Bubble noise and wavelet spills recorded 1 m below the ocean surface

A remote instrument has been used to record the sound and environment of small surface spills in light winds from a depth of approximately 1 m in the open ocean. Recordings from the instrument indicate that these small breaks have no correlation with the amplitude or phase of long-period swells moving faster than prevailing winds. The sound from the spills, which is composed of a number of distinct resonant bubble oscillations, is very similar to that described by Medwin and Beaky [J. Acoust. Soc. Am. 86, 1124–1130 (1989)] for windless artificial wave breaks. Peak oscillation source pressures range up to 1.2 Pa. The average of several acoustic spectra from a single energetic spill has shown a slope of −5 dB per octave over the frequency range of the instrument, roughly 500–8000 Hz. The unique frequency for each oscillating bubble within a spill indicates that bubbles are “rung” as they are formed during entrainment, die out exponentially within milliseconds, and then no longer contribute actively to the acoustic record. Analysis of the acoustic energy generated by a number of bubbles versus frequency suggests that the −5 dB per octave wind-dependent ambient noise slopes of the Knudsen curves [J. Mar. Res. 7, 410–429 (1948)] are caused by the shorter lifetimes of high-frequency bubbles, rather than significantly lower peak pressures.