(Also called acoustic wave, sonic wave.) A mechanical disturbance advancing with finite velocity through an elastic medium and consisting of longitudinal displacements of the ultimate particles of the medium, that is, 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
See ultrasonic, infrasonic, pressure wave.
where ∇2 is the Laplace operator, φ the velocity potential, t the time, and c the speed of sound, the density variations and velocities being taken small. As so defined, this includes waves outside the frequency limits of human hearing, which limits customarily define sound. 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; 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.
See ultrasonic, infrasonic, pressure wave.
