Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No, 6945
BBN Systems and Technologies Corporation
of man-associated activities such as vessels and aircraft are presented in
Section 3.3 on a 1/3 octave band basis.
3.2.1 Meteorological sources (wind, rain, sleet)
An early major summary of ocean ambient noise, published by Knudsen et
al. (1944), has become a common baseline for comparison of ambient noise
conditions in both the deep water and shallow water environment. However, the
standard "Knudsen curves", which provide a means for estimating background
noise levels to be expected for particular wind or sea state conditions, apply
most reliably to deep ocean conditions, and then most effectively for frequencies
between 500 Hz and 50 kHz. Shallow water ambient noise levels, the focus
of this study, tend to agree with the Knudsen curves for frequencies above
1000 Hz but can vary considerably from site-to-site in continental shelf and
near-shore areas. Wenz (1962) and Urick (1983) provide useful ambient noise
summaries for the shallow water environment (as well as deep water). Figure
3.1 includes average shallow water spectra for typical wind and rain
conditions obtained from their summaries.
- Wind
On a 1 &octave basis, wind-related ambient noise in shallow water (Fig.
3.1) tends to peak at about 1 kHz. Levels in 1/3 octave bands generally
decrease at a rate of 3-4 dB per octave at progressively higher frequencies
and at about 6 dB per octave at progressively lower frequencies. Sound levels
increase at a rate of 5-6 dB per doubling of wind speed. Maximum 1/3-octave
band levels of about 95 dB referenced to 1 pPa are frequently observed at
about 1 kHz for sustained winds of 17-21 m/sec (34-40 knots) and about 82 dB
also at 1 kHz when the winds are in the 3.4 - 5.4 m/s or 7-10 knot range.
Since ambient noise related to wind is caused primarily by wave action and
spray (and possibly to some extent to acoustic and pressure fluctuation
coupling effects from air to water), the wind related noise component is
strongly dependent on wind duration and fetch as well as water depth, bottom
topography and proximity to topographic features such as islands and shore. A
sea state scale which is related to sea surface conditions as a function of
wind conditions is commonly used in categorizing wind-related ambient noise
Table 3.1). The curves for wind-related ambient noise shown in Fig. 3.1 are
reasonable averages, although relatively large departures from these curves
can be experienced depending on site location and other factors such as bottom
topography- and proximity to island or land features. Statistical estimates of
ambient noise conditions along the coast of the-Alaskan Beaufort Sea (Miles et
al. 1987) predict that the 95th percentile and 5th percentile levels of
ambient noise (due primarily to wind) are 10 to 20 dB above and -below the
median level respectively. The median levels in the Beaufort Sea, as shown by
-the* and o symbols in Fig. 3.1 and by Greene ( 1987), are close to the Sea
State 2 curve.
- Rain
Water droplets impacting the ocean surface can be a major high frequency
source of ambient noise in the ocean, depending on precipitation rate. As
described by Wenz (1962) and Urick (1983) and based on their review of