The expression of the phase decoupling frequency given by Zwikker and Kosten [ZK49] is:

APMR

Acoustical Porous Material Recipes

The expression of the phase decoupling frequency given by Zwikker and Kosten [ZK49] is:

\[
f_d=\frac{\sigma \phi^2}{2 \pi \rho_1}
\]

where $\sigma$ is the static air flow resistivity of the material, $\phi$ its open porosity and $\rho_1$ its mass density.