A fundamental principle in the field of acoustics, the acoustic approximation states that an acoustic wave is created by a small, adiabatic, pressure ripple riding on a comparatively large equilibrium (bias) pressure. Typically, the acoustic pressure is on the order of a few ppm of the equilibrium pressure.
By extension, the acoustic approximation also guarantees that an acoustic wave travels at a speed exactly equal to the local speed of sound, such that the Mach number:
M=1
See also
Sound
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