The attenuation of acoustic signals by aqueous and particulate foams

Abstract A study was undertaken to determine how the moisture content of an aqueous foam affects its ability to attenuate an acoustic signal. The energy losses from an acoustic signal, for the series of foams considered in this study, are believed to arise principally from two causes: an increase in the density of the medium through which the signal passes (resulting in a decrease in the velocity of sound) and a viscous loss mechanism. The experimental results presented indicate that both these processes are dependent on the density or liquid content of the foam. In addition to this, the viscous loss mechanism displays a bubble size and liquid viscosity dependence. The incorporation of talc particles into the aqueous foams was found to greatly enhance the stability of the foam and its attenuation characteristics. This is believed to arise from an increase in the viscosity of the liquid phase of the foam and the enhanced signal/air and particle/liquid interactions, both of which result from the presence of the fine particles in the foam.