The scattering of multipole near-field sound by a gas bubble

Abstract

A gas bubble very close to an underwater sound source can have a profound influence on the sound radiated by the source. It is well known that when source and bubble are far apart, the bubble is a strong scattering centre for the incident sound, but when they are close together the bubble responds also to the near-field flow. This near field is scattered by the bubble from a non-propagating to a propagating mode so that the acoustic output of a multipole source, which has a relatively high near-field, can be considerably increased by the scattering process. A theory is developed for the scattering by a single spherical gas bubble near ( a ) a compressible spherical monopole source and ( b ) a compressible spherical dipole source, but it can be applied to non-spherical sources and bubbles by ascribing to them an ‘equivalent spherical radius’. The theory is in good agreement with experiments using bubbles near a spherical monopole source and in fair agreement with experiments on bubbles near non-spherical monopole and dipole sources.