Inphysics,sound is avibration that propagates as anacoustic wave through atransmission medium such as a gas, liquid or solid.In humanphysiology andpsychology, sound is thereception of such waves and theirperception by thebrain.[1] Only acoustic waves that havefrequencies lying between about 20 Hz and 20 kHz, theaudio frequency range, elicit an auditory percept in humans. In air at atmospheric pressure, these represent sound waves withwavelengths of 17 meters (56 ft) to 1.7 centimeters (0.67 in). Sound waves above 20 kHz are known asultrasound and are not audible to humans. Sound waves below 20 Hz are known asinfrasound. Different animal species have varyinghearing ranges, 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)."[2] 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 asensation.
Acoustics is the interdisciplinary science that deals with the study ofmechanical waves in gasses, liquids, and solids includingvibration, sound, ultrasound, and infrasound. A scientist who works in the field ofacoustics is anacoustician, while someone working in the field ofacoustical engineering may be called anacoustical engineer.[3] Anaudio engineer, on the other hand, is concerned with the recording, manipulation, mixing, and reproduction of sound.
Experiment using twotuning forksoscillating usually at the samefrequency. 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 anacoustic 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 aslongitudinal waves and also as atransverse wave insolids. The sound waves are generated by a sound source, such as the vibratingdiaphragm 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 thespeed of sound, thus forming the sound wave. At a fixed distance from the source, thepressure,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 theaverage position of the particles over time does not change). During propagation, waves can bereflected,refracted, orattenuated by the medium.[5]
The behavior of sound propagation is generally affected by three things:
A complex relationship between thedensity 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. 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 does not have constant physical properties, it may berefracted (either dispersed or focused).[5]
Spherical compression (longitudinal) waves
The mechanical vibrations that can be interpreted as sound can travel through allforms of matter: gases, liquids, solids, andplasmas. The matter that supports the sound is called themedium. Sound cannot travel through avacuum.[6][7]
Studies has shown that sound waves are able to carry a tiny amount of mass and is surrounded by a weak gravitational field.[8]
Waves
Sound is transmitted through gases, plasma, and liquids aslongitudinal waves, also calledcompression waves. It requires a medium to propagate. Through solids, however, it 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 at right angle to the direction of propagation.
Sound waves may be viewed using parabolic mirrors and objects that produce sound.[9]
The energy carried by an oscillating sound wave converts back and forth between the potential energy of the extracompression (in case of longitudinal waves) or lateral displacementstrain (in case of transverse waves) of the matter, and the kinetic energy of the displacement velocity of particles of the medium.
Longitudinal plane wave
Transverse plane wave
Longitudinal and transverse plane wave
A 'pressure over time' graph of a 20 ms recording of a clarinet tone demonstrates the two fundamental elements of sound: Pressure and Time.Sounds can be represented as a mixture of their componentSinusoidal 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).[10][11][12]
Soundwaves are often simplified to a description in terms ofsinusoidalplane waves, which are characterized by these generic properties:
Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. In air atstandard temperature and pressure, the corresponding wavelengths of sound waves range from 17 m (56 ft) to 17 mm (0.67 in). Sometimes speed and direction are combined as avelocityvector; wave number and direction are combined as awave vector.
Transverse waves, also known asshear waves, have the additional property,polarization, which is not a characteristic of longitudinal sound waves.[13]
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 frequencies between approximately 20 Hz and 20,000 Hz (20 kHz),[18]: 382 The upper limit decreases with age.[18]: 249 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 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.[28][29][30] 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
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.[10][11][12] 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.
^Breinig, Marianne."Polarization".Elements of Physics II. The University of Tennessee, Department of Physics and Astronomy. Retrieved4 March 2024.
^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.
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