How to Model the Acoustic Properties of a Solid Foam with Thin Membranes?

Porous materials used for soundproofing, like solid foams, usually have an open-porosity: no obstacle should prevent the sound to penetrate inside the medium, otherwise the acoustic energy would be reflected back instead of being absorbed. But what happens if the foam's cells are closed by thin membranes (a few microns thick)? We performed acoustic experiments on two types of polyurethane foams: some with membranes, and some for which the membranes were removed by the manufacturer during the production process. We found that the presence of membranes affects the foam's effective density (even though the added mass due to membranes is negligible) but its effective compressibility remains the same. Surprisingly, we also evidenced that acoustic absorption is larger for the closed-cell foams than for the open-cell ones. In fact, since the membranes are thin, they transmit the acoustic perturbation, so the sound can still propagate inside the fluid phase. In addition, the membranes may increase the coupling between the vibration of the air and the vibration of the solid skeleton (which is usually supposed to be rigid and motionless), leading to an increase of the dissipation phenomena. Solid foams with membranes thus offer new perspectives to design materials for acoustic absorption.