Frame decoupling at low frequency in thin porous layers saturated by air

According to the Biot theory and other models of propagation, two compressional waves can propagate in a porous medium saturated by air. At low frequencies each wave creates displacements of the same order of magnitude in the fluid and the frame. It is shown that for the case of thin materials bonded onto a rigid impervious backing, at low frequencies the acoustic field can nevertheless be described by only one wave which propagates in the air. This simple description is related to a mixing of both compressional waves which leaves the frame motionless.