In Situ Investigation of Sea Surface Noise from a Depth of One Meter
SCRIPPS INSTITUTION OF OCEANOGRAPHY LA JOLLA CA MARINE PHYSICAL LAB
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The mechanism responsible for audible underwater surface noise, particularly at low wind speeds, is not well understood. Because most surface noise recording instruments are deployed at depths of tens to thousands of meters, they can only characterize the behavior of a population of noise sources, rather than individual noise mechanisms. For this reason, an instrument has been built and deployed at a depth of 1 meter to record the sounds and images of the sea surface during light winds. This self-suspending instrument contains an upward looking video camera, four hydrophones digitized at 20 kHz each, and positional sensors. A surface buoy, tethered 30 meters from the submerged instrument, continuously supplies the wind speed and direction. Recordings from the instrument indicate that surface noise produced during light winds is generated by small waelet spills which have no correlation with the amplitude or phase of long period swells moving faster than prevailing winds. The averaged acoustic spectra from a single energetic spill showed a slope of -5 dB per octave, matching the spectral slope first reported by Knudsen, Alford, Emling in 1948 for wind generated surface noise.