Bubble‐related ambient noise in the ocean

Several mechanisms by which bubbles can contribute to ambient noise in the ocean are described and their effectiveness estimated. At low frequency, up to a few tens of Hz, bubbles are driven into oscillation by oceanic turbulence. The normal quadrupole radiation mechanism of turbulence thus acquires a monopole character. At frequencies from around 1 to a few kHz single bubbles formed by breaking waves radiate in free oscillation. In the range of hundreds of Hz the acoustic emission may be due to collective oscillations of systems composed of many bubbles. The efficiency of all these mechanisms is estimated on the basis of an adaptation of Lighthill's theory of aerodynamic noise. Finally, at frequencies above several kHz, drop impact and free oscillations of bubbles thereby produced appear to be responsible for the ambient noise. [Work supported by SACLANT ASWR, La Spezia, Italy.]