The absorption of sound in cellular foams

Porous materials are often used as sound absorbers in a variety of situations including architectural and industrial applications. In many cases it is advantageous for the material to be both lightweight and rigid. Metal foams, originally developed for use in catalytic converters in car exhaust systems, offer an attractive mix of properties being both lightweight and rigid. In addition they have good sound absorbing properties and are good heat conductors giving rise to the possibility of enhanced sound absorption through heat transfer. In this paper, we review the use and acoustic modelling of these materials. We compare the predictions made by a number of viscous models developed by the authors for the propagation of sound through open-cell metal foams with an experiment both for the metal foams and for the polymer substrates used to manufacture the foam. All models are valid in the limit of low Reynold's number which is valid for the typical ligament dimensions found in metal foams provided the amplitude of the waves is below 160dB. The first model considers the drag experienced by acoustic waves as they propagate passed rigid cylinders parallel to their axes, the second considers propagation normal to their axes, and the third considers the propagation passed the spherical joints. All three are combined together to give a general model of the acoustic behaviour of the foams. In particular, the sound absorption is found to be significant and well predicted by the combined model. In addition we describe a post-processing technique for the experiment used to extract the fundamental wave propagation characteristics of the material. Copyright © 2004 by ASME.