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. Soc. Am. 96, 1755–1765 (1994)] reported the measurements of the ambient sound generated by laboratory breaking waves over the range 100–20 000 Hz in fresh water. Those observations from both spilling- and plunging-type breakers have been repeated in the same manner and wavemaker/anechoic tank with water that had 25‰ salt in its content. The observations in salt water, just like those in fresh water, reveal that the sources of sound in laboratory spilling breakers are due primarily to single bubble oscillations that can have frequencies lower than a few hundred Hertz. In the case of weak spilling breakers, the sound spectra level in fresh water was due primarily to single bubble oscillation, while the same breakers in salt water have introduced smaller size bubbles with large density. The relatively high-density populated bubble cloud generated by weak breakers shows the evidence of the onset of collective oscillation that was absent for the same breakers in the fresh water. In the case of moderate spilling and plunging breakers, it appears that both individual bubbles and bubble clouds can contribute to the acoustic emissions in fresh and salt water. The average sound 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/oct, 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 level in all observed frequency range. The ionic structure of the medium alters the sound radiation from bubbles. In this paper some of the observed acoustic signatures from breaking waves are discussed and a plausible explanation of how salt can effect the sound radiation from bubbles is given.