A description, using transfer matrices, is given of the propagation of the three inhomogeneous Biot waves in layered porous media. This is applied to predict the surface impedance of porous layered materials at oblique incidence, and an example is presented.

A matrix representation of the acoustical properties of porous layers at oblique incidence is used to predict the surface impedance of layered porous materials with impervious screens. The screens can be located inside the material or at the surface. Comparisons between prediction and measurement achieved with a new technique are given.

A fast variation of the surface impedance of two layers of different thicknesses of the same material has been observed at low frequencies. These variations, which are predicted by the Biot theory, appear at the λ/4 resonance of the frame.