According to the technical standard established by ANSI/ASA S1.1-2013, the American National Standard for Acoustical Terminology, sound is defined as:
"(a)Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation.
(b) Auditory sensation evoked by the oscillation described in (a)."[2]
This two-part definition of sound states that sound can be taken as a wave motion in an elastic medium, making it also astimulus, or as an excitation of the hearing mechanism that results in the perception of sound, making it asensation.
Acoustics is the interdisciplinary scientific study ofmechanical waves,vibrations,sound,ultrasound, andinfrasound in gaseous, liquid, or solid media. A scientist who works in the field ofacoustics is called anacoustician, while an individual specialising inacoustical engineering may be referred to as anacoustical engineer.[3] Anaudio engineer, by contrast, is concerned with the recording, manipulation, mixing, and reproduction of sound.
Twotuning forks of the same frequency demonstratingacoustic resonance. Striking one fork induces vibrational motion in the other through the production of air‑pressure oscillations.
Sound travels as a mechanical wave through a medium (e.g. water, crystals, air). Sound waves are generated by a sound source, such as a vibratingdiaphragm of a loudspeaker. As the sound source vibrates the surrounding medium, mechanical disturbances propagate away from the source at the localspeed of sound, thus resulting in a sound wave. At a fixed distance from the source, thepressure,velocity, and displacement of the medium's particles vary intime. At an instant in time, the pressure, velocity, and displacement vary spatially. The particles of the medium do not travel with the sound wave; instead, the disturbance and itsmechanical energy propagate through the medium. Though intuitively obvious for solids, this also applies for liquids and gases. During propagation, waves can bereflected,refracted, orattenuated by the medium.[5]
The matter that supports the transmission of a sound is named thetransmission medium. Media may be anyform of matter, whether solids, liquids, gases orplasmas. However, sound cannot propagate through avacuum because there is no medium to support mechanical disturbances.[6][7]
The propagation of sound in a medium is influenced primarily by:
A complicated relationship between thedensity and pressure of the medium. This relationship, also affected by temperature, determines the speed of sound within the medium.
Motion of the medium itself. If the medium is moving, this movement may increase or decrease the absolute speed of the sound wave depending on the direction of the movement. For example, sound moving through wind will have its speed of propagation increased by the speed of the wind if the sound and wind are moving in the same direction. If the sound and wind are moving in opposite directions, the speed of the sound wave will be decreased by the speed of the wind.
The viscosity of the medium. Mediumviscosity determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.
When sound is moving through a medium that isn't uniform in its physical properties, it may berefracted (either dispersed or focused).[5]
Some theoretical work suggests that sound waves may carry an extremely small effective mass and be associated with a weakgravitational field.[8]
Waves
Sound is transmitted through fluids (e.g. gases, plasmas, and liquids) aslongitudinal waves, also calledcompression waves. Through solids, however, sound can be transmitted as both longitudinal waves andtransverse waves. Longitudinal sound waves are waves of alternatingpressure deviations from theequilibrium pressure, causing local regions ofcompression andrarefaction, whiletransverse waves (in solids) are waves of alternatingshear stress perpendicular to the direction of propagation. Unlike longitudinal sound waves, transverse sound waves have the property ofpolarisation.[9]
Circular, longitudinal waves propagating from a source and causing particle vibration.
A longitudinal plane wave, the horizontally distorted region moving across the grid.
A transverse plane wave, the vertically distorted region moving across the grid.
Sound waves may be viewed using parabolic mirrors and objects that produce sound.[10]
The energy carried by aperiodic sound wave alternates between the potential energy of the extracompression (in the case of longitudinal waves) or lateral displacementstrain (in the case of transverse waves) of the matter, and the kinetic energy of the particles' displacement velocity in the medium.
Although sound transmission involves many physical processes, thesignal received at a point (such as a microphone or the ear) can be fully described as a time‑varying pressure. This pressure‑versus‑time waveform provides a complete representation of any sound or audio signal detected at that location.
Soundwaves are often simplified assinusoidalplane waves, which are characterized by these generic properties:
U.S. NavyF/A-18 approaching the speed of sound. The white halo is formed by condensed water droplets thought to result from a drop in air pressure around the aircraft (seePrandtl–Glauert singularity).[14]
The speed of sound depends on the medium the waves pass through, and is a fundamental property of the material. The first significant effort towards measurement of the speed of sound was made byIsaac Newton. He believed the speed of sound in a particular substance was equal to the square root of the pressure acting on it divided by its density:
This was later proven wrong and the French mathematicianLaplace corrected the formula by deducing that the phenomenon of sound travelling is not isothermal, as believed by Newton, butadiabatic. He added another factor to the equation—gamma—and multipliedby,thus coming up with the equation.Since,the final equation came up to be,which is also known as the Newton–Laplace equation. In this equation,K is the elastic bulk modulus,c is the velocity of sound, and is the density. Thus, the speed of sound is proportional to thesquare root of theratio of thebulk modulus of the medium to its density.
Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in gases depends on temperature. In 20 °C (68 °F) air at sea level, the speed of sound is approximately 343 m/s (1,230 km/h; 767 mph) using the formulav [m/s] = 331 + 0.6 T [°C]. The speed of sound is also slightly sensitive, being subject to a second-orderanharmonic effect, to the sound amplitude, which means there are non-linear propagation effects, such as the production of harmonics and mixed tones not present in the original sound (seeparametric array). Ifrelativistic effects are important, the speed of sound is calculated from therelativistic Euler equations.
In fresh water the speed of sound is approximately 1,482 m/s (5,335 km/h; 3,315 mph). In steel, the speed of sound is about 5,960 m/s (21,460 km/h; 13,330 mph). Sound moves the fastest in solid atomic hydrogen at about 36,000 m/s (129,600 km/h; 80,530 mph).[15][16]
Sound pressure is the difference, in a given medium, between average local pressure and the pressure in the sound wave. A square of this difference (i.e., a square of the deviation from the equilibrium pressure) is usually averaged over time and/or space, and a square root of this average provides aroot mean square (RMS) value. For example, 1Pa RMS sound pressure (94 dBSPL) in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm Pa) and (1 atm Pa), that is between 101323.6 and 101326.4 Pa.As the human ear can detect sounds with a wide range of amplitudes, sound pressure is often measured as a level on a logarithmicdecibel scale. Thesound pressure level (SPL) orLp is defined as
wherep is theroot-mean-square sound pressure and is areference sound pressure. Commonly used reference sound pressures, defined in the standardANSIS1.1-1994, are 20μPa in air and 1μPa in water. Without a specified reference sound pressure, a value expressed in decibels cannot represent a sound pressure level.
Since the human ear does not have a flatspectral response, sound pressures are oftenfrequency weighted so that the measured level matches perceived levels more closely. TheInternational Electrotechnical Commission (IEC) has defined several weighting schemes.A-weighting attempts to match the response of the human ear to noise and A-weighted sound pressure levels are labeled dBA. C-weighting is used to measure peak levels.
A distinct use of the termsound from its use in physics is that in physiology and psychology, where the term refers to the subject ofperception by the brain. The field ofpsychoacoustics is dedicated to such studies. Webster's dictionary defined sound as: "1. The sensation of hearing, that which is heard; specif.: a.Psychophysics. Sensation due to stimulation of the auditory nerves and auditory centers of the brain, usually by vibrations transmitted in a material medium, commonly air, affecting the organ of hearing. b. Physics. Vibrational energy which occasions such a sensation. Sound is propagated by progressive longitudinal vibratory disturbances (sound waves)."[17] This means that the correct response to the question: "if a tree falls in a forest and no one is around to hear it, does it make a sound?" is "yes", and "no", dependent on whether being answered using the physical, or the psychophysical definition, respectively.
The physical reception of sound in any hearing organism is limited to a range of frequencies. Humans normally hear sound as pitch for frequencies between approximately 20 Hz and 20,000 Hz (20 kHz),[18]: 382 The upper limit decreases with age.[18]: 249 Below 20 Hz, sound waves are heard as discrete stuttering sounds (for discrete pulses) or fast 'wow-wow-wow' sounds(for continuous sounds like sine waves). Sometimessound refers to only those vibrations withfrequencies that are within thehearing range for humans[19] or sometimes it relates to a particular animal. Other species have different ranges of hearing. For example, dogs can perceive vibrations higher than 20 kHz.
As a signal perceived by one of the majorsenses, sound is used by many species fordetecting danger,navigation,predation, and communication. Earth'satmosphere,water, and virtually anyphysical phenomenon, such as fire, rain, wind,surf, or earthquake, produces (and is characterized by) its unique sounds. Many species, such as frogs, birds,marine and terrestrialmammals, have also developed specialorgans to produce sound. In some species, these producesong andspeech. Furthermore, humans have developed culture and technology (such as music, telephone and radio) that allows them to generate, record, transmit, and broadcast sound.
Noise is a term often used to refer to an unwanted sound. In science and engineering, noise is an undesirable component that obscures a wanted signal. However, in sound perception it can often be used to identify the source of a sound and is an important component of timbre perception (see below).
Soundscape is the component of the acoustic environment that can be perceived by humans. The acoustic environment is the combination of all sounds (whether audible to humans or not) within a given area as modified by the environment and understood by people, in context of the surrounding environment.
Pitch perception. During the listening process, each sound is analysed for a repeating pattern (orange arrows) and the results forwarded to the auditory cortex as a single pitch of a certain height (octave) and chroma (note name).
Pitch is perceived as how "low" or "high" a sound is and represents the cyclic, repetitive nature of the vibrations that make up sound. For simple sounds, pitch relates to the frequency of the slowest vibration in the sound (called the fundamental harmonic). In the case of complex sounds, pitch perception can vary. Sometimes individuals identify different pitches for the same sound, based on their personal experience of particular sound patterns. Selection of a particular pitch is determined by pre-conscious examination of vibrations, including their frequencies and the balance between them. Specific attention is given to recognising potential harmonics.[24][25] Every sound is placed on a pitch continuum from low to high.
For example:white noise (random noise spread evenly across all frequencies) sounds higher in pitch thanpink noise (random noise spread evenly across octaves) as white noise has more high frequency content.
Duration
Duration perception. When a new sound is noticed (Green arrows), a sound onset message is sent to the auditory cortex. When the repeating pattern is missed, a sound offset messages is sent.
Duration is perceived as how "long" or "short" a sound is and relates to onset and offset signals created by nerve responses to sounds. The duration of a sound usually lasts from the time the sound is first noticed until the sound is identified as having changed or ceased.[26] Sometimes this is not directly related to the physical duration of a sound. For example; in a noisy environment, gapped sounds (sounds that stop and start) can sound as if they are continuous because the offset messages are missed owing to disruptions from noises in the same general bandwidth.[27] This can be of great benefit in understanding distorted messages such as radio signals that suffer from interference, as (owing to this effect) the message is heard as if it was continuous.
Loudness
Loudness perception involves the integration of sound energy over a brief time window (around 200 ms), during which greater basilar‑membrane displacement and increased nerve‑firing rates contribute to a stronger loudness signal.
Loudness is perceived as how “loud” or “soft” a sound is, and reflects the overall pattern ofauditory‑nerve activity produced by a sound. In general, louder sounds create greater displacement of thebasilar membrane, which stimulates more auditory‑nerve fibres and results in a stronger neural representation of loudness.[28]
Perceived loudness also depends on how sound energy is distributed over time. When a sound is very brief, the auditory system does not fully integrate its energy, so it is heard as softer than a longer sound presented at the same physical intensity. This process, known astemporal summation, operates over a window of roughly 200 ms.[29] Beyond this duration, increasing the length of the sound no longer increases its perceived loudness.
The spectral complexity of a sound can also influence loudness perception. Complex tones, which activate a broader range of auditory‑nerve fibres, are often judged as louder than simple tones (such as sine waves) even when matched for physical amplitude.[30]
Timbre
Timbre perception, showing how a sound changes over time. Despite a similar waveform, differences over time are evident.
Timbre is perceived as the quality of different sounds (e.g. the thud of a fallen rock, the whir of a drill, the tone of a musical instrument or the quality of a voice) and represents the pre-conscious allocation of a sonic identity to a sound (e.g. "it's an oboe!"). This identity is based on information gained from frequency transients, noisiness, unsteadiness, perceived pitch and the spread and intensity of overtones in the sound over an extended time frame.[11][12][13] The way a sound changes over time provides most of the information for timbre identification. Even though a small section of the wave form from each instrument looks very similar, differences in changes over time between the clarinet and the piano are evident in both loudness and harmonic content. Less noticeable are the different noises heard, such as air hisses for the clarinet and hammer strikes for the piano.
Texture
Sonic texture relates to the number of sound sources and the interaction between them.[31][32] The wordtexture, in this context, relates to the cognitive separation of auditory objects.[33] In music, texture is often referred to as the difference betweenunison,polyphony andhomophony, but it can also relate (for example) to a busy cafe; a sound which might be referred to ascacophony.
Spatial location represents the cognitive placement of a sound in an environmental context; including the placement of a sound on both the horizontal and vertical plane, the distance from the sound source and the characteristics of the sonic environment.[33][34] In a thick texture, it is possible to identify multiple sound sources using a combination of spatial location and timbre identification.
Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications
Ultrasound is sound waves with frequencies higher than 20,000 Hz. Ultrasound is not different from audible sound in its physical properties, but cannot be heard by humans. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz.
Infrasound is sound waves with frequencies lower than 20 Hz. Although sounds of such low frequency are too low for humans to hear as a pitch, these sound are heard as discrete pulses (like the 'popping' sound of an idling motorcycle). Whales, elephants and other animals can detect infrasound and use it to communicate. It can be used to detect volcanic eruptions and is used in some types of music.[35]
^abKendall, R.A. (1986). The role of acoustic signal partitions in listener categorization of musical phrases. Music Perception, 185–213.
^abMatthews, M. (1999). Introduction to timbre. In P.R. Cook (Ed.), Music, cognition, and computerized sound: An introduction to psychoacoustic (pp. 79–88). Cambridge, Massachusetts: The MIT press.
^Burton, R.L. (2015).The elements of music: what are they, and who cares?Archived 2020-05-10 at theWayback Machine In J. Rosevear & S. Harding. (Eds.), ASME XXth National Conference proceedings. Paper presented at: Music: Educating for life: ASME XXth National Conference (pp. 22–28), Parkville, Victoria: The Australian Society for Music Education Inc.
^Kamien, R. (1980). Music: an appreciation. New York: McGraw-Hill. p. 62
^abCariani, Peter; Micheyl, Christophe (2012). "Toward a Theory of Information Processing in Auditory Cortex".The Human Auditory Cortex. Springer Handbook of Auditory Research. Vol. 43. pp. 351–390.doi:10.1007/978-1-4614-2314-0_13.ISBN978-1-4614-2313-3.
^Levitin, D.J. (1999). Memory for musical attributes. In P.R. Cook (Ed.), Music, cognition, and computerized sound: An introduction to psychoacoustics (pp. 105–127). Cambridge, Massachusetts: The MIT press.
_-_filtered.jpg)
