Nonlinear acoustics retains finite-amplitude effects in sound waves, including amplitude-dependent propagation speed, distortion and possible shock waves. For a one-dimensional polytropic flow with , a rightgoing simple wave has characteristic speed relative to a quiescent reference. Thus small-amplitude linear acoustics need not be uniform when a background velocity nearly cancels the speed of sound.
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Nonlinear acoustics is a branch of acoustics that deals with the behavior of sound waves in media where the relationships between pressure, density, and particle velocity are nonlinear. In contrast to linear acoustics, where sound waves are assumed to propagate in a medium under the assumption that changes in pressure and density are small, nonlinear acoustics considers scenarios where these changes are significant and can lead to more complex wave behavior.