Noise generation by newly created bubbles

Abstract

Air bubbles trapped at a free surface by breaking waves, or emerging from an underwater nozzle, or splitting, or coalescing in various ways, all undergo strong “shape oscillations” before settling into a spherical shape. In a fully nonlinear theory (not the usual linearized approximation), the shape oscillations can be shown to emit a strong monopole radiation of sound. Second‐order calculations suggest that this is a significant, perhaps dominant, source of bubble noise in the ocean. Resonance between the second harmonic of each shape oscillation and the radial “breathing” mode of a given bubble leads to enhanced emission of sound at or near the breathing‐mode frequency. There can also be coupling between a bubble and the free surface from which it is detached. Oceanic data and laboratory experiments relating to these predictions will be discussed, particularly the well‐known experiments by Fitzpatrick and Strasberg (1957); also, broadband spectra of oceanic noise from a variety of locations will be discussed.