Abstract
The average compressibility of an emulsion acquires a frequency-dependent, relaxing behavior due to the thermoconduction between adjacent phases heated through thermo-mechanical coupling. Introducing the relaxing compressibility into the sound propagation equations, we extend Isakovitch's theory of sound absorption to emulsions of an arbitrary number of liquids with the same density. The sound propagation speed and attenuation are found to be isotropic, even if the emulsion morphology is anisotropic. In the limit of frequencies greater than the inverse heat diffusion time, both the relaxing part of the compressibility and the sound attenuation are proportional to a single parameter depending linearly on the emulsion interfacial area per unit volume, thus giving easy access to this quantity in non-transparent systems.