Underwater sound generation by breaking wind waves

The problem of identifying the source of wind-generated underwater sound is reviewed. Amalgamated observations of the ambient noise reveal a similarity structure, both in the acoustical spectrum and wind dependency, which allows for a considerable simplification of the problem. Mechanisms of sound generation are discussed with particular reference to oscillating bubbles. Air entrainment by breaking waves and the probabilistic distribution of bubbles are discussed. A model for underwater noise generation by bubbles oscillating in a breaking wave is proposed. It is argued that the most intense sound is associated with bubbles the radius of which is comparable to the Kolmogorov scale length. A slight three-eighths wind dependency is predicted for the frequency of the maximum intensity. Several arguments, one based on the number density of bubbles in a breaking wave, another based on the areal coverage of whitecaps, lead to the deduction that the sound intensity from a field of breaking waves varies as friction velocity to the three-halves power. Agreement in structure and order of magnitude is demonstrated.