Frequency dependence of elastic properties of acoustic foams

Polyurethane (PU) and other plastic foams are widely used as passive acoustic absorbers. For optimal design, it is often necessary to know the viscoelastic properties of these materials in the frequency range relevant to their application. A nonresonance technique (dynamic stiffness) based on a forced vibrations procedure is used to investigate the frequency dependent complex shear modulus of a PU foam. This modulus is first measured, in a quasistatic configuration, in the frequency range (0.016–16 Hz) at different temperatures between 0 and 20 °C. It is afterwards predicted over a wide frequency range (0.01–3000 Hz) using the frequency-temperature superposition principle. The validation of this principle is discussed through quasistatic experiments. Under the assumption that Poisson’s ratio of polymeric foams is real and frequency independent on the frequency range used, the frequency dependence of the complex shear modulus obtained is used to predict the complex stiffness of the acoustic foam on a wide frequency range.