Sound

This article is about compression waves. For other meanings, see sound (disambiguation).

In regular usage, the term sound is applied to any stimulus that excites our sense of hearing. The cause of sound is vibratory movement from a disturbance, communicated to the ear through a medium such as air. Scientists group all such vibratory phenomena under the general category of "sound," even when they lie outside the range of human hearing. The scientific study of sound is known as acoustics.

A schematic representation of hearing. (Blue: sound waves. Red: eardrum. Yellow: cochlea. Green: auditory receptor cells. Purple: frequency spectrum of hearing response. Orange: nerve impulse.)

Explanation

Solids, liquids, and gases are all capable of transmitting sound. For example, the practice of putting one's ear to the ground to listen for an approaching train is based on the fact that solids can transmit sound. Likewise, one can hear sounds when one's head is submerged in a swimming pool, thus demonstrating the ability of a liquid to carry sound. The matter that supports the transmission of sound is called the medium.

Sound is transmitted by means of a sound wave, much as a pebble thrown into a lake generates waves on the surface of the water. In air, a sound wave is a disturbance that creates a region of high pressure (compression) followed by one of low pressure (rarefaction). These variations in pressure are transferred to adjacent regions of the air in the form of a spherical wave radiating outward from the disturbance. Sound is therefore characterized by the properties of waves, such as their frequency, wavelength, period, amplitude, and velocity (or speed).

Sound waves are longitudinal waves, meaning that the compression and rarefaction of the medium occur in the direction in which the wave moves. By contrast, a boat on the water bobs up and down at right angles to waves on the surface—an example of transverse waves.

The properties of a sound wave depend upon the springiness, or elasticity, of the material that the sound travels through. In a gas, stresses and strains are manifested as changes in pressure and density. The movement of a sound wave is accompanied by the transmission of energy that is spread over the spherical wave front. Assuming the total energy remains constant, the energy intensity (measured in watts per square meter) at any point decreases as the square of the distance from the source. The decrease in energy intensity is accompanied by a proportional softening of the sound.

Noise and sound often mean the same thing; when they differ, a noise is an unwanted sound. In science and engineering, noise is an undesirable component that obscures a signal. What is noise and what is signal depends on your point of view.

Perception of sound

Humans and many animals use their ears to hear sound, but loud sounds and low-frequency sounds can be perceived by other parts of the body as well, through the sense of touch. Sounds are used in several ways, most notably for communication through speech or music. Sound can also be used to acquire information about the surrounding environment, such as the presence and distances of other animals or objects. For example, bats use echolocation and ships and submarines use sonar to obtain spatial information about the surroundings.

The range of frequencies that humans can hear is approximately between 20 and 20,000 Hertz (Hz). This range constitutes the audible spectrum, but some people (particularly women) can hear above 20,000 Hz. This range varies from one individual to the next and generally shrinks with age, mostly in the upper part of the spectrum. The ear is most sensitive to frequencies around 3,500 Hz. Sounds above 20,000 Hz are classified as ultrasound; sounds below 20 Hz, as infrasound.

The amplitude of a sound wave is specified in terms of its pressure, measured in pascal (Pa) units. The human ear can detect a wide range of amplitudes of sound, and a logarithmic decibel (dB) amplitude scale is used. The quietest sounds that humans can hear have an amplitude of approximately 20 μPa (micropascals), or a sound pressure level (SPL) of 0 dB re 20 μPa (often incorrectly abbreviated as 0 dB SPL). Prolonged exposure to a sound pressure level exceeding 85 dB can permanently damage the ear, sometimes resulting in tinnitus and hearing impairment. Sound levels in excess of 130 dB are considered above of what the human ear can withstand and may result in serious pain and permanent damage. At very high amplitudes, sound waves exhibit nonlinear effects, including shock.

Speed of sound

The speed of sound has been a subject of study since the days of the philosopher Aristotle (384–322 B.C.E.). In his writings, Aristotle discussed the time lapse between the sighting of an event and detection of the sound it produces. A cannon, for example, will be seen to flash and smoke before the sound of the explosive powder reaches an observer.

The speed at which sound travels depends on the medium through which the sound waves pass, and is often quoted as a fundamental property of the material. The speed of sound in air or a gas depends on the temperature and pressure of the gas. In air at room temperature and 1 atmosphere pressure, the speed of sound is approximately 345 meters per second (ms^-1); in water, 1500 ms^-1; and in a bar of steel, 5000 ms^-1.

Based on the dynamic properties of matter, Isaac Newton (1642-1727) was able to derive a mathematical expression for the speed of sound waves in an elastic or compressible medium. For a gas, this expression reduces to v = (P/)1/2 (P=pressure; =density), and yields a number that is short of the true velocity. The formula was improved upon by Eighteenth Century mathematician-physicist Pierre-Simon Laplace (1749-1827), who took into consideration the temperature effects of the compression of the air at the front of a sound wave and derived the equation:

v = ( P/)1/2

where  is a constant that depends on the heat-retaining properties of the gas. The speed of sound in a gas rises with the temperature.

An equivalent expression for the velocity of sound waves in a gas can be derived from the molecular theory of gases, based on the velocity of molecules, their size, weight and concentration. Seen in terms of the molecular theory, statistical variations in the energy of billions of molecules at the wave front are communicated to adjacent portions of the gas, thus moving the wave front forward.

Sound pressure

Sound pressure is the pressure deviation from the local ambient pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. The SI unit for sound pressure is the pascal (symbol: Pa). The instantaneous sound pressure is the deviation from the local ambient pressure caused by a sound wave at a given location and given instant in time. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time. In a sound wave, the complementary variable to sound pressure is the acoustic particle velocity. For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the acoustic impedance. The acoustic impedance depends on both the characteristics of the wave and the medium. The local instantaneous sound intensity is the product of the sound pressure and the acoustic partical velocity and is, therefore, a vector quantity.

Sound pressure level

As the human ear can detect sounds with a very wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale.

The sound pressure level (SPL) or L_p is defined as

L_{\mathrm {p} }=10\,\log _{10}\left({\frac {{p}^{2}}{{p_{0}}^{2}}}\right)=20\,\log _{10}\left({\frac {p}{p_{0}}}\right){\mbox{ dB}}

where p is the root-mean-square sound pressure and p₀ is a reference sound pressure. (When using sound pressure levels, it is important to always quote the reference sound pressure used.) Commonly used reference sound pressures, defined in the standard ANSI S1.1-1994, are 20 µPa in air and 1 µPa in water.

Since the human ear does not have a flat spectral response, sound pressure levels are often frequency weighted so that the measured level will match perceived sound level. 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 sound levels.

Examples of sound pressure and sound pressure levels

Source of sound	sound pressure	sound pressure level
	pascal	dB re 20µPa
threshold of pain	100 Pa	134 dB
hearing damage during short term effect	20 Pa	approx. 120 dB
jet, 100 m distant	6 – 200 Pa	110 – 140 dB
jack hammer, 1 m distant / discotheque	2 Pa	approx. 100 dB
hearing damage during long-term effect	6×10⁻¹ Pa	approx. 90 dB
major road, 10 m distant	2×10⁻¹ – 6×10⁻¹ Pa	80 – 90 dB
passenger car, 10 m distant	2×10⁻² – 2×10⁻¹ Pa	60 – 80 dB
TV set at home level, 1 m distant	2×10⁻² Pa	ca. 60 dB
normal talking, 1 m distant	2×10⁻³ – 2×10⁻² Pa	40 – 60 dB
very calm room	2×10⁻⁴ – 6×10⁻⁴ Pa	20 – 30 dB
leaves noise, calm breathing	6×10⁻⁵ Pa	10 dB
auditory threshold at 2 kHz	2×10⁻⁵ Pa	0 dB

Acoustics

Acoustics is a branch of physics that studies sound, namely mechanical waves in gases, liquids, and solids. A scientist that works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering. There is often much overlap and interaction between the interests of acousticians and acoustical engineers.

...[A]coustics is characterized by its reliance on combinations of physical principles drawn from other sources; and that the primary task of modern physical acoustics is to effect a fusion of the principles normally adhering to other sciences into a coherent basis for understanding, measuring, controlling, and using the whole gamut of vibrational phenomena in any material.
Origins in Acoustics, F.V. Hunt, Yale University Press, 1978.

Sub-disciplines of acoustics

The following are the main sub-disciplines of acoustics.[1]

Acoustical measurements and instrumentation.
Acoustic signal processing.
Aeroacoustics is the study of aerodynamic sound, generated when a fluid flow interacts with a solid surface or with another flow. It has particular application to aeronautics, examples being the study of sound made by flying jets and the physics of shock waves (sonic booms).
Architectural acoustics is the study of how sound and buildings interact including the behavior of sound in concert halls and auditoriums but also in office buildings, factories and homes.
Bioacoustics is the study of the use of sound by animals such as whales, dolphins and bats.
Biomedical acoustics is the study of the use of sound in medicine, for example the use of ultrasound for diagnostic and therapeutic purposes.
Environmental noise is the study of the sound propagation in the human environment, noise health effects and noise mitigation analysis.
Psychological acoustics is the study of how people react to sound, hearing, perception, and localization.
Physiological acoustics is the study of the mechanical, electrical and biochemical function of hearing in living organisms.
Physical acoustics is the study of the detailed interaction of sound with materials and fluids and includes, for example, sonoluminescence (the emission of light by bubbles in a liquid excited by sound) and thermoacoustics (the interaction of sound and heat).
Speech communication is the study of how speech is produced, the analysis of speech signals and the properties of speech transmission, storage, recognition and enhancement.
Structural acoustics and vibration is the study of how sound and mechanical structures interact; for example, the transmission of sound through walls and the radiation of sound from vehicle panels.
Transduction is the study of how sound is generated and measured by loudspeakers, microphones, sonar projectors, hydrophones, ultrasonic transducers and sensors.
Ultrasonics is the study of high frequency sound, beyond the range of human hearing.
Musical acoustics is the study of the physics of musical instruments.
Underwater acoustics is the study of the propagation of sound in the oceans. Closely associated with sonar research and development.

Measurement of sound

Decibel, sone, mel, phon
Sound pressure, acoustic pressure, sound pressure level
Particle velocity, acoustic velocity, sound velocity
Particle displacement, particle amplitude, particle acceleration
Sound power, acoustic power, sound power level
Sound intensity, acoustic intensity, sound intensity level
Acoustic impedance, sound impedance, characteristic impedance
Speed of sound, amplitude
Sound energy flux
See also Template:Sound measurements

References
ISBN links support NWE through referral fees

Olson, Harry F., "Acoustical Engineering" (1957) cited in Roads, Curtis (2001). Microsound. MIT. ISBN 0262182157.
Roederer, Juan C., Introduction to the Physics and Psychophysics of Music (2nd ed.) New York: Springer-Verlag, 1979.
Dodge, Charles, and Jerse, Thomas A., Computer Music, New York: Schirmer Books, 1997. ISBN 0028646827
Grey, J. M., "An Exploration of Musical Timbre." Doctoral dissertation, Stanford University, 1975.
Beranek, Leo L., "Acoustics" (1993) Acoustical Society of America. ISBN 0-88318-494-X
Hund, F.V., Origins in Acoustics, Yale University Press, 1978

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