Low‐frequency noise: Wave turbulence interaction and bubble cloud oscillations

Abstract

Low‐frequency ocean ambient noise data, when not dominated by shipping noise, show evidence for wind‐dependent noise at frequencies less than 500 Hz. Vertical directionality measurements have a horizontal component with a broad frequency characteristic. This effect is partly due to the coupling of wind‐generated noise into the sound channel by either a shallowing sound channel or a down‐slope conversion process due to basin boundaries and sea mounts. Omnidirectional measurements made below the sound channel critical depth, in sparsely shipped basins, and at high sea states indicate two distinct regions divided by the occurrence of breaking waves. Prior to wave breaking, a possible sound generation mechanism is the interaction of surface waves and turbulence in the near‐surface layer [I. Z. V. Gonchavov, Atmos. Oceanic Phys. 6(11) (1970); Yen and Pertone NUSC TR5833 (1979)]. Wave breaking produces bubble clouds. The dynamic evolution of these bubble clouds is a mechanism for the production of sound of frequencies less than 500 Hz. These clouds of micron‐size bubbles are regions of low sonic velocity described by Wood's volume fraction equations. These regions can be treated as a compressible body with a composite mixture speed and density that can exhibit a collective resonant oscillation and radiate as monopole and dipole sources. However, due to the proximity of the sea surface, only the monopole and its image, an effective dipole, would be of importance. When driven by the wave breaking vorticity and turbulence, these regions are shown to result in sufficient radiated sound to produce noise levels comparable to those observed and are also pronounced scatterers of low‐frequency sound.