Acoustic absorption of solid foams with thin membranes

We measured the acoustic absorption, on the 0.5-6 kHz frequency range, of polyurethane foams with mean pore diameters between 0.6 and 3.2 mm. Two types of foams were investigated: classical open-cells ones versus membrane foams, in which thin polyurethane membranes were preserved during solidification. Interestingly , the latter presented better absorption abilities, indicating that membranes could be an asset for sound absorption. Noise pollution has become a major problem in our modern life. Industrial and academic research has continuously tried to design more efficient soundproofing materials. Recently, exotic stuctures have been considered, using the concept of double-porosity 1,2 or introducing low frequency resonators to enhance the dissipation. 3–5 Traditionally , sound absorbers have been porous media, such as mineral wools or foams. A first general rule for their efficiency is that no obstacle should prevent the sound from propagating in the medium, otherwise the acoustic energy is reflected back instead of being absorbed. For foams, for example, it means that open-cell structures are preferred. On the other hand, the physical picture is that when sound penetrates such a medium, it loses a lot of energy because of the large surface area it can interact with. As a rule of thumb, one finds that good absorption is obtained when the typical pore size corresponds to the heat and viscous diffusive length in air, which is of the order of 50 µm at 2 kHz, for example. Hence, open-cell porous materials with pores sizes of tens of micrometers are good candidates for efficient sound absorption and are therefore used extensively for sound insulation. 6,7 In this letter we show that there are exceptions to these established rules: closed-cell foams with millimeter-sized pores can actually be good sound absorbers. The foams we studied were provided by the company Foampartner. They were made of polyurethane, with a porosity (air volume fraction) of 98%. Their most interesting feature, for us, was that most of the membranes which separate neighbouring pores (Fig. 1) were preserved during solidification. 8 As these membranes are not desired for most of the applications, the manufacturer employs a technique by which the membranes a) Electroni