Wave propagation in a viscoelastic medium containing fluid-filled microspheres

Effective medium techniques have been developed by many authors to model the acoustic propagation through layers of visco-elastic materials which contain fluid-filled or elastic cavities. This paper extends such work to the case of materials containing air-filled microspheres whose thin glassy shell separates the interior air from the surrounding polymer. Such composites have been found particularly useful in conditions of high hydrostatic loading where the presence of the reinforcing shells delays the onset of the hole collapse and the consequent degradation in the acoustic performance of the composite. In the present paper, a model is presented which includes the effect of shell wall thickness variability but excludes the depth dependency—the latter will be presented in a sequel. Theoretical predictions are compared with experimentally measured values of transmission loss for both stiff and soft polymer substrates and, in each case, good agreement with experimental data is shown.