an internal air pressure of the audio transducer; and applying a DC offset to the first signal provided to the first coil.

37. The method of claim 35 or 36, wherein the first signal comprises at least one of: an inaudible pilot tone; and an audio signal.

38. The method of one of claims 35 to 37, wherein the first coil comprises a voice coil.

39. The method of one of claims 35 to 38, wherein the first coil extends circumferentially around the second coil.

40. The method of one of claims 35 to 38, wherein the second coil extends circumferentially around the first coil.

41. The method of one of claims 35 to 40, wherein the second coil comprises a planar coil.

42. The method of one of claims 35 to 41, wherein the second coil comprises a conductive trace disposed on a substrate.

43. The method of one of claims 35 to 42, wherein the first coil does not axially overlap the second coil at any point over the first excursion range.

44. A method, comprising: providing an audible first signal and an inaudible second signal to a voice coil of an audio transducer, the voice coil electromagnetically coupled to one or more magnets such that the first signal causes the voice coil to move axially within a first excursion range; detecting a third signal generated in a second coil of the transducer, the second coil arranged substantially coaxially with the voice coil; and based on the second signal and the third signal, estimating a relative displacement of the voice coil along the first excursion range.

45. The method of claim 44, wherein the voice coil extends circumferentially around the second coil.

46. The method of claim 44, wherein the second coil extends circumferentially around the voice coil.

47. The method of one of claims 44 to 46, wherein the second coil comprises a planar coil.

48. The method of one of claims 44 to 47, wherein the second coil comprises a conductive trace disposed on a substrate.

49. The method of one of claims 44 to 48, wherein the voice coil does not axially overlap the second coil at any point over the first excursion range.