Mitigation of low-frequency underwater sound using large encapsulated bubbles and freely-rising bubble clouds.

Abstract

Low-frequency anthropogenic noise may affect marine life, motivating the need to minimize its potential impact. Bubbles cause significant dispersion and attenuation of underwater sound at frequencies near the individual bubble resonance and can potentially be used to abate this noise. Such effects have been reported for large encapsulated bubbles with resonance frequencies below 100 Hz, and significant attenuation due to bubble resonance phenomena and acoustic impedance mismatch was observed in a tank experiment [J. Acoust. Soc. Am. 127, 2015 (2010); 128, 2279 (2010)]. Both of these mechanisms were found to significantly reduce down-range radiated acoustic pressure, as much as 40 dB, at low frequencies (60 to 1000 Hz) in a series of lake experiments where a sound source was surrounded by an array of tethered resonant toroidal air bubbles, a cloud of freely-rising sub-resonant bubbles, and various combinations of the two. Hydrophones were placed at various depths and ranges to determine the effect of the bubbles on the radiated field. The effects of void fraction and bubble size variation on the spectrum of the radiated sound were also investigated. [Work supported by Shell.]