Search

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.

An underwater package has been developed and deployed to determine the cause of surface noise generated at low wind speeds. The self-suspending instrument, tethered 1 km from a support ship, contains a video camera and four hydrophones, as well as orientation sensors. The signal from each hydrophone is filtered and digitized to cover a frequency range of 800–8000 Hz. Data recorded from a depth of 1–2 m indicates that considerable noise is generated by tiny surface spills. The sound from each spill is composed of distinct decaying oscillations which only last for a few milliseconds. These oscillations match the resonant bubble signatures described by Strasberg [J. Acoust. Soc. Am. 28, 20–26 (1956)] and Medwin and Beaky [J. Acoust. Soc. Am. 86, 1124–1130 (1989)].

A series of experiments conducted during low sea state conditions show that surface-related ambient noise is significantly reduced beneath monomolecular films. Although the amount of noise reduction varies between experiments, the highest attenuations reach 8 dB for frequencies of 1 kHz and higher. The ambient noise spectra beneath these films generally resemble those of nonfilmed lower sea states. Preliminary low sea state experiments, providing simultaneous aural and visual monitoring of the sea surface from a depth of one meter, suggest that, in the absence of whitecapping, films attenuate surface noise by dramatically reducing the number of microbreaks which are associated with bubble entraining noise.

In sea states, two‐six monomolecular films on the ocean surface have been previously reported to dramatically reduce the ambient noise field beneath them. The mechanism was presumed to be associated with ocean whitecapping. A recent series of low sea state tests with films, in the absence of whitecapping, has found an even more pronounced ambient noise reduction if the ocean surface agitation is above a certain critical level. The mechanism through which the films reduce surface noise at low sea states is explored.