Low‐frequency noise and bubble plume oscillations

The evidence for a wind dependence in low‐frequency ambient noise (< 200 Hz) is sparse due to the corrupting influences of the radiated sound from ships. Measurements made with hydrophones below critical depth, in sparse shipped basins, or at high sea states show indications of two distinct regimes associated with the occurrence of breaking waves and a variation of root‐mean‐square pressure with the square of the local wind speed. In a previous paper [W. Carey, J. Acoust. Soc. Am. Suppl. 1 78, S1 (1985)], the generation of sound by wave turbulence interaction at low sea states (WS < 10 m/s) and collective bubble oscillations driven by turbulence at high sea states was proposed (WS > 10 m/s). This paper shows that recently observed bubble clouds that penetrate to tens of meters below the surface of the sea result in regions of low sonic velocity described by Wood's treatment of air‐bubble water mixtures, but with a density close to that of water. It is shown that such a region can be treated as a flexible body with mixture speed and density. The radiation from such a body would be monopole and dipole in nature; but due to the proximity of the sea surface, only the resulting dipole source would be of importance. These regions could also have a resonant characteristic, and when driven by the turbulence result in sufficient radiated sound. These results are similar to and consistent with the analysis by A. Prosperetti [Nato AS1, “Natural Mechanisms of Surface Generated Noise in the Ocean” (1987)1.