 Acoustic impedance

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, $Z\cos \theta$.

Acoustic impedance is a fundamental property of a medium that describes how much resistance it offers to the propagation of sound waves. It is defined as the ratio of the acoustic pressure (the sound pressure level) to the particle velocity (the speed of the particles in the medium due to the sound wave) at a specific frequency.