= Acoustic wave
{wiki}
= Sound wave
{synonym}
An <acoustic wave> is a propagating disturbance in a material's <pressure>, <mass density> and particle motion. Compressing a small fluid region raises its <pressure>; the resulting <pressure gradient> accelerates adjacent material, while <inertia> carries the motion onward. This restoring mechanism permits <longitudinal waves> in an ordinary fluid.
For small perturbations about a uniform resting state, the linear <continuity equation> and <momentum conservation> combine into a <wave equation>. The squared <speed of sound> is the relevant derivative of <pressure> with respect to <mass density>; rapid locally reversible compression usually uses the isentropic derivative. A <microphone> records the pressure variation, while a travel-time measurement probes propagation speed. The particle <velocity> can remain much smaller than the <speed of sound>: local material oscillation and movement of the wave pattern are different quantities. <Acoustic energy flux> relates the pressure and particle-velocity fields to transported <power>.
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