= Normal acoustic impedance
{title2=$Z_n$}
The normal acoustic impedance of a progressive <acoustic wave> at a planar boundary is the pressure amplitude divided by the normal-velocity amplitude. For propagation angle $\theta$ from the normal,
$$
Z_n=\frac{\rho c}{\cos\theta}=\frac{\rho\omega}{k_n}.
$$
The distinction from the <acoustic impedance> $\rho c$ matters at oblique incidence: interface reflection depends on the ratio of normal impedances. For an evanescent outgoing wave $k_n$ is imaginary and the normal impedance is reactive, giving no mean normal <acoustic energy flux>.
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