Ambient noise and surface wave dissipation in the ocean
Citable URI
https://hdl.handle.net/1912/5695Location
OregonDOI
10.1575/1912/5695Keyword
Ocean waves; Underwater acoustics; NoiseAbstract
There is a growing consensus that the sound generated by breaking waves is
responsible for much of the ambient noise level in the ocean. While numerous field
measurements have shown a strong correlation between the ambient noise spectrum
level (N) in the range 100Hz to 25kHz and wind speed in the ocean, very little has
been done to establish a comparable correlation between the ambient noise spectrum
level and surface wave field parameters. The difficulty in establishing this relationship
is remarkable given that the frequency and intensity of wave breaking are dependent
on the characteristics of the wave field.
In Fall 1991, an experiment was conducted from the research platform Flip 130
kilometers off the coast of Oregon, where the ambient noise between 2.5 and 25 kHz,
the wind speed, and the sea surface elevation using wire wave gauges were measured.
The correlation between N and the root mean square wave amplitude a was
found to be poor but could be improved if the swell was filtered out from the wave
elevation time series. The influence of swell on the value of a was disproportionate to
the level of ambient noise since its characteristics were not directly due to the local
wind-wave conditions. Observations of the dependence of the high frequency wind
waves and the directional wave spectrum under turning winds suggested that the high
frequency wave components responded more quickly to changes in the wind speed and
wind direction than the energy-containing frequencies.
The ambient noise level also correlated well with the root mean square wave
slopes. This is consistent with previous laboratory measurements which showed that
the steepness of a packet of waves correlates with the strength of wave breaking and
with characteristics of breaking waves such as loss of momentum flux, dissipation,
initial volume of air entrained, mixing, and sound generation.
Comparisons of surface wave dissipation estimates using field measurements
and models developed by Phillips (1985) and Hasselmann (1974) show that although
the two models have very different forms, they give values that are comparable in
magnitude. The relationship between the ambient noise level and log of dissipation give
correlation coefficients (0.93-0.95) that are comparable to those between ambient noise
and wind speed. The mean square acoustic pressure was shown to vary with the
dissipation, with p2 ∝ D0.6-0.8. The results suggest that measurements of ambient sound
may prove to be useful in inferring surface wave dissipation.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1993
Suggested Citation
Thesis: Felizardo, Francis C., "Ambient noise and surface wave dissipation in the ocean", 1993-06, DOI:10.1575/1912/5695, https://hdl.handle.net/1912/5695Related items
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