Acoustic hot-spots and breaking wave noise in the surf zone

Measurements of the horizontal directivity of the underwater sound made beh ind individual breaking waves in the surf zone are presented. To an observe r behind a breaking wave, the breaking noise appears to radiate from acoust ic hot-spots, which are compact sources at the ends of the breaking wavecre st. The hot-spots move through the surf zone as the length of breaking wave crest increases, leading to systematic temporal variations in the wave nois e directionality. Measurements of the acoustical properties of the mixture of water and bubbles left behind the breaking wave are presented. Sound spe eds as low as 500 m/s and acoustic absorption up to 50 dB per mere; occur i n the bubbly region. Based on these measurements and a propagation model fo r the surf noise, it is proposed that the observed horizontal directivity i s caused by the interaction between the noise radiated by the breaking wave crest and the residue of bubbly water left behind the wave. (C) 1999 Acoust ical Society of America. [S0001-4966(99)02206-7].