Acoustic attenuation in mixtures of air and water droplets

Sound propagation through mixtures of air and water-droplets occurs in many natural systems such as in fogs and clouds, where it has been observed that acoustic waves are attenuated to a greater degree than in the absence of water droplets. This effect has been exploited at space shuttle launches with the sound suppression system, where water sprayed around the shuttle significantly reduces the acoustic power radiated from the rocket engines. Theoretical studies have investigated various mechanisms causing acoustic attenuation in air-water droplet mixtures: heat transfer, mass transfer and momentum transfer. More recent studies have incorporated other mechanisms into analytical models so that they are more representative of sound propagation through fogs or clouds. The aim of this paper is to review the different mechanisms causing attenuation of acoustic waves, and to present a comparison of the different analytical models to illustrate the differences in predicted acoustic attenuation and dispersion. Some limitations of the different models are identified, and proposals for experimental work to validate analytical results are discussed.