An acoustic wave is a type of mechanical wave that propagates through a medium (such as air, water, or solids) due to the oscillation of particles in that medium. These waves transmit energy and information by causing local compression and rarefaction of the medium's particles. Acoustic waves can be categorized into two main types: 1. **Longitudinal Waves**: In these waves, the particle displacement is parallel to the direction of wave propagation.
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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.