EP2472905A1 - Electroacoustic transducer - Google Patents

Abstract

An electroacoustic transducer (speaker or microphone) has a coil attached to a diaphragm and a permanent magnet arrangement. The diaphragm comprises a flexible circuit board, wherein electrical and mechanical connection is made between the flexible circuit board and the coil. This provides a simple mechanical and electrical connection for the coil with low component count and cost and with good reliability.

Description

• The invention relates to an electro-acoustic transducer comprising a membrane and a magnet system.
• In particular, the invention relates to the construction of micro speakers. Micro speakers are loudspeakers mainly used in mobile applications such as mobile phones, mp3 players, navigation systems, laptops etc.
• The key to the success of micro speakers is high performance and high reliability at a very low total cost level.
• One difficulty in the miniaturisation and cost reduction results from the electrical connection that needs to be made to a moving component (the voice coil). Typically the connection is made with loop wires. The voice coil is glued to the membrane. The loop wires at the ends of the voice coils are formed and guided to the fixed part of the speaker, where they are soldered/thermo bonded to contact springs.
• The loop wires are formed from the same material as the voice coil and move during the operation of the speaker. These moving loop wires are a critical aspect of the reliability of the speaker. This loop wire arrangement also requires a number of components dedicated specifically to the electrical connection.
• According to the invention, there is provided an electro-acoustic transducer comprising a diaphragm, a coil attached to the diaphragm and a permanent magnet arrangement, wherein the magnetic field of the permanent magnet arrangement interacts with the coil electromagnetic field,
  wherein the diaphragm comprises a flexible circuit board, wherein electrical and mechanical connection is made between the flexible circuit board and the coil.
• In this arrangement, a flexible circuit board enables the diaphragm movement, and also enables fixed electrical connections to be made to the coil.
• The flexible circuit board preferably comprises a flexible foil printed circuit board.
• The flexible circuit board can comprise an outer frame, an inner diaphragm part and suspension members extending between the outer frame and the inner diaphragm part. The suspension members thus provide the physical flexibility as well as providing conductor tracks to the outer frame. The electrical connection off the flexible board can then be at the static outer frame part of the board.
• The inner diaphragm part can be a solid shape. This means the flexible board itself is providing the mechanical diaphragm. In another arrangement, the inner diaphragm part has a central opening over which is provided a plate. The plate then functions as the mechanical diaphragm.
• There can be four suspension members extending between the corners of a generally rectangular inner diaphragm part and corners of a larger generally rectangular outer frame.
• The coil is preferably annular and positioned at the edge of the inner diaphragm part.
• In a preferred arrangement, there are two concentric annular coils, adjacent each other, and both positioned at the edge of the inner diaphragm part.
• The invention will now be described with reference to the accompanying drawings, in which:
• Figure 1 shows, in cross-section, a known electroacoustic transducer;
• Figure 2 shows a first example of voice coil suspension arrangement for use in a transducer of the invention;
• Figure 3 shows a second example of voice coil suspension arrangement for use in a transducer of the invention;
• Figure 4 shows the arrangement of components in cross section;
• Figure 5 shows a dual voice coil arrangement;
• Figure 6 shows a third example of voice coil suspension arrangement for use in a transducer of the invention;
• Figure 7 shows a fourth example of voice coil suspension arrangement for use in a transducer of the invention; and
• Figure 8 shows a fifth example of voice coil suspension arrangement for use in a transducer of the invention.
• The invention provides an electroacoustic transducer (speaker or microphone) having a coil attached to a diaphragm and a permanent magnet arrangement. The diaphragm comprises a flexible circuit board, wherein electrical and mechanical connection is made between the flexible circuit board and the coil. This provides a simple mechanical and electrical connection for the coil with low component count and cost and with good reliability.
• Figure 1 shows a known electroacoustic transducer 1 as described inW02004/014104, which in this case is a loudspeaker 1. The transducer 1 is essentially centrally symmetrical in design, and has a transducer axis 2.
• The transducer 1 has a pot-shaped housing 3, which is made of metal and which has a base wall 4 and a side wall 5. The base wall 4 is provided with a circular elevation 6, in which sound transmission openings are provided (not shown). Four strip-shaped lugs 7, running in radial directions in relation to transducer axis 2, protrude from the side wall 5. The lugs 7 are used to secure the components of the transducer 1 in thehousing 3.
• In the area of transducer axis 2, the pot-shaped housing 3 can have a height of 1-5mm and the diameter ofhousing 3 in the direction running perpendicular to the direction of transducer axis 2 is for example in the range 5-20mm. The transducer 1 is for example provided for use in a mobile telephone or in similar small scale telecommunications apparatus.
• The transducer 1 contains a membrane 8, which is essentially U-shaped in its peripheral area 9. The U-shaped peripheral area 9 is connected to aring 10, which is connected to the membrane 8 when the membrane 8 is produced. Thering 10 is for transport purposes and also for assembly purposes. When the transducer 1 is produced, thering 10, including the membrane 8 which is connected to it, is inserted into thehousing 3 in parallel with the direction of transducer axis 2, before the lugs 7 have been bent over. The outer part of the membrane is fixed in position.
• The membrane 8 has an intermediate area 11 adjacent to the U-shaped peripheral area 9, and an annular securing area 12 adjacent to intermediate area 11. Acentral area 13 is located within the securing area 12, and thecentral area 13 is for sound generation. The membrane 8 as a whole is arranged parallel to transducer axis 2 so as to be oscillatory.
• The transducer 1 is further equipped with a magnet system 14. The magnet system 14 comprises apermanent magnet 15 and an annularfirst yoke 16, whichyoke 16 is located tight against thepermanent magnet 15. Asecond yoke 17 has an L-shaped cross-sectional shape, and a disk-shapedfirst section 18 of thesecond yoke 17 is also located tight against thepermanent magnet 15. An annularsecond section 19 protrudes fromfirst section 18 parallel with transducer axis 2.
• When the transducer 1 is produced, thefirst yoke 16, then thepermanent magnet 15 and then thesecond yoke 17 are inserted into thehousing 3 parallel with the direction of transducer axis 2. Once the threecomponents 16, 15 and 17 of the magnet system have been inserted, the lugs 7 ofhousing 3 are bent over into their positions as shown inFigure 1, as a result of which the membrane 8 and the magnet system 14 are fixed.
• Anair gap 20 is formed between the free end ofsecond section 19 of thesecond yoke 17 and the inner edge of thefirst yoke 16, whichair gap 20 is bounded by the two components of the magnet system, namely thefirst yoke 16 and thesecond yoke 17. The magnet system 14 encloses aninner space 22. Fixed in theinner space 22 is acontact carrier 24 made of plastic and provided to hold two moving-coil contacts 25, only one of which moving-coil contacts 25 is visible inFigure 1 owing to the sectional view.
• The moving-coil contacts 25 are connected to thecontact carrier 24 by means of extrusion.
• The transducer 1 contains a moving coil 29 (the voice coil), which is partly arranged inair gap 20 and is connected to the membrane 8 in its securing area 12 using an adhesive connection. The movingcoil 29 interacts with the magnet system 14, wherein an electrical signal representing a signal to be reproduced acoustically is sent to the movingcoil 29, as a result of which the movingcoil 29 is caused to oscillate, which results in the membrane 8 bringing about the signal to be reproduced acoustically.
• The electrical signal sent to movingcoil 29 must be amplified and, if applicable, its signal waveform must be influenced, before being sent to the movingcoil 29. Anintegrated circuit 31 for this purpose is indicated schematically inFigure 1.
• The invention relates particularly to the way thevoice coil 29 is mounted and electrically connected.
• The invention can be applied to a microphone similar in design to that shown inFigure 1, but it can be applied to other designs.Figure 1 is provided simply to show an example of the typical arrangement of components.
• The relevant aspects of the design ofFigure 1 are that there is a voice coil attached to a diaphragm, and a permanent magnet arrangement. The magnetic field of the permanent magnet arrangement interacts with the voice coil electromagnetic field.
• The invention provides a transducer in which the diaphragm comprises a flexible circuit board, wherein electrical and mechanical connection is made between the flexible circuit board and the voice coil.
• Figure 2 shows a first design from the underside. Theflexible circuit board 40 preferably comprises a flexible foil printed circuit board.
• The flexible circuit board comprises anouter frame 42, aninner diaphragm part 44 andsuspension members 46 extending between the outer frame and the inner diaphragm part. Thesuspension members 46 thus provide the physical flexibility as well as providing conductor tracks to the outer frame. The corners of the outer frame can be provided with electrical contacts to the control circuitry, and theflexible circuit board 40 provides the electrical connections to thevoice coil 29. The outer frame is fixed as in the example ofFigure 1.
• The electrical connection off the flexible board is thus at the static outer frame part of the board.
• The inner diaphragm part can be a solid shape, as shown inFigure 2. This means the flexible board itself is providing the mechanical diaphragm. In another arrangement shown inFigure 3, theinner diaphragm part 44 has acentral opening 50 over which is provided a plate. The plate then functions as the mechanical diaphragm. InFigure 3, electrical connections from thevoice coil 29 to the circuit board are at connection points 52 which are positioned within the shape of the voice coil.
• In the examples shown, there are four suspension members extending between the corners of a generally rectangularinner diaphragm part 44 and corners of a larger generally rectangularouter frame 42. However, a different shape can be used.
• Thevoice coil 29 is annular and positioned around the edge of theinner diaphragm part 44. There may be two concentric annular voice coils, adjacent each other, and both positioned around the edge of the inner diaphragm part. Dual voice coil speakers are known. The use of dual voice coils enables different amplification systems to be used, for example boost bridge amplifiers. The additional connections required to two voice coils has been a problem in the past.
• The simple connection to the flexible PCB, with the PCB functioning as physical carrier as well as system for routing electrical signals, makes the implementation of a dual voice coil system very simple. The two voice coils are driven with signals that are phase inverted with respect to each other.
• Figure 4 shows the arrangement of components in cross section.
• In this example, thevoice coil 29 is provided with short circuit rings 33. The voice coil is surrounded by thepermanent magnet 15, thefirst yoke 16 which functions as a pole plate, and thesecond yoke 17. In the rest position (shown) or with low excursion of the voice coil, the short circuit rings 33 play no role. When the short circuit rings enter the air gap between thepole plate 16 andyoke 17, they increase the electrical damping. There may be only a lower short circuit ring, only an upper short circuit ring, or both as shown. The upper short circuit ring can be part of the structure of the PCB. The short circuit rings are not electrically connected to the voice coil. The short circuit rings are low weight electrically conducting material, such as aluminium.
• Figure 5 shows two voice coils - aninner coil 29a and anouter coil 29b suspended by theflexible circuit board 40. The short circuit rings 33 are also shown.
• The specific design of the suspension arms may vary to obtain the desired flexibility and elasticity. Some variations are shown inFigures 5 to 7.
• InFigure 6, the suspension members have a lateral widening 60 which includes an opening. This shape increases the flexibility of thesuspension members 46. There may be multiple widenings on each suspension member in series.
• Figures 7 and 8 show further variations more schematically.
• InFigure 7, thesuspension members 46 are curved instead of straight.
• InFigure 8, the length of thesuspension members 46 is increased by coupling each corner of theinner diaphragm portion 44 not to the closest corner of the outer frame, but to the next corner (clockwise in this example) around the outer frame.
• The invention has been described with reference to a speaker, but it may be applied more generally to electro acoustic transducers, including microphones.
• The PCB can be single sided or double sided. Solder bonds can be used for the mechanical and electrical connection, but additional mechanical coupling can also be used (such as fixings or adhesives). The PCB can be of known design, with flexible conductive tracks such as copper on a flexible carrier.
• Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims (9)

1. An electroacoustic transducer comprising a diaphragm (40), a coil (29) attached to the diaphragm and a permanent magnet arrangement (15,16,17), wherein the magnetic field of the permanent magnet arrangement (15,16,17) interacts with the coil (29) electromagnetic field,
   wherein the diaphragm (40) comprises a flexible circuit board, wherein electrical and mechanical connection is made between the flexible circuit board (40) and the coil (29).
2. An electroacoustic transducer as claimed in claim 1, wherein the flexible circuit board comprises a flexible foil printed circuit board.
3. An electroacoustic transducer as claimed in any preceding claim, wherein the flexible circuit board comprises an outer frame (42), an inner diaphragm part (44) and suspension members (46) extending between the outer frame (42) and the inner diaphragm part (44).
4. An electroacoustic transducer as claimed in claim 3, wherein the inner diaphragm part (44) is a solid shape.
5. An electroacoustic transducer as claimed in claim 3, wherein the inner diaphragm part (44) has a central opening (50) over which is provided a plate.
6. An electroacoustic transducer as claimed in claim 3, 4 or 5 comprising four suspension members (46) extending between the corners of a generally rectangular inner diaphragm part (44) and corners of a larger generally rectangular outer frame (42).
7. An electroacoustic transducer as claimed in any preceding claim, wherein the coil (29) is annular and positioned around the edge of the inner diaphragm part (44).
8. An electroacoustic transducer as claimed in claim 7, comprising two concentric annular coils (29a,29b), adjacent each other, and both positioned around the edge of the inner diaphragm part (44).
9. An electroacoustic transducer as claimed in any preceding claim comprising a speaker, and the or each coil (29,29a,29b) comprises a voice coil.

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