Bubble-related ambient noise in the ocean

Abstract

An analysis is presented of the mechanisms by which bubbles can generate ambient noise in the ocean and the resulting noise levels are estimated. Bubbles can be extremely efficient amplifiers of water turbulence noise up to 100–200 Hz. At higher frequencies, the Lagrangian spectral intensity of the turbulence is too poor for this mechanism to contribute. Above 1–2 kHz, however, the oscillations by which newly formed bubbles dispose of their initial energy is shown to lead to substantial noise levels. This same process cannot account for the noise in the frequency range intermediate between these two because it would require unrealistically large bubbles, with a diameter of 1 cm or more. A possible mechanism active in this intermediate range, in which relatively large levels of ambient noise are observed, is that of collective oscillations of bubble clouds. In all cases the results obtained by the formal derivations (which are based on an adaptation of Lighthill’s theory of aerodynamic noise) are substantiated by simple physical arguments. Other possible noise mechanisms in which bubbles are involved are also briefly considered.