OBSERVATIONS OF UNDERWATER SOUND FROM LABORATORY BREAKING WAVES AND THE IMPLICATIONS CONCERNING AMBIENT NOISE IN THE OCEAN

The results of an experiment to characterize the underwater sound fiel d radiated by various breaking waves intensities in fresh water in the range from 0.1 to 20 kHz are described. Waves are generated by a comp uter-controlled plunging-type wave maker and propagate along a 12.7-m- long channel where they are made to break at the mid-surface of a 3-X3 -X2.5-m anechoic water tank. The individual bubbles and bubble clouds entrained by the breaking wave provide a mechanism for sound productio n. Using high-speed cinephotography, correlations were established bet ween the hydrodynamic evolution of the cloud and the radiated acoustic emissions. The bubble size distributions inside the cloud were measur ed with the aid of a high-speed video camera and a fiber optic cable. These measurements indicate that single bubbles with radii as large as 7-8 mm may be entrained in this fresh-water system by moderate spilli ng breakers. Detailed measurements of the bubble size distribution of the bubble cloud enabled us also to obtain a measurement of the averag e void fraction in the cloud. These observations reveal that the sourc es of sound in laboratory spilling breakers is due mostly to single bu bble oscillations that can have frequencies as low as 400 Hz; in the c ase of plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions. The acoustic r adiation from bubble clouds is the result of collective oscillations o f the bubbles, stimulated by large scale vortices created by the plung ing breaker. The sound spectra, averaged over 100 breakers, reveal the following observations. First, the peak frequencies of the average so und spectra shifts from few Mit (weak, spilling breaker) to few hundre d Hz (plunging breaker). Second, the sound pressure levels increase in all frequency bands with increasing breaker severity. Lastly, the hig h-frequency portions of the sound spectra have slopes of about 5-6 dB/ oct, which are the slopes observed for the noise spectra of the ocean. These results provide considerable insight into the likely source mec hanisms for ocean ambient noise.