Acoustic plane wave
= Acoustic plane wave
{title2=$\tilde{\mathbf u}=\hat{\mathbf k}\tilde p/(\rho_0c_0)$}
A progressive small-amplitude pressure perturbation in a uniform inviscid fluid depends on one travelling coordinate. Linear momentum balance gives its velocity parallel to propagation and proportional to pressure with impedance $\rho_0c_0$.