Acoustic impedance
= Acoustic impedance
{title2=$Z=\rho c$}
{wiki}
The acoustic impedance of a fluid is $Z=\rho c$. For a progressive plane acoustic wave, pressure amplitude $P$ and velocity amplitude $U$ in its propagation direction satisfy $P=ZU$. At an oblique planar interface, zero reflection requires a matching condition involving the normal impedances $Z/\cos\theta$ or, equivalently for displacement amplitudes, $Z\cos\theta$.