Sound

Sound measurements
	Sound pressure p
	Sound pressure level (SPL)
	Particle velocity v
	Particle velocity level (SVL)
	(Sound velocity level)
	Particle displacement ξ
	Sound intensity I
	Sound intensity level (SIL)
	Sound power Pac
	Sound power level (SWL)
	Sound energy density E
	Sound energy flux q
	Acoustic impedance Z
	Speed of sound c

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

File:Processing of sound.jpg

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.)

Sound is a disturbance of mechanical energy that propagates through matter as a wave. Humans perceive sound by the sense of hearing.

By sound is generally meant the vibrations that are transmitted through air and can be heard by humans. However, scientists and engineers use a wider definition of sound that includes low and high frequency vibrations in air that cannot be heard, and vibrations that travel through all forms of matter, gases, liquids and solids. The matter that supports the sound is called the medium. Sound propagates as waves of alternating pressure, causing local regions of compression and rarefaction. Particles in the medium are displaced by the wave and oscillate.

As a wave, sound is characterized by the properties of waves including frequency, wavelength, period, amplitude and velocity or speed.

The scientific study of sound is called acoustics.

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

Sound is perceived through the sense of hearing. 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 through the sense of touch. Sounds are used in several ways, most notably for communication through speech or, for example, music. Sound can also be used to acquire information about the surrounding environment in properties such as spatial characteristics and presence of other animals or objects. For example, bats use echolocation, ships and submarines use sonar, and humans can determine spatial information by the way in which they perceive sounds.

The range of frequencies that humans can hear is approximately between 20 Hz and 20,000 Hz. This range is by definition the audible spectrum, but some people (particularly women) can hear above 20,000 Hz. This range varies by individual and generally shrinks with age, mostly in the upper part of the spectrum. The ear is most sensitive to frequencies around 3,500 Hz. Sound above 20,000 Hz is known as ultrasound; sound below 20 Hz as infrasound.

The amplitude of a sound wave is specified in terms of its pressure. The human ear can detect sounds with a very wide range of amplitudes and a logarithmic decibel 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 and sometimes resulting in tinnitus and hearing impairment. Sound levels in excess of 130 dB are considered upward of what the human ear can withstand and may result in serious pain and permanent damage. At very high amplitudes, sound waves exhibit non-linear effects including shock.

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

Divisions 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.
Charles Dodge and Thomas A. Jerse, Computer Music, New York: Schirmer Books, 1997. ISBN 0028646827
Grey, J. M. "An Exploration of Musical Timbre." Doctoral dissertation, Stanford University, 1975.
Hunt, F.V., Origins in Acoustics, Yale University Press, 1978.

External links

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Sound

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