TheInfoList

In
physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succession of eve ...

, sound is a
vibration Vibration is a mechanical phenomenon whereby oscillation Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. The term '' vib ...

that propagates as an
acoustic wave Acoustic waves are a type of energy propagation through a medium by means of adiabatic compression and decompression. Important quantities for describing acoustic waves are acoustic pressure, particle velocity, particle displacement and acoustic ...
, through a
transmission medium A transmission medium is a system or substance that can mediate the propagation of signal In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such ...
such as a gas, liquid or solid. In human
physiology Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. ...
and
psychology Psychology is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. ...

, sound is the ''reception'' of such waves and their ''perception'' by the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tiss ...

. Only acoustic waves that have
frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparent ...

lying between about 20 Hz and 20 kHz, the
audio frequency An audio frequency or audible frequency (AF) is a periodic vibration Vibration is a mechanical phenomenon whereby oscillation Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point o ...
range, elicit an auditory percept in humans. In air at atmospheric pressure, these represent sound waves with
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adja ...

s of to . Sound waves above 20
kHz The hertz (symbol: Hz) is the unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatric ...
are known as
ultrasound Ultrasound is s with higher than the upper audible limit of human . Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies from person to person and is appro ...

and are not audible to humans. Sound waves below 20 Hz are known as
infrasound Infrasound, sometimes referred to as low status sound , describes sound waves with a frequency below the lower limit of human audibility (generally 20 Hz). Hearing becomes gradually less sensitive as frequency decreases, so for humans to per ...
. Different animal species have varying
hearing range Hearing range describes the range of frequencies that can be Hearing (sense), heard by humans or other animals, though it can also refer to the Sound pressure, range of levels. The human range is commonly given as 20 to 20,000 hertz, Hz, a ...
s.

Acoustics

Acoustics is the interdisciplinary science that deals with the study of
mechanical wave A mechanical wave is a wave In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics) ...
s in gases, liquids, and solids including
vibration Vibration is a mechanical phenomenon whereby oscillation Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. The term '' vib ...

, sound, ultrasound, and infrasound. A scientist who works in the field of
acoustics Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician w ...
is an ''acoustician'', while someone working in the field of
acoustical engineering Acoustical engineering (also known as acoustic engineering) is the branch of engineering dealing with sound and oscillation, vibration. It includes the application of acoustics, the science of sound and vibration, in technology. Acoustical engineer ...
may be called an ''acoustical engineer''. An
audio engineer An audio engineer (also known as a sound engineer or recording engineer) helps to produce a sound recording, recording or a live performance, balancing and adjusting sound sources using equalization (audio), equalization, Dynamic range compressi ...
, 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 Aeroacoustics is a branch of acoustics that studies noise generation via either turbulent fluid motion or aerodynamic forces interacting with surfaces. Noise generation can also be associated with periodically varying flows. A notable example of th ...
,
audio signal processing Audio signal processing is a subfield of that is concerned with the electronic manipulation of s. Audio signals are electronic representations of s—s which travel through air, consisting of compressions and rarefactions. The energy contained ...
,
architectural acoustics Architectural acoustics (also known as building acoustics) is the science and engineering of achieving a good sound within a building and is a branch of acoustical engineering Acoustical engineering (also known as acoustic engineering) is the br ...
,
bioacoustics Bioacoustics is a cross-disciplinary that combines and . Usually it refers to the investigation of production, dispersion and reception in s (including ). This involves and basis of sound production and detection, and relation of acoustic to ...
, electro-acoustics,
environmental noise Environmental noise is an accumulation of noise pollution that occurs outside. This noise can be caused by transport, industrial, and Sport, recreational activities. Noise is frequently described as 'unwanted sound'. Within this context, environ ...
,
musical acousticsMusical acoustics or music acoustics is a multidisciplinary field that combines knowledge from physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and beh ...
,
noise control Noise control or noise mitigation is a set of strategies to reduce noise pollution Noise pollution, also known as environmental noise Environmental noise is an accumulation of noise pollution that occurs outside. This noise can be caused b ...
,
psychoacoustics Psychoacoustics is the branch of psychophysics Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they produce. Psychophysics has been described as "the scientific study of the ...
,
speech Speech is human vocal communication Communication (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, ...

,
ultrasound Ultrasound is s with higher than the upper audible limit of human . Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies from person to person and is appro ...

,
underwater acoustics Image:SCM4 fig11.jpg, 300px, Output of a computer model of underwater acoustic propagation in a simplified ocean environment.Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that cons ...
, and
vibration Vibration is a mechanical phenomenon whereby oscillation Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. The term '' vib ...

.

Definition

Sound is defined as "(a)
Oscillation Oscillation is the repetitive variation, typically in time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, irreversible succession from the pas ...

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)." 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
sensationSensation refers to the processing of sense Sense relates to any of the systems and corresponding organs involved in sensation, i.e. the physical process of responding to Stimulus (physiology), stimuli and providing data for perception. During sensa ...

.

Physics

Sound can propagate through a medium such as air, water and solids as
longitudinal wave Longitudinal waves are waves in which the vibration of the medium is parallel to the direction the wave travels and displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical wave, Mechanical longitudinal ...

s and also as a in
solids Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) re ...
. The sound waves are generated by a sound source, such as the vibrating
diaphragm Diaphragm may refer to: * Diaphragm (anatomy) or thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Diaphragm (optics), a stop in the light path of a lens, having an aperture that regulates the amount of light that pass ...
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 The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elasticity (solid mechanics), elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends s ...
, thus forming the sound wave. At a fixed distance from the source, the
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

,
velocity The velocity of an object is the Time derivative, rate of change of its Position (vector), position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed and direction ...

, and displacement of the medium vary in time. At an instant in time, the pressure, velocity, and displacement vary in space. Note that 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 reflected,
refracted In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior throu ...

, or by the medium. The behavior of sound propagation is generally affected by three things: * A complex relationship between the
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

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 The viscosity of a is a measure of its to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, has a higher viscosity than . Viscosity can be conceptualized as quantifying the inter ...

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 In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior throu ...
(either dispersed or focused). The mechanical vibrations that can be interpreted as sound can travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the
medium Medium may refer to: Science and technology Aviation *Medium bomber, a class of war plane *Tecma Medium, a French hang glider design Communication * Media (communication), tools used to store and deliver information or data * Medium of i ...
. Sound cannot travel through a
vacuum A vacuum is a space Space is the boundless three-dimensional Three-dimensional space (also: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Gree ...

.

Waves

Sound is transmitted through gases, plasma, and liquids as
longitudinal wave Longitudinal waves are waves in which the vibration of the medium is parallel to the direction the wave travels and displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical wave, Mechanical longitudinal ...

s, also called
compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression *Gas compression *Compression ratio, of a co ...
waves. It requires a medium to propagate. Through solids, however, it can be transmitted as both longitudinal waves and s. Longitudinal sound waves are waves of alternating
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

deviations from the
equilibrium List of types of equilibrium, the condition of a system in which all competing influences are balanced, in a wide variety of contexts. Equilibrium may also refer to: Film and television * Equilibrium (film), ''Equilibrium'' (film), a 2002 scien ...
pressure, causing local regions of
compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression *Gas compression *Compression ratio, of a co ...
and
rarefaction 305px, An example of rarefaction is also as a phase in a sound wave or phonon. Half of a sound wave is made up of the compression of the Transmission medium, medium, and the other half is the decompression or ''rarefaction'' of the medium. Ra ...
, while s (in solids) are waves of alternating
shear stress Shear stress, often denoted by (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approx ...

at right angle to the direction of propagation. Sound waves may be viewed using parabolic mirrors and objects that produce sound. The energy carried by an oscillating sound wave converts back and forth between the potential energy of the extra
compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression *Gas compression *Compression ratio, of a co ...
(in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter, and the kinetic energy of the displacement velocity of particles of the medium. 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).Handel, S. (1995)
Timbre perception and auditory object identification
. Hearing, 425–461.
Kendall, R.A. (1986). The role of acoustic signal partitions in listener categorization of musical phrases. Music Perception, 185–213.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. Sound
wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

s are often simplified to a description in terms of
sinusoidal A sine wave or sinusoid is any of certain mathematical curves that describe a smooth periodic oscillation Oscillation is the repetitive variation, typically in time Time is the indefinite continued sequence, progress of existence and ev ...

plane wave In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Ph ...

s, which are characterized by these generic properties: *
Frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparen ...

, or its inverse,
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase (waves), phase on the wave, such as two adja ...

*
Amplitude The amplitude of a ic is a measure of its change in a single (such as or ). There are various definitions of amplitude (see below), which are all s of the magnitude of the differences between the variable's . In older texts, the of a period f ...

,
sound pressure Sound pressure or acoustic pressure is the local pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from '' ...
or Intensity *
Speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elasticity (solid mechanics), elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends s ...
*
Direction Direction may refer to: *Relative direction, for instance left, right, forward, backwards, up, and down ** Anatomical terms of location for those used in anatomy *Cardinal direction Mathematics and science *Direction vector, a unit vector that ...
Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. In air at
standard temperature and pressure Standard temperature and pressure (STP) are standard Standard may refer to: Flags * Colours, standards and guidons * Standard (flag), a type of flag used for personal identification Norm, convention or requirement * Standard (metrology), a ...
, the corresponding wavelengths of sound waves range from to . Sometimes speed and direction are combined as a
velocity The velocity of an object is the Time derivative, rate of change of its Position (vector), position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed and direction ...

vector Vector may refer to: Biology *Vector (epidemiology) In epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and risk factor, determinants of health and disease conditions in defined pop ...
; wave number and direction are combined as a
wave vector In , a wave vector (also spelled wavevector) is a which helps describe a . Like any vector, it has a , both of which are important. Its magnitude is either the or of the wave (inversely proportional to the ), and its direction is ordinarily the ...
. s, also known as
shear Shear may refer to: Textile production * Animal shearing, the collection of wool from various species **Sheep shearing *The removal of Nap (textile), nap during wool cloth production Science and technology Engineering *Shear strength (soil), the ...

waves, have the additional property, ''
polarization Polarization or polarisation may refer to: In the physical sciences *Polarization (waves), the ability of waves to oscillate in more than one direction, in particular polarization of light, responsible for example for the glare-reducing effect of ...
'', and are not a characteristic of sound waves.

Speed

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 Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics a ...

. 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: : $c = \sqrt.$ This was later proven wrong and the French mathematician
Laplace Pierre-Simon, marquis de Laplace (; ; 23 March 1749 – 5 March 1827) was a French scholar and polymath A polymath ( el, πολυμαθής, ', "having learned much"; Latin Latin (, or , ) is a classical language belonging to the I ...

corrected the formula by deducing that the phenomenon of sound travelling is not isothermal, as believed by Newton, but . He added another factor to the equation— ''gamma''—and multiplied $\sqrt$ by $\sqrt$, thus coming up with the equation $c = \sqrt$. Since $K = \gamma \cdot p$, the final equation came up to be $c = \sqrt$, 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 $\rho$ is the density. Thus, the speed of sound is proportional to the
square root In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis) ...

of the
ratio In mathematics, a ratio indicates how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8∶6, which is equivalent to ...

of the
bulk modulus The bulk modulus (K or B) of a substance is a measure of how resistant to compression that substance is. It is defined as the ratio of the infinitesimal In mathematics, infinitesimals or infinitesimal numbers are quantities that are closer to ze ...
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 air at sea level, the speed of sound is approximately using the formula . The speed of sound is also slightly sensitive, being subject to a second-order 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 A parametric array, in the field of acoustics Acoustics is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science th ...
). If relativistic effects are important, the speed of sound is calculated from the
relativistic Euler equations In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, more specifically the relationships among forc ...
. In fresh water the speed of sound is approximately . In steel, the speed of sound is about . Sound moves the fastest in solid atomic hydrogen at about .

Sound Pressure Level

Sound pressure Sound pressure or acoustic pressure is the local pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from '' ...
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 In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
(RMS) value. For example, 1 Pa RMS sound pressure (94 dBSPL) in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm $-\sqrt$ Pa) and (1 atm $+\sqrt$ 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 The decibel (symbol: dB) is a relative equal to one tenth of a bel (B). It expresses the ratio of two values of a on a . Two signals whose differ by one decibel have a power ratio of 101/10 (approximately ) or root-power ratio of 10 (approxim ...

scale. The sound pressure level (SPL) or ''L''p is defined as :$L_\mathrm=10\, \log_\left\left(\frac\right\right) =20\, \log_\left\left(\frac\right\right)\mbox\,$ :where ''p'' is the
root-mean-square In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
sound pressure and $p_\mathrm$ is a reference sound pressure. Commonly used reference sound pressures, defined in the standard
ANSI The American National Standards Institute (ANSI ) is a private non-profit organization A nonprofit organization (NPO), also known as a non-business entity, not-for-profit organization, or nonprofit institution, is a legal entity organized ...
S1.1-1994, are 20 µPa in air and 1 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 spectral response, sound pressures are often
frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparen ...

weighted so that the measured level matches perceived levels more closely. The
International Electrotechnical Commission The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization A standards organization, standards body, standards developing organization (SDO), or s ...
(IEC) has defined several weighting schemes.
A-weighting A-weighting is the most commonly used of a family of curves defined in the International standard IECIEC may refer to: Businesses and organisations * International Electrotechnical Commission The International Electrotechnical Commission ...
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

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 Psychoacoustics is the branch of psychophysics Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they produce. Psychophysics has been described as "the scientific study of the ...
is dedicated to such studies. Webster's 1936 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)." This means that the correct response to the question: " if a tree falls in the forest with no one to hear it fall, 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  and 20,000 Hz (20
kHz The hertz (symbol: Hz) is the unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatric ...
), The upper limit decreases with age. Sometimes ''sound'' refers to only those vibrations with
frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparent ...

that are within the
hearing range Hearing range describes the range of frequencies that can be Hearing (sense), heard by humans or other animals, though it can also refer to the Sound pressure, range of levels. The human range is commonly given as 20 to 20,000 hertz, Hz, a ...
for humans 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 major
sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world and responding to Stimulus (physiology), stimuli. (For example, in the human body, the brain receives signals from the senses ...

s, sound is used by many species for detecting danger,
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...

,
predation Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical en ...

, and communication. Earth's
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

,
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...
, and virtually any
physical phenomenon A phenomenon (; plural phenomena) is an observable In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies mat ...
, such as fire, rain, wind, , or earthquake, produces (and is characterized by) its unique sounds. Many species, such as frogs, birds,
marine Marine is an adjective meaning of or pertaining to the sea or ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
and terrestrial
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s, have also developed special
organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is the bulk of functional ...
to produce sound. In some species, these produce
song A song is a musical composition Musical composition can refer to an piece or work of , either or , the of a musical piece or to the process of creating or writing a new piece of music. People who create new compositions are called s ...
and
speech Speech is human vocal communication Communication (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''c ...
. Furthermore, humans have developed culture and technology (such as music, telephone and radio) that allows them to generate, record, transmit, and broadcast sound.
Noise Noise is unwanted sound 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 the ...

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 above).
Soundscape A soundscape is the acoustic environment as perceived by humans, in context. The term was originally coined by Michael Southworth, and popularised by R. Murray Schafer. There is a varied history of the use of soundscape depending on discipline, ra ...
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 Pitch may refer to: Acoustic frequency * Pitch (music), the perceived frequency of sound including "definite pitch" and "indefinite pitch" ** Absolute pitch or "perfect pitch" ** Pitch class, a set of all pitches that are a whole number of octaves ...
,
duration Duration may refer to: * The amount of Time#Terminology, time elapsed between two events * Duration (music) – an amount of time or a particular time interval, often cited as one of the fundamental aspects of music * Duration (philosophy) – a th ...
,
loudness In acoustics Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is ...
,
timbre In music, timbre ( ), also known as tone color or tone quality (from ), is the perceived sound quality of a , sound or . Timbre distinguishes different types of sound production, such as choir voices and musical instruments. It also enables li ...

, sonic texture and
spatial location In geography, location or place are used to denote a region (geometry), regions (point, line, or area) on the earth's surface or elsewhere. The term ''location'' generally implies a higher degree of certainty than ''place'', the latter often indica ...
. 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 temporal fine structure as perceptually relevant analyses.

Pitch

Pitch Pitch may refer to: Acoustic frequency * Pitch (music), the perceived frequency of sound including "definite pitch" and "indefinite pitch" ** Absolute pitch or "perfect pitch" ** Pitch class, a set of all pitches that are a whole number of octaves ...
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. Every sound is placed on a pitch continuum from low to high. For example:
white noise In signal processing Signal processing is an electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, elect ...

(random noise spread evenly across all frequencies) sounds higher in pitch than
pink noise Pink noise or noise is a signal (information theory), signal or process with a frequency spectrum such that the power spectral density (power per frequency interval) is inversely proportional to the frequency of the signal. In pink noise, each ...
(random noise spread evenly across octaves) as white noise has more high frequency content. Figure 1 shows an example of pitch recognition. During the listening process, each sound is analysed for a repeating pattern (See Figure 1: orange arrows) and the results forwarded to the auditory cortex as a single pitch of a certain height (octave) and chroma (note name).

Duration

Duration Duration may refer to: * The amount of Time#Terminology, time elapsed between two events * Duration (music) – an amount of time or a particular time interval, often cited as one of the fundamental aspects of music * Duration (philosophy) – a th ...
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. 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. 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. Figure 2 gives an example of duration identification. When a new sound is noticed (see Figure 2, Green arrows), a sound onset message is sent to the auditory cortex. When the repeating pattern is missed, a sound offset messages is sent.

Loudness

Loudness In acoustics Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is ...
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. 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. Figure 3 gives an impression of how 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.

Timbre

Timbre In music, timbre ( ), also known as tone color or tone quality (from ), is the perceived sound quality of a , sound or . Timbre distinguishes different types of sound production, such as choir voices and musical instruments. It also enables li ...

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. The way a sound changes over time (see figure 4) provides most of the information for timbre identification. Even though a small section of the wave form from each instrument looks very similar (see the expanded sections indicated by the orange arrows in figure 4), 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. The word ''texture'', in this context, relates to the cognitive separation of auditory objects. In music, texture is often referred to as the difference between
unison Unison, stylised as UNISON, is the second largest trade union A trade union (or a labor union in American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of v ...
,
polyphony Polyphony is a type of musical consisting of two or more simultaneous lines of independent melody, as opposed to a musical texture with just one voice, , or a texture with one dominant melodic voice accompanied by , . Within the context of the ...
and
homophony In music, homophony (;, Greek language, Greek: ὁμόφωνος, ''homóphōnos'', from ὁμός, ''homós'', "same" and φωνή, ''phōnē'', "sound, tone") is a Texture (music), texture in which a primary Part (music), part is supported by o ...
, but it can also relate (for example) to a busy cafe; a sound which might be referred to as ''
cacophony Phonaesthetics (also spelled phonesthetics in North America North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere. It can also be described as the northern subcontinent of the A ...
''.

Spatial location

Spatial location (see:
Sound localization Sound localization is a listener's ability to identify the location or origin of a detected sound in direction and distance. The sound localization mechanisms of the mammalian auditory system have been extensively studied. The auditory system us ...
) 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. In a thick texture, it is possible to identify multiple sound sources using a combination of spatial location and timbre identification. This is the main reason why we can pick the sound of an oboe in an orchestra and the words of a single person at a cocktail party.

Ultrasound

Ultrasound Ultrasound is s with higher than the upper audible limit of human . Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies from person to person and is appro ...

is sound waves with frequencies higher than 20,000 Hz. Ultrasound is not different from audible sound in its physical properties it just cannot be heard by humans. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz.
Medical ultrasound Medical ultrasound includes diagnostic Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different academic discipline, disciplines, with variations in the use of logic, analytics, and ...

is commonly used for diagnostics and treatment.

Infrasound

Infrasound Infrasound, sometimes referred to as low status sound , describes sound waves with a frequency below the lower limit of human audibility (generally 20 Hz). Hearing becomes gradually less sensitive as frequency decreases, so for humans to per ...
is sound waves with frequencies lower than 20 Hz. Although sounds of such low frequency are too low for humans to hear, 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.

;Sound sources *
Earphones Headphones are a pair of small worn on or around the head over a user's ears. They are s, which convert an to a corresponding . Headphones let a single user listen to an audio source privately, in contrast to a , which emits sound into th ...

*
Musical instrument A musical instrument is a device created or adapted to make musical sounds. In principle, any object that produces sound can be considered a musical instrument—it is through purpose that the object becomes a musical instrument. A person who play ...
*
Sonar Sonar (sound navigation and ranging) is a technique that uses sound 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 box A sound box or sounding box (sometimes written soundbox) is an open chamber in the body of a musical instrument which modifies the sound of the instrument, and helps transfer that sound to the surrounding air. Objects respond more strongly to vibra ...

*
Sound reproduction 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 br ...
;Sound measurement *
Acoustic impedance Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The SI unit The International System of Units, known ...
* Acoustic velocity * Characteristic impedance * Mel scale * Particle acceleration * Particle amplitude * Particle displacement * Particle velocity * Phon * Sone * Sound energy flux * Sound impedance * Sound intensity level * Sound power * Sound power level ;General: * Acoustic theory * Beat (acoustics), Beat * Doppler effect * Echo (phenomenon), Echo *
Infrasound Infrasound, sometimes referred to as low status sound , describes sound waves with a frequency below the lower limit of human audibility (generally 20 Hz). Hearing becomes gradually less sensitive as frequency decreases, so for humans to per ...
— sound at extremely low frequencies * List of unexplained sounds * Musical tone * Resonance * Reverberation * Sonic weaponry * Sound synthesis * Soundproofing * Structural acoustics

References

*
Sounds Amazing; a KS3/4 learning resource for sound and waves (uses Flash)

Audio for the 21st Century

Audio Check: a free collection of audio tests and test tones playable on-line

More Sounds Amazing; a sixth-form learning resource about sound waves
{{Authority control Sound, Hearing Waves Qualia Acoustics