Low-frequency sound generation from breaking waves: Experimental measurements and modeling.

The first at-sea measurements capable of spatially resolving the individual breaking wave contributions at frequencies below 400 Hz, where collective bubble oscillations are believed to be the dominant sound generation mechanism, were conducted in shallow water (200 m) near San Clemente Island in March/April 1995 using a vertical array deployed from the RP/FLIP. The time-frequency structure of the noise generated from individual breaking waves was obtained using the endfire beam of a vertical array along with optical measurements of the locations, sizes, and lifetimes of those breaking waves. Strong acoustic bursts in time coincidence with the larger breaking wave events were observed. Approximately one-third of breaking wave source spectra contained large peaks at 40–60 Hz. Additionally, a theoretical model based on the excitation of an entrained bubble cloud by individual point sources is presented to explain the low-frequency peaks evident in the measured source spectra. Statistical characterizations of the association of breaking-wave size and the broadband acoustic energy will be presented. [Work supported by ONR base funding at NRL.]