: By ambient noise we mean the prevailing, sustained unwanted background of sound at some spot in the ocean. It excludes momentary, occasional sounds, such as the noise of a close-by passage of a ship or of an occasional rain squall. It is the background of noise, typical of the location and depth where a measuring hydrophone is located, against which a signal," such as the sound of a submarine or the echo from a target, must be detected. Ambient noise also excludes all forms of self-noise, such as the noise of current flow around the measurement hydrophone and its supporting structure, and obviously must exclude all forms of electrical noise. Thus, ambient noise is what is left over, so to speak, after identifiable, occasional noise sources are accounted for.

Ambient noise in the sea

Hildebrand, J. A. Impacts of Anthropogenic Sound in Marine Mammal Research: Conservation beyond Crisis. Edited by J.E. Reynolds III, W. F. Perrin, R. R. Reeves, S. Montgomery, and T. J. Ragen The Johns Hopkins University Press, Baltimore, Maryland. Pages 101-124 (2005).

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Impacts of Anthropogenic Sound

The seasonal sea ice cover of the northern North Atlantic is reviewed, comprising the ice of the East Greenland Current, the ice cover around Svalbard, and the ice of the Barents Sea as far east as Franz Josef Land. The review considers various types of ice cover: (1) the fast ice along the coasts and in fjords, including ‘sikussak,’ or very old fast ice of exceptional thickness, (2) the transition zone between the fast ice and the main pack, including the positions and causes of recurring winter polynyas and the nature of the shear zone in NE Greenland, (3) the constitution, dynamics, and seasonal extent of the drifting pack ice, including interannual variations, (4) the processes of air-ice-ocean interaction which occur in the marginal ice zone, where the pack ice is influenced by an open ocean boundary.

The ice cover in the Greenland and Norwegian seas

From September 2006 to June 2009, an autonomous acoustic recorder measured ambient noise north of Barrow, Alaska on the continental slope at 235 m depth, between the Chukchi and Beaufort Seas. Mean monthly spectrum levels, selected to exclude impulsive events, show that months with open-water had the highest noise levels (80–83 dB re: 1 μPa2/Hz at 20–50 Hz), months with ice coverage had lower spectral levels (70 dB at 50 Hz), and months with both ice cover and low wind speeds had the lowest noise levels (65 dB at 50 Hz). During ice covered periods in winter-spring there was significant transient energy between 10 and 100 Hz from ice fracture events. During ice covered periods in late spring there were significantly fewer transient events. Ambient noise increased with wind speed by ∼1 dB/m/s for relatively open-water (0%–25% ice cover) and by ∼0.5 dB/m/s for nearly complete ice cover (> 75%). In September and early October for all years, mean noise levels were elevated by 2–8 dB due to the presence of seismic surveys in the Chukchi and Beaufort Seas.

Underwater ambient noise on the Chukchi Sea continental slope from 2006-2009.

Implications of Arctic industrial growth and strategies to mitigate future vessel and fishing gear impacts on bowhead whales

Quasisynoptic measurements were made to determine the spatial variability of ambient noise at the Arctic ice‐water boundary over a frequency range from 100 to 1000 Hz. The results of this investigation show that the ice‐water boundary acts as a spatially well‐defined source of ambient noise with measured noise levels near a compact edge about 12 dB higher than open water levels and about 20 dB higher than levels far in the ice field. Measured noise levels near a diffuse ice edge were about 4 dB higher than open water levels, and about 10 dB higher than levels far in the ice field. The relatively high noise levels at the ice edge are probably generated by wave and swell interactions with individual ice floes.

R. S. Winokur

Papers

Spatial variability of underwater ambient noise at the Arctic ice‐water boundary