Bubble acoustics in shallow water: Possible applications in nature

Leighton, T.G., White, P.R. and Finfer, D.C. (2005) Bubble acoustics in shallow water: Possible applications in nature. International Conference on Boundary Influences in High Frequency, Shallow Water Acoustics, Bath. 04 - 08 Sep 2005. pp. 433-40 .

Abstract

Gas bubbles are the most potent naturally-occurring entities that influence the acoustic environment in liquids. Upon entrainment under breaking waves, waterfalls, or rainfall over water, each bubble undergoes small amplitude decaying pulsations with a natural frequency that varies approximately inversely with the bubble radius, giving rise to the 'plink' of a dripping tap or the roar of a cataract. When they occur in their millions per cubic metre in the top few metres of the ocean, bubbles can dominate the underwater sound field. Similarly, when driven by an incident sound field, bubbles exhibit a strong pulsation resonance. This paper discusses three examples of how bubble acoustics may find applications in Nature. The first of these is the determination of bubble size distributions through inversion of the sound fields that bubbles generate on entrainment. This can be used not only in testing models of bubble cloud evolution under breaking waves, but also in extraterrestrial environmental assessment. The second application lies in the possible enhancement by humpback whales of the efficiency of the bubble nets they use in fishing. The third speculates on the apparent conundrum, that unless dolphins employ better signal processing than humans currently do, then when they use bubble nets to hunt they are, in this visually confusing environment, nullifying their own most spectacular sensory apparatus.

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