Acoustic characteristics of low density foams

Plastic foams are used extensively in architectural (room and building acoustics) and noise control (acoustic insulation and absorption of sound sources) applications. The desired acoustic properties of the foam differ for each application. It goes without saying that the understanding of the relation between the physical parameters of the foam and its acoustic behaviour is of general interest. The study of the acoustic properties of porous materials has occupied scientists since the work of Beranek [1,2], Morse et al. [3] and Zwikker and Kosten [4] in the 1940s. A myriad of models describing sound propagation in porous materials have been published since. An excellent review can be found in [5]. However, most of these models require the introduction of parameters which cannot be measured independently, making them inappropriate for the design of sound absorbing materials. Since 1956, the Biot theory allows in a very general and rigorous way the description of sound propagation in porous materials. This theory has been used amply, including in underwater acoustics and sound propagation through liquid filled rocks. Recently, the Biot theory has been used to calculate the acoustic properties of plastic foams, and it has been proven that in a number of cases the Biot theory is indispensable for explaining the acoustic behaviour of certain foam types.