View source for Sound

{{Short description|Vibration that travels via pressure waves in matter}}
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{{About|audible acoustic waves}}
[[File:Thoth08BigasDrumEvansChalmette.jpg|thumb|A [[drum]] produces sound via a vibrating [[membrane]].]]
{{Listen
| filename    = Drum - Cadence A.ogg
| title       = Drum - Cadence A
| description = [[Drum cadence]]s performed by the [[United States Navy Band]]
}}

In [[physics]], '''sound''' is a [[vibration]] that propagates as an [[acoustic wave]] through a [[transmission medium]] such as a gas, liquid or solid.
In human [[physiology]] and [[psychology]], sound is the ''reception'' of such waves and their ''perception'' by the [[brain]].<ref>{{cite book |publisher=Western Electrical Company |title=Fundamentals of Telephone Communication Systems |date=1969 |page=2.1}}</ref> Only acoustic waves that have [[frequency|frequencies]] lying between about 20&nbsp;Hz and 20&nbsp;kHz, the [[audio frequency]] range, elicit an auditory percept in humans. In air at atmospheric pressure, these represent sound waves with [[wavelength]]s of {{convert|17|meters|feet}} to {{convert|1.7|cm|inch|sp=us}}. Sound waves above 20&nbsp;[[kHz]] are known as [[ultrasound]] and are not audible to humans. Sound waves below 20&nbsp;Hz are known as [[infrasound]]. Different animal species have varying [[hearing range]]s, allowing some to even hear ultrasounds.

==Definition==
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)."<ref>[[ANSI/ASA S1.1-2013]]</ref> Sound can be viewed as a wave motion in  air or other elastic media. In this case, sound is a stimulus. Sound can also be viewed as an excitation of the hearing mechanism that results in the perception of sound. In this case, sound is a [[sense|sensation]].

==Acoustics==
{{main|Acoustics}}
Acoustics is the interdisciplinary science that deals with the study of [[mechanical wave]]s in gasses, liquids, and solids including [[vibration]], sound, ultrasound, and infrasound. A scientist who works in the field of [[acoustics]] is an ''acoustician'', while someone working in the field of [[acoustical engineering]] may be called an ''acoustical engineer''.<ref>ANSI S1.1-1994. American National Standard: Acoustic Terminology. Sec 3.03.</ref> An [[audio engineer]], on the other hand, is concerned with the recording, manipulation, mixing, and reproduction of sound.

Applications of acoustics are found in almost all aspects of modern society, subdisciplines include [[aeroacoustics]], [[audio signal processing]], [[architectural acoustics]], [[bioacoustics]], electro-acoustics, [[environmental noise]], [[musical acoustics]], [[noise control]], [[psychoacoustics]], [[speech]], [[ultrasound]],  [[underwater acoustics]], and [[vibration]].<ref>{{cite web |last=Acoustical Society of America |title=PACS 2010 Regular Edition—Acoustics Appendix |url=http://www.aip.org/pacs/pacs2010/individuals/pacs2010_regular_edition/reg_acoustics_appendix.htm |access-date=22 May 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130514111126/http://www.aip.org/pacs/pacs2010/individuals/pacs2010_regular_edition/reg_acoustics_appendix.htm |archive-date=14 May 2013 }}</ref>

==Physics==
[[File:23. Звучни вилушки.ogv|thumb|upright=1.2|Experiment using two [[tuning fork]]s [[acoustic oscillations|oscillating]] usually at the same [[frequency]]. One fork is hit with a rubberized mallet, causing the second fork to become visibly excited due to the oscillation caused by the periodic change in the pressure and density of the air. This is an [[acoustic resonance]]. When an additional piece of metal is attached to a prong, the effect becomes less pronounced as resonance is not achieved as effectively.]]
Sound can propagate through a medium such as air, water and solids as [[longitudinal wave]]s and also as a [[transverse wave]] in [[solids]]. The sound waves are generated by a sound source, such as the vibrating [[Diaphragm (acoustics)|diaphragm]] of a stereo speaker. The sound source creates vibrations in the surrounding medium. As the source continues to vibrate the medium, the vibrations propagate away from the source at the [[speed of sound]], thus forming the sound wave. At a fixed distance from the source, the [[pressure]], [[velocity]], and displacement of the medium vary in time. At an instant in time, the pressure, velocity, and displacement vary in space. The particles of the medium do not travel with the sound wave. This is intuitively obvious for a solid, and the same is true for liquids and gases (that is, the vibrations of particles in the gas or liquid transport the vibrations, while the ''average'' position of the particles over time does not change).  During propagation, waves can be [[Reflection (physics)|reflected]], [[Refraction|refracted]], or [[Attenuation|attenuated]] by the medium.<ref name="JHU">{{cite web |url=http://pages.jh.edu/~virtlab/ray/acoustic.htm |title=The Propagation of sound |access-date=26 June 2015 |url-status=live |archive-url=https://web.archive.org/web/20150430054640/http://pages.jh.edu/~virtlab/ray/acoustic.htm |archive-date=30 April 2015 }}</ref>

The behavior of sound propagation is generally affected by three things:
* A complex relationship between the [[density]] and pressure of the medium. This relationship, 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. Medium [[viscosity]] 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 does not have constant physical properties, it may be [[refracted]] (either dispersed or focused).<ref name="JHU" />

[[File:Spherical pressure waves.gif|thumb|Spherical compression (longitudinal) waves]]

The mechanical vibrations that can be interpreted as sound can travel through all [[State of matter|forms of matter]]: gases, liquids, solids, and [[plasma (physics)|plasmas]]. The matter that supports the sound is called the [[Transmission medium|medium]]. Sound cannot travel through a [[vacuum]].<ref>[http://www.qrg.northwestern.edu/projects/vss/docs/space-environment/1-is-there-sound-in-space.html Is there sound in space?] {{webarchive|url=https://web.archive.org/web/20171016010300/http://www.qrg.northwestern.edu/projects/vss/docs/space-environment/1-is-there-sound-in-space.html |date=2017-10-16 }} Northwestern University.</ref><ref>[http://curious.astro.cornell.edu/about-us/150-people-in-astronomy/space-exploration-and-astronauts/general-questions/918-can-you-hear-sounds-in-space-beginner Can you hear sounds in space? (Beginner)] {{webarchive|url=https://web.archive.org/web/20170618062312/http://curious.astro.cornell.edu/about-us/150-people-in-astronomy/space-exploration-and-astronauts/general-questions/918-can-you-hear-sounds-in-space-beginner |date=2017-06-18 }}. Cornell University.</ref>

Studies has shown that sound waves are able to carry a tiny amount of mass and is surrounded by a weak gravitational field.<ref>[https://phys.org/news/2019-03-evidence-mass.html Beyond cloning: Harnessing the power of virtual quantum broadcasting]</ref>

===Waves===
Sound is transmitted through gases, plasma, and liquids as [[longitudinal wave]]s, also called [[compression (physical)|compression]] waves. It requires a medium to propagate. Through solids, however, it can be transmitted as both longitudinal waves and [[transverse wave]]s. Longitudinal sound waves are waves of alternating [[pressure]] deviations from the [[Mechanical equilibrium|equilibrium]] pressure, causing local regions of [[compression (physical)|compression]] and [[rarefaction]], while [[transverse wave]]s (in solids) are waves of alternating [[shear stress]] at right angle to the direction of propagation.

Sound waves may be viewed using parabolic mirrors and objects that produce sound.<ref>{{cite web |title=What Does Sound Look Like? |url=https://www.youtube.com/watch?v=px3oVGXr4mo |work=NPR | date=9 April 2014 |publisher=YouTube |access-date=9 April 2014 |url-status=live |archive-url=https://web.archive.org/web/20140410064648/http://www.youtube.com/watch?v=px3oVGXr4mo |archive-date=10 April 2014 }}</ref>

The energy carried by an oscillating sound wave converts back and forth between the potential energy of the extra [[compression (physical)|compression]] (in case of longitudinal waves) or lateral displacement [[Strain (materials science)|strain]] (in case of transverse waves) of the matter, and the kinetic energy of the displacement velocity of particles of the medium.

{{multiple image
 | width = 250
 | image1 = Onde compression impulsion 1d 30 petit.gif
 | alt1 = Longitudinal plane pressure pulse wave
 | caption1 = Longitudinal plane wave
 | image2 = Onde cisaillement impulsion 1d 30 petit.gif
 | alt2 = Transverse plane wave in linear polarization, i.e. oscillating only in the y-direction
 | caption2 = Transverse plane wave
 | footer = Longitudinal and transverse plane wave
}}

[[File:The Elements of Sound jpg.jpg|thumb|A 'pressure over time' graph of a 20 ms recording of a clarinet tone demonstrates the two fundamental elements of sound: Pressure and Time.]]
[[File:Sine waves different frequencies.svg|thumb|Sounds can be represented as a mixture of their component [[Sine wave|Sinusoidal waves]] of different frequencies. The bottom waves have higher frequencies than those above. The horizontal axis represents time.]]
Although there are many complexities relating to the transmission of sounds, at the point of reception (i.e. the ears), sound is readily dividable into two simple elements: pressure and time. These fundamental elements form the basis of all sound waves. They can be used to describe, in absolute terms, every sound we hear.

In order to understand the sound more fully, a complex wave such as the one shown in a blue background on the right of this text, is usually separated into its component parts, which are a combination of various sound wave frequencies (and noise).<ref name="Handel, S. 1995">Handel, S. (1995). [https://books.google.com/books?id=OywDx9pxCMYC&dq=%22Timbre+perception+and+auditory+object+identification%22&pg=PA425 Timbre perception and auditory object identification] {{Webarchive|url=https://web.archive.org/web/20200110135929/https://books.google.com/books?hl=en&lr=&id=OywDx9pxCMYC&oi=fnd&pg=PA425&dq=%22Timbre+perception+and+auditory+object+identification%22&ots=P_6L53f1rX&sig=UF2k3GyEzCF1rOnDKHhgeA2MyJc |date=2020-01-10 }}. Hearing, 425–461.</ref><ref name="Kendall, R. A. 1986">Kendall, R.A. (1986). The role of acoustic signal partitions in listener categorization of musical phrases. Music Perception, 185–213.</ref><ref name="Matthews, M. 1999 pp. 79-88">Matthews, 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.</ref>

Sound [[wave]]s are often simplified to a description in terms of [[Sine wave|sinusoidal]] [[plane wave]]s, which are characterized by these generic properties:
* [[Frequency]], or its inverse, [[wavelength]]
* [[Amplitude]], [[sound pressure]] or [[Sound intensity|Intensity]]
* [[Speed of sound]]
* [[Direction (geometry, geography)|Direction]]

Sound that is perceptible by humans has frequencies from about 20&nbsp;Hz to 20,000&nbsp;Hz. In air at [[standard temperature and pressure]], the corresponding wavelengths of sound waves range from {{convert|17|m|ft|abbr=on|sigfig=2}} to {{convert|17|mm|in|abbr=on|sigfig=2}}.  Sometimes speed and direction are combined as a [[velocity]] [[Vector (geometric)|vector]]; wave number and direction are combined as a [[wave vector]].

[[Transverse wave]]s, also known as [[Shear stress|shear]] waves, have the additional property, ''[[Polarization (waves)|polarization]]'', which is not a characteristic of longitudinal sound waves.<ref>{{cite web|url=http://labman.phys.utk.edu/phys222core/modules/m6/polarization.html|title=Polarization|website=Elements of Physics II|publisher=The University of Tennessee, Department of Physics and Astronomy|last=Breinig|first=Marianne|access-date=4 March 2024}}</ref>

===Speed===
{{main|Speed of sound}}
[[File:FA-18 Hornet breaking sound barrier (7 July 1999) - filtered.jpg|right|thumb|U.S. Navy [[F/A-18 Hornet|F/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 (see [[Prandtl–Glauert singularity]]).<ref name=nasa>{{Cite APOD |title=A Sonic Boom|access-date=26 June 2015 |date=19 August 2007}}</ref>]]

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 by [[Isaac 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:

: <math>c = \sqrt{\frac{p}{\rho}}.</math>

This was later proven wrong and the French mathematician [[Pierre-Simon Laplace|Laplace]] corrected the formula by deducing that the phenomenon of sound travelling is not isothermal, as believed by Newton, but [[Adiabatic process|adiabatic]].  He added another factor to the equation—[[heat capacity ratio|''gamma'']]—and multiplied
<math>\sqrt{\gamma}</math>
by
<math>\sqrt{p/\rho}</math>,
thus coming up with the equation
<math>c = \sqrt{\gamma \cdot p/\rho}</math>.
Since
<math>K = \gamma \cdot p</math>,
the final equation came up to be
<math>c = \sqrt{K/\rho}</math>,
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 <math>\rho</math> is the density.  Thus, the speed of sound is proportional to the [[square root]] of the [[ratio]] of the [[bulk 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 {{convert|20|C|F}} air at sea level, the speed of sound is approximately {{convert|343|m/s|km/h mph|abbr=on|sigfig=3}} using the formula {{nobr|''v'' [m/s] {{=}} 331 + 0.6 ''T'' [°C]}}.  The speed of sound is also slightly sensitive, being subject to a second-order [[Anharmonicity|anharmonic]] 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 (see [[parametric array]]). If [[special relativity|relativistic]] effects are important, the speed of sound is calculated from the [[relativistic Euler equations]].

In fresh water the speed of sound is approximately {{convert|1482|m/s|km/h mph|abbr=on|sigfig=4}}. In steel, the speed of sound is about {{convert|5960|m/s|km/h mph|abbr=on|sigfig=4}}. Sound moves the fastest in solid atomic hydrogen at about {{convert|36000|m/s|km/h mph|abbr=on|sigfig=4}}.<ref>{{Cite web |url=https://phys.org/news/2020-10-scientists-upper-limit.html |title=Scientists find upper limit for the speed of sound |access-date=2020-10-09 |archive-date=2020-10-09 |archive-url=https://web.archive.org/web/20201009190631/https://phys.org/news/2020-10-scientists-upper-limit.html |url-status=live }}</ref><ref>{{Cite journal |title=Speed of sound from fundamental physical constants |year=2020 |doi=10.1126/sciadv.abc8662 |last1=Trachenko |first1=K. |last2=Monserrat |first2=B. |last3=Pickard |first3=C. J. |last4=Brazhkin |first4=V. V. |journal=Science Advances |volume=6 |issue=41 |pages=eabc8662 |pmid=33036979 |pmc=7546695 |arxiv=2004.04818 |bibcode=2020SciA....6.8662T }}</ref>

===Sound pressure level===
{{Sound measurements}}
''[[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 a [[root mean square]] (RMS) value. For example, 1 [[Pascal (unit)|Pa]] RMS sound pressure (94 dBSPL) in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm <math>-\sqrt{2}</math> Pa) and (1 atm <math>+\sqrt{2}</math> 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 logarithmic [[decibel]] scale. The ''sound pressure level'' (SPL) or ''L''<sub>p</sub> is defined as
:<math>
L_\mathrm{p}=10\, \log_{10}\left(\frac{{p}^2}{{p_\mathrm{ref}}^2}\right) =20\, \log_{10}\left(\frac{p}{p_\mathrm{ref}}\right)\mbox{ dB}\,
</math>

:where ''p'' is the [[root-mean-square]] sound pressure and <math>p_\mathrm{ref}</math> is a ''[[reference sound pressure]]''. Commonly used reference sound pressures, defined in the standard [[American National Standards Institute|ANSI]] [[ANSI S1.1-1994|S1.1-1994]], are 20 [[micropascal|μPa]] in air and 1 [[micropascal|μPa]] 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 flat [[Responsivity|spectral response]], sound pressures are often [[frequency]] weighted so that the measured level matches perceived levels more closely. The [[International 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.

==Perception==
{{main|Psychoacoustics}}
A distinct use of the term ''sound'' from its use in physics is that in physiology and psychology, where the term refers to the subject of ''perception'' by the brain. The field of [[psychoacoustics]] 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)."<ref>{{cite book |last1=Webster |first1=Noah |title=Sound. In Webster's Collegiate Dictionary |date=1936 |publisher=The Riverside Press |location=Cambridge, Mass. |pages=950–951 |edition= Fifth}}</ref> 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 frequencies between approximately 20&nbsp;[[Hertz|Hz]] and 20,000&nbsp;Hz (20&nbsp;[[kHz]]),{{r|Olson1967|page1=382}} The upper limit decreases with age.<ref name="Olson1967">{{cite book |url=https://archive.org/details/musicphysicsengi0000olso |url-access=registration |title=Music, Physics and Engineering |publisher=Dover Publications |isbn=9780486217697 |last1=Olson |first1=Harry F. Autor |date=1967 |page=[https://archive.org/details/musicphysicsengi0000olso/page/249 249]}}</ref>{{rp|249}} Sometimes ''sound'' refers to only those vibrations with [[frequencies]] that are within the [[hearing range]] for humans<ref>{{cite web|title=The American Heritage Dictionary of the English Language |edition= Fourth |publisher=Houghton Mifflin Company |date=2000 |url=http://www.bartleby.com/61/65/S0576500.html |archive-url=https://web.archive.org/web/20080625012016/http://www.bartleby.com/61/65/S0576500.html |archive-date=June 25, 2008 |access-date=May 20, 2010}}</ref> or sometimes it relates to a particular animal. Other species have different ranges of hearing. For example, dogs can perceive vibrations higher than 20&nbsp;kHz.

As a signal perceived by one of the major [[sense]]s, sound is used by many species for [[defence mechanism (biology)|detecting danger]], [[navigation]], [[predation]], and communication. Earth's [[atmosphere]], [[hydrosphere|water]], and virtually any [[physical phenomenon]], such as fire, rain, wind, [[ocean surface wave|surf]], or earthquake, produces (and is characterized by) its unique sounds. Many species, such as frogs, birds, [[Marine mammals|marine]] and terrestrial [[mammal]]s, have also developed special [[Organ (anatomy)|organs]] to produce sound. In some species, these produce [[Bird vocalization|song]] and [[Speech communication|speech]]. 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.

There are, historically, six experimentally separable ways in which sound waves are analysed. They are:  [[Pitch (music)|pitch]], [[Duration (music)|duration]], [[loudness]], [[timbre]], [[Texture (music)|sonic texture]] and  [[Sound localization|spatial location]].<ref>Burton, R.L. (2015). [https://search.informit.com.au/documentSummary;dn=649996699786780;res=IELHSS The elements of music: what are they, and who cares?] {{Webarchive|url=https://web.archive.org/web/20200510200056/https://search.informit.com.au/documentSummary;dn=649996699786780;res=IELHSS |date=2020-05-10 }} 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.</ref> Some of these terms have a standardised definition (for instance in the ANSI Acoustical Terminology [[ANSI/ASA S1.1-2013]]). More recent approaches have also considered [[Temporal envelope and fine structure|temporal envelope and temporal fine structure]] as perceptually relevant analyses.<ref>{{Citation|last1=Viemeister|first1=Neal F.|title=Time Analysis|date=1993|work=Springer Handbook of Auditory Research|pages=116–154|publisher=Springer New York|language=en|doi=10.1007/978-1-4612-2728-1_4|isbn=9781461276449|last2=Plack|first2=Christopher J.}}</ref><ref>{{Cite journal|last=Rosen|first=Stuart|date=1992-06-29|title=Temporal information in speech: acoustic, auditory and linguistic aspects|journal=Phil. Trans. R. Soc. Lond. B|language=en|volume=336|issue=1278|pages=367–373|doi=10.1098/rstb.1992.0070|issn=0962-8436|pmid=1354376|bibcode=1992RSPTB.336..367R}}</ref><ref>{{Cite journal|last=Moore|first=Brian C.J.|date=2008-10-15|title=The Role of Temporal Fine Structure Processing in Pitch Perception, Masking, and Speech Perception for Normal-Hearing and Hearing-Impaired People|journal=Journal of the Association for Research in Otolaryngology|language=en|volume=9|issue=4|pages=399–406|doi=10.1007/s10162-008-0143-x|issn=1525-3961|pmc=2580810|pmid=18855069}}</ref>

===Pitch===
[[File:Pitch perception.png|thumb|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 (music)|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.<ref>De Cheveigne, A. (2005). Pitch perception models. Pitch, 169-233.</ref><ref>{{cite journal |last1=Krumbholz |first1=K. |last2=Patterson |first2=R. |last3=Seither-Preisler |first3=A. |last4=Lammertmann |first4=C. |last5=Lütkenhöner |first5=B. |year=2003 |title=Neuromagnetic evidence for a pitch processing center in Heschl's gyrus |journal=Cerebral Cortex |volume=13 |issue=7 |pages=765–772 |doi=10.1093/cercor/13.7.765|pmid=12816892 |doi-access=free }}</ref>  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 than [[pink noise]] (random noise spread evenly across octaves) as white noise has more high frequency content.

===Duration===
[[File:Duration perception.png|thumb|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 (music)|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.<ref>{{cite journal |last1=Jones |first1=S. |last2=Longe |first2=O. |last3=Pato |first3=M.V. |year=1998 |title=Auditory evoked potentials to abrupt pitch and timbre change of complex tones: electrophysiological evidence of streaming? |journal=Electroencephalography and Clinical Neurophysiology |volume=108 |issue=2 |pages=131–142 |doi=10.1016/s0168-5597(97)00077-4|pmid=9566626 }}</ref> 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.<ref>{{cite journal |last1=Nishihara |first1=M. |last2=Inui |first2=K. |last3=Morita |first3=T. |last4=Kodaira |first4=M. |last5=Mochizuki |first5=H. |last6=Otsuru |first6=N. |last7=Kakigi |first7=R. |year=2014 |title=Echoic memory: Investigation of its temporal resolution by auditory offset cortical responses |journal=PLOS ONE |volume=9 |issue=8 |page=e106553 |doi=10.1371/journal.pone.0106553 |pmid=25170608 |pmc=4149571 |bibcode=2014PLoSO...9j6553N|doi-access=free }}</ref> 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===
[[File:Loudness perception v5.gif|thumb|Loudness information is summed over a period of about 200 ms before being sent to the auditory cortex. Louder signals create a greater 'push' on the Basilar membrane and thus stimulate more nerves, creating a stronger loudness signal. A more complex signal also creates more nerve firings and so sounds louder (for the same wave amplitude) than a simpler sound, such as a sine wave.]]
[[Loudness]] is perceived as how "loud" or "soft" a sound is and relates to the totalled number of auditory nerve stimulations over short cyclic time periods, most likely over the duration of theta wave cycles.<ref>{{citation |author=Corwin, J. |date=2009 |title=The auditory system |url=http://www.medicine.virginia.edu/basic-science/departments/neurosci/research/corwin-lab/educationresearch/handouts/Auditory-System-handout-revised-for-2009-by-JTC.pdf |access-date=2013-04-06 |url-status=live |archive-url=https://web.archive.org/web/20130628061206/http://www.medicine.virginia.edu/basic-science/departments/neurosci/research/corwin-lab/educationresearch/handouts/Auditory-System-handout-revised-for-2009-by-JTC.pdf |archive-date=2013-06-28 }}</ref><ref>{{cite journal |last1=Massaro |first1=D.W. |year=1972 |title=Preperceptual images, processing time, and perceptual units in auditory perception |journal=Psychological Review |volume=79 |issue=2 |pages=124–145 |doi=10.1037/h0032264|pmid=5024158 |citeseerx=10.1.1.468.6614 }}</ref><ref>{{cite journal |last1=Zwislocki |first1=J.J. |year=1969 |title=Temporal summation of loudness: an analysis |journal=The Journal of the Acoustical Society of America |volume=46 |issue=2B |pages=431–441 |doi=10.1121/1.1911708 |pmid=5804115 |bibcode=1969ASAJ...46..431Z}}</ref> This means that at short durations, a very short sound can sound softer than a longer sound even though they are presented at the same intensity level. Past around 200 ms this is no longer the case and the duration of the sound no longer affects the apparent loudness of the sound.

===Timbre===
[[File:Timbre perception.png|thumb|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.<ref name="Handel, S. 1995"/><ref name="Kendall, R. A. 1986"/><ref name="Matthews, M. 1999 pp. 79-88"/> 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===
[[texture (music)|Sonic texture]] relates to the number of sound sources and the interaction between them.<ref>{{citation |author1=Cohen, D. |author2=Dubnov, S. |date=1997 |title=Gestalt phenomena in musical texture |journal=Journal of New Music Research |volume=26 |issue=4 |pages=277–314 |url=https://www.researchgate.net/publication/233076257 |access-date=2015-11-19 |url-status=live |archive-url=https://web.archive.org/web/20151121084708/http://www.researchgate.net/profile/Shlomo_Dubnov/publication/233076257_Polyspectra_as_measures_of_sound_texture_and_timbre*/links/0f31752f6ce3c49334000000.pdf |archive-date=2015-11-21 |doi=10.1080/09298219708570732 }}</ref><ref>Kamien, R. (1980). Music: an appreciation. New York: McGraw-Hill. p. 62</ref> The word ''texture'', in this context, relates to the cognitive separation of auditory objects.<ref name="Cariani, P. 2012 pp. 351-390">{{Cite book | doi=10.1007/978-1-4614-2314-0_13| chapter=Toward a Theory of Information Processing in Auditory Cortex| title=The Human Auditory Cortex| volume=43| pages=351–390| series=Springer Handbook of Auditory Research| year=2012| last1=Cariani| first1=Peter| last2=Micheyl| first2=Christophe| isbn=978-1-4614-2313-3}}</ref> In music, texture is often referred to as the difference between [[unison]], [[polyphony]] and [[homophony]], but it can also relate (for example) to a busy cafe; a sound which might be referred to as ''[[cacophony]]''.

===Spatial location===
{{Main|Sound localization}}
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.<ref name="Cariani, P. 2012 pp. 351-390"/><ref>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.</ref> In a thick texture, it is possible to identify multiple sound sources using a combination of spatial location and timbre identification.

==Frequency==
{{see also|Audio frequency}}

===Ultrasound===
[[File:Ultrasound range diagram.svg|thumb|upright=1.3|Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications]]
[[Ultrasound]] is sound waves with frequencies higher than 20,000&nbsp;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&nbsp;kHz up to several gigahertz.

[[Medical ultrasound]] is commonly used for diagnostics and treatment.

=== Infrasound ===
{{see also|Perception of infrasound}}
[[Infrasound]] is sound waves with frequencies lower than 20&nbsp;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.<ref>{{Cite journal|last=Leventhall|first=Geoff|date=2007-01-01|title=What is infrasound?|journal=Progress in Biophysics and Molecular Biology|series=Effects of ultrasound and infrasound relevant to human health|volume=93|issue=1|pages=130–137|doi=10.1016/j.pbiomolbio.2006.07.006|pmid=16934315|issn=0079-6107|doi-access=}}</ref>

==See also==
{{div col|colwidth=20em}}
;Sound sources
* [[Earphones]]
* [[Musical instrument]]
* [[Sonar]]
* [[Sound box]]
* [[Sound reproduction]]
;Sound measurement
* [[Acoustic impedance]]
* [[Acoustic velocity]]
* [[Characteristic impedance]]
* [[Mel scale]]
* [[Particle acceleration]]
* [[Particle amplitude]]
* [[Particle displacement]]
* [[Particle velocity]]
* [[Sound energy flux]]
* [[Sound impedance]]
* [[Sound intensity level]]
* [[Sound power]]
* [[Sound power level]]

;<span class="anchor" id="Units"></span>Units:
*[[Decibel|dB]], decibel
*[[sone]] - perceived [[loudness]]
*[[phon]] - subjective loudness
*[[hertz|Hz]] - unit of frequency

;General:
* [[Acoustic theory]]
* [[Beat (acoustics)|Beat]]
* [[Doppler effect]]
* [[Echo (phenomenon)|Echo]]
* [[Infrasound]] — sound at extremely low frequencies
* [[List of unexplained sounds]]
* [[Musical tone]]
* [[Resonance]]
* [[Reverberation]]
* [[Sonic weaponry]]
* [[Sound synthesis]]
* [[Soundproofing]]
* [[Structural acoustics]]
{{div col end}}

==References==
{{Reflist}}

==External links==
{{wikiquote}}
{{wikibooks}}
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{{Wikisource}}
{{Library resources box
|by=no
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|label=Sound }}
*{{cite web
  | author  =  Eric Mack
  | title = Stanford scientists created a sound so loud it instantly boils water
  | website = CNET
  | date = 20 May 2019
  | url = https://www.cnet.com/news/stanford-scientists-created-a-sound-so-loud-it-instantly-boils-water/}}
* [http://www.acoustics.salford.ac.uk/schools/index1.htm Sounds Amazing; a KS3/4 learning resource for sound and waves (uses Flash)] {{Webarchive|url=https://web.archive.org/web/20120313122023/http://www.acoustics.salford.ac.uk/schools/index1.htm |date=2012-03-13 }}
* [http://hyperphysics.phy-astr.gsu.edu/hbase/sound/soucon.html HyperPhysics: Sound and Hearing]
* [http://podcomplex.com/guide/physics.html Introduction to the Physics of Sound]
* [http://www.phys.unsw.edu.au/~jw/hearing.html Hearing curves and on-line hearing test]
* [http://www.audiodesignline.com/howto/audioprocessing/193303241 Audio for the 21st Century] {{Webarchive|url=https://web.archive.org/web/20090123120804/http://www.audiodesignline.com/howto/audioprocessing/193303241 |date=2009-01-23 }}
* [http://www.sengpielaudio.com/calculator-soundlevel.htm Conversion of sound units and levels]
* [http://www.sengpielaudio.com/Calculations03.htm Sound calculations]
* [http://www.audiocheck.net Audio Check: a free collection of audio tests and test tones playable on-line]
* [http://salfordacoustics.co.uk/sound-waves More Sounds Amazing; a sixth-form learning resource about sound waves]

{{Acoustics}}

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[[Category:Hearing]]
[[Category:Waves]]
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