Characterization of the Bulk Elastic Properties of Expanding Foams from Impedance Tube Absorption Tests

A method is presented to determine the bulk elastic properties of isotropic elastic closed-cell foams from impedance tube sound absorption tests. For such foams, resonant sound absorption is generally observed, where acoustic energy is transformed into mechanical vibration, which in turn is dissipated into heat due to structural damping. This paper shows how the bulk Young's modulus and damping loss factor can be deduced from the resonant absorption. Also, an optimal damping loss factor yielding 100% of absorption at the first resonance is defined from the developed theory. It is shown how this optimal factor can be used to properly design efficient sound absorbing treatments. The method is experimentally tested on one expanding closed-cell foam to find its elastic properties. Using the found properties, sound absorption predictions using an equivalent solid model with and without surface absorption are compared to measurements. Excellent correlations are obtained when considering surface absorption.