Low-frequency sound radiation and scattering from bubble clouds.

Abstract

Recent experimental evidence has shown that when wave breaking occurs, low-frequency (LF∼200 Hz) sound is produced and LF scatter has a different characteristic than expected from rough sea surface scattering. These effects have been attributed to the bubbles produced during wave breaking, which are convected to depth by the breaking turbulence, vorticity and Langmuir circulation as observed by Thorpe [S. Thorpe, OceanicWhiteCaps, edited by E. Monahan and G. MacNiocaill (Reidel, Boston, 1986), pp. 57–58]. While the radiation and scattering characteristics at frequencies greater than 1 kHz are explained by incoherent scatter from the observed bubble size and space distributions the lower frequency phenomena are not easily explained. However if bubble plumes and clouds produced in the wave breaking have appreciable volume fractions (≳10−5), then LF sound radiation and scattering can be explained by classical theories. This paper reviews the scattering and radiation from bubble clouds in water as a function of volume fraction. When the cloud is compact, coherent and collective scatter are shown to occur. The natural frequency of radiation is shown to be described by a modified Minnaert result while the backscatter target strength is described by the first-order volume mode. These analytical results agree with experimental sound radiation and scatter measurements. Finally, the collective radiation of bubble plumes and clouds is discussed as a possible explanation of the observed ocean low-frequency scattering and radiation phenomena. [Work sponsored by ONR 11250A and NUSC IR.]