SOUND RADIATION BY VARIOUS TYPES OF LABORATORY BREAKING WAVES IN FRESH AND SALT-WATER

A recent article [A. R. Kolaini and L. A. Crum, J. Acoust. Sec. Am. 96 , 1755-1765 (1994)] reported the measurements of the ambient sound gen erated by laboratory breaking waves over the range 100-20 000 Hz in fr esh water. Those observations from both spilling-and plunging-type bre akers have been repeated in the same manner and wavemaker/anechoic tan k with water that had 25 parts per thousand salt in its content. The o bservations in salt water, just like those in fresh water, reveal that the sources of sound in laboratory spilling breakers are due primaril y to single bubble oscillations that can have frequencies lower than a few hundred Hertz. In the case of weak spilling breakers, the sound s pectra level in fresh water was due primarily to single bubble oscilla tion, while the same breakers in salt water have introduced smaller si ze bubbles with large density. The relatively high-density populated b ubble cloud generated by weak breakers shows the evidence of the onset of collective oscillation that was absent for the same breakers in th e fresh water. In the case of moderate spilling and plunging breakers, it appears that both individual bubbles and bubble clouds can contrib ute to the acoustic emissions in fresh and salt water. The average sou nd spectra reveal that the peak frequencies of the spectra shift from a few kHz (weak, spilling breaker) to few hundred Hz (plunging breaker ), and the high-frequency portions have slopes approximately 5-6 dB/oc t, which are the slopes observed from the noise spectra of the ocean. Besides the high bubble density and smaller bubble sizes in salt water , all breakers experienced a significant increase in sound-pressure le vel in all observed frequency range. The ionic structure of the medium alters the sound radiation from bubbles. In this paper some of the ob served acoustic signatures from breaking waves are discussed and a pla usible explanation of how salt can effect the sound radiation from bub bles is given. (C) 1998 Acoustical Society of America.