On mechanism of wind wave noise generation

The classification of underwater sound sources requires detailed knowledge of ambient noise structure at different hydrodynamic and meteorological conditions. This includes a better understanding of the mechanism of ambient sound generation and attenuation. It is generally recognised that breaking wind-waves are the dominant source of natural ambient noise in the ocean. The noise intensity is usually characterised by the Knudsen frequency spectra with the typical slope of -5-6 dB/octave. First measurements of the Knudsen spectra revealed strong correlation of the wind speed to underwater ambient noise. Recent studies suggested that the wind is the source of the Knudsen spectra only through the mediation of the spilling breakers, which are the sources of the sound-radiating air bubbles. Calculating the amount of dissipated wave energy that is spent on the bubble entrainment against buoyancy, a conclusion on the bubble cloud parameters can be made. In the paper, a mathematical model of ambient noise generation due to wave breaking is presented. The acoustic energy is generated by the collective oscillation of the bubbles in the bubble cloud, resulting in a much lower eigenfrequency than that of a single bubble. The model leads to an expression for noise intensity spectra due to wind-wave breaking. The model suggests that observed shape of the Knudsen spectra is defined mainly by the structure of bubble population and lowest eigenfrequency of collective bubble oscillation.