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for frequencies > 1000 Hz. The increase in spectral level with decreasing frequency does not continue; instead a spectral plateau, or even a slight decrease in spectral level with decreasing frequency (i.e., the opposite trend), is characteris- tic of purely wind-speed dependent noise, although this is difficult to see in data owing to shipping related noise that tends to dominate lower frequencies. However, this is clearly evident in the wind-related noise data curve corresponding to a wind speed of 5 m/s21 taken in Australian waters where shipping traffic is much reduced. Furthermore, a recent refined analysis of the wind dependence of low-frequency (< 500 Hz) ambient noise measurements from the 1975 Church Opal data set taken midway between Hawaii and California22 displays the same trend.
Rain can produce a peak in the ambient sound pressure
spectral density in the vicinity of 15 kHz, as is shown by the
range of data depicted by the green shaded area, correspon-
ding to rain rates from 2 to 5 mm/hr measured at different
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2 24
may reach levels of 80 dB re 1 μPa /Hz at 10 kHz. The Wenz
curves do not reflect the contribution from rain per se, and thus the explicit contribution from rain is not included in our simplified descriptive curves for high and low noise pressure spectral density.
Underwater ambient sound from commercial shipping is typically quite variable given that this contribution is strong- ly modulated by both shipping activity and environmental conditions for long range acoustics propagation. A maximum in spectral level, in the vicinity of 50–100 Hz, is often observed with a decay in frequency thereafter at a rate some- what higher than that observed for wind-related noise;6 this is apparent in analysis of data taken from the bottom mount- ed SOSUS array (depth ~2000 m) located on the continental
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over the years 1963–1965. A more recent study, also
involving continental slope waters off California, reports a similar increase in noise level believed to be associated with increases in commercial shipping activity. Not necessarily comparable to these data, but still of interest, is the data curve in Fig. 2 representing measurements taken in the Norwegian Sea over 40 years ago28 using bottom mounted hydrophones in waters of depth 600 m. The noise spectral levels are a com- bination of “moderately heavy traffic composed of diesel- engine trawlers and smaller fishing craft,” and rough seas and high winds.
Ambient noise levels readily exceeding the nominal high spectral density curve in Fig. 2 were measured by three experi- mental groups29 involved in the 2001 Asian Seas International Acoustics Experiment (ASIAEX) in the East China Sea in waters of depth ~100 m. A composite pressure spectral density composed of the three different but overlapping frequency
The spectral peak is associated with acoustic resonance from the dominant bubble sizes created through the impact of rain drops;23 at higher rain rates, however, this peak tends to be obscured and the spectrum flattens out and
wind speeds.
These data, from 1995–2001, represent long-term averages that can reflect seasonal and yearly trends. Their higher level is postulated to be associat- ed with increases in anthropogenic noise due to shipping over recent decades as evidenced by a comparison (also in Fig. 2) of measurements made using the same receiver but
slope off Point Sur, California.
ranges measured by each group is shown by the three blue-col-
ored lines of varying thickness. In this case the primary source
of noise is a fleet of ~30-m-long fishing trawlers (as wind speed
during the observation period was less than 3 m/s) operating
constantly in the immediate experimental area with a vessel
density of approximately 1–2 vessels per square kilometer. Like
intense fishing activity, busy harbors in shallow water invariably
possess high ambient noise levels, as seen in the data taken in
the Korean Straits (depth ~100 m) within the shipping lanes of
the large port of Busan,30 representing a spectral average over 30
min, during which the wind speed was 2 m/s. Also shown in
Fig. 2 are measurements made at very high frequency (30 and
90 kHz) within the confines of the port of Long Beach,
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California. Interestingly, similar high frequency measure-
ments from other ports are reported in Ref. 31 and the general tendency of the noise level to increase with decreasing port lat- itude was observed and presumed to be associated with a greater abundance of snapping shrimp (more on this below). We nevertheless anticipate that noise spectral levels from a typ- ical port environment, or otherwise from waters of concentrat- ed commercial shipping or fishing activity, and derived from a suitably long-term average (say, 30 min or more), to exceed our nominal high spectral density curve by ~10–20 dB.
A view of underwater ambient noise based on the com- bination of sea state and commercial shipping contributions (including fishing or recreational boating) alone is, of course, far from complete. Biological noise from fish, invertebrates
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Biological noise may form the major back- ground noise in some areas. The “tropical biological back- ground” in Fig. 2 shows the range of background noise in waters north of Australia where the ambient noise is mostly from biological sources and is dominated by snapping shrimps above 2 kHz. Biological choruses that result when large numbers of animals are calling, typically of a few hours duration, are common, especially following sunset. In the range of a few hundred hertz to a few kilohertz, levels over a
wide area can be as high as the highest levels in Fig. 2.
In ice-covered waters that are often far removed from distant shipping sources, a very different picture emerges. In the central Arctic, low frequency (10–20 Hz) noise is corre- lated with stresses applied to the pack ice by a combination of winds, currents, and pack ice drift,36,37 and levels near the spectral peak (~15 Hz) can reach those produced by shipping sources. In conditions where these stresses are reduced, under-ice conditions can produce very low levels of under- water ambient sound. For example, measurements made under smooth, 3-m thick Antarctic sea ice in McMurdo Sound during the austral summer of 196938 fall well below our nominal low noise spectral density curve. In this case, the smooth surface of the McMurdo ice sheet is thought to have minimized the effect of the wind, and wind speed itself was minimal during the measurement period. The spectral peak near 400 Hz is a biological component associated with phonations of Weddell seals. Were we to remove the light shipping activity contribution from our low noise curve, the result would be closer to the Antarctic measurements for the
and whales is an important component of ambient noise. The most ubiquitous biological contribution is that produced by snapping shrimp, which abound in shallow temperate and
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tropical waters.
Underwater Ambient Noise 27