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A mechanical disturbance advancing with infinite velocity through an elastic medium and consisting of longitudinal displacements of the medium, i.e., consisting of compressional and rarefactional displacements parallel to the direction of advance of the disturbance; a longitudinal wave. Sound waves are small-amplitude adiabatic oscillations. The wave equation governing the motion of sound waves has the form

2φ = (1/c2)(δ2φ/δt2)

where ▼2 is the Laplace operator, φ is the velocity potential, c is the speed of sound, and t is the time; the density variations and velocities are small. As so defined, this includes waves outside the frequency limits of human hearing, which limits customarily define sound. Also called acoustic wave, sonic wave.

Gases, liquids, and solids transmit sound waves, and the propagation velocity is characteristic of the nature and physical state of each of these media. In those cases where a steadily vibrating sound generator acts as a source of waves, one may speak of a uniform wave train; but in other cases (explosions, lightning discharges) a violent initial disturbance sends out a principal wave, followed by waves of more or less rapidly diminishing amplitude.

An oscillation of air pressure. One wave is measured from the point of null (average room) pressure past both points of greatest (crest) and least (trough) pressure to the point of normal pressure. The length of the wave is measured in feet. The frequency is measured in Hertz (when time is factored in). Source: Church Audio & Acoustics Glossary

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