Attenuation of low-frequency underwater sound using bubble resonance phenomena and acoustic impedance mismatching
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Air bubbles can be a significant source of attenuation in underwater sound propagation, but such effects have not been experimentally verified for low frequencies in part due to the difficulty in creating large stable bubbles. This work is in part an extension of a previously reported study in the acoustic effects of large resonant encapsulated air bubbles in a 1D waveguide [J. Acoust. Soc. Am. 127 2015 (2010)]. Now, both bubble resonance effects and acoustic impedance mismatching effects are investigated in a large-scale tank setup. Both of these mechanisms are shown to attenuate sound at low frequencies (50 to 200 Hz) through experiments in which a sound source is surrounded by a column of freely-rising sub-resonant bubbles, a matrix of tethered resonant air balloons, and a combination of the two. Experiments with a matrix of thicker-shelled encapsulated bubbles demonstrate decreased attenuation due to weaker resonant interaction and the use of a polydipserse versus monodisperse encapsulated bubble size distribution demonstrates a flattened frequency response for fixed global void fraction. Finite-element models of both an effective medium bubble cloud and discrete stationary bubbles are discussed and compared with experiment. 2011 Acoustical Society of America.
