Ali R. Kolaini
Lawrence A. Crum
Natl. Ctr. for Phys. Acoust., Univ. of Mississippi, Oxford, MS 38677
Results of a laboratory experiment to characterize the underwater sound field radiated from breaking waves are described. Waves are generated by a computer controlled plunging-type wave maker that propagate along a 40-ft-long channel and break at the surface of 12x12x8-ft anechoic water tank. The wave-maker parameters are controlled to produce spilling-type breakers with different breaker intensity (i.e., bubble cloud size). At smaller wave-maker amplitudes, a ``gentle'' spilling breaker can be produced to create a discrete number of bubbles. The number of bubbles and the sounds emitted by them can be measured and their type can be identified [H. Medwin and A. C. Daniel, J. Acoust. Soc. Am. 88, 408--412 (1990)]. Increasing the amplitude of the wave maker causes the breaker intensity to increase. The evolution of the bubble cloud size and its acoustic emissions are monitored by a high-speed video camera. The acoustic noise emitted at different breaker intensities by individual bubbles and by bubble clouds is presented and scaled to ocean breakers. Backscattering from bubble clouds was also observed using a parametric source. The scattered signal level was measured in the presence and absence of breaking waves as the insonifying frequency approached the collective oscillation frequencies of the bubble clouds. [Work supported by ONR.]