Measurements of the horizontal directivity of the underwater sound made behind individual breaking waves in the surf zone are presented. To an observer behind a breaking wave, the breaking noise appears to radiate from acoustic hot-spots, which are compact sources at the ends of the breaking wavecrest. The hot-spots move through the surf zone as the length of breaking wavecrest increases, leading to systematic temporal variations in the wave noise directionality. Measurements of the acoustical properties of the mixture of water and bubbles left behind the breaking wave are presented. Sound speeds as low as 500 m/s and acoustic absorption up to 50 dB per meter occur in the bubbly region. Based on these measurements and a propagation model for the surf noise, it is proposed that the observed horizontal directivity is caused by the interaction between the noise radiated by the breaking wavecrest and the residue of bubbly water left behind the wave.
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Grant B. Deane (1999) studied this question.
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