Mitigation of low-frequency underwater noise generated by rotating machinery on a mobile work barge using large tethered encapsulated bubbles

Abstract

Collections of bubbles cause significant dispersion and attenuation of underwater sound at frequencies near the individual bubble resonance and can potentially be used to mitigate low-frequency anthropogenic underwater noise. Such effects have been reported for large encapsulated bubbles with resonance frequencies below 100 Hz [J. Acoust. Soc. Am. 130:3325-3332 (2011)] and significant attenuation due to bubble resonance phenomena and acoustic impedance mismatching was observed in experiments using a compact electromechanical acoustic source [J. Acoust. Soc. Am. 128:2279 (2010); J. Acoust. Soc. Am. 129:2462 (2011)]. In the present study, screens of tethered resonant encapsulated air bubbles were used to surround a mechanically-vibrated mobile work barge in a lake experiment to demonstrate their potential as a mitigation strategy for such noise sources. Conventional screens of freely-rising bubbles were also deployed for comparison. The radiated acoustic pressure was measured at various water depths and ranges to determine the effect of the bubble screens on the noise field. Compared to the freely rising bubbles, the tethered encapsulated bubbles yielded a significant increase in noise reduction below 1 kHz, demonstrating their efficacy for abatement of low-frequency underwater rotating machinery noise. [Work supported by Shell Global Solutions.]