Variations in signal phase and beamformer gain due to bubble scattering

We have investigated experimentally variations in the phase of acoustic signals propagating through bubble clouds and received at a three-element hydrophone array. The effect of bubbles in water to attenuate acoustic signals is well known, but the interest here is on the phase variation across the array introduced by bubble scattering, potentially decorrelating the signals across the array and reducing the gain that can be achieved by a beamformer. In this paper we report measurements made at the AB Wood acoustic water tank located at the Institute of Sound and Vibration Research (ISVR), University of Southampton, in June 2008. The distribution of signal phase among the individual hydrophones is seen to become more spread out as bubble density increases. As a result, beamformer gain is degraded. It seems that phase shifts caused by scattering from nearby bubbles are responsible for this effect.