ASA 126th Meeting Denver 1993 October 4-8

3aUW1. Shallow-water noise---A review.

Robert J. Urick

11701 Berwick Rd., Silver Spring, MD 20904

Like most aspects of underwater sound, the noise in the sea received its first quantitative attention during World War II, when a team under the venerable acoustician V. O. Knudsen made measurements in the shallow coastal waters off Southern California and Florida---ostensibly for the purposes of acoustic mines and harbor protection sonars. At that time too, the pernicious snapping shrimp of the warm shallow tropic waters of the Pacific were investigated. One of the principal features of shallow water noise is its variability---from place to place and time to time. Yet it may be said that there are only three major noise sources in shallow water: ships and other man-made activities at moderately close ranges, biologics, and the wind or waves. Strangely, when the first two of these are absent, the noise levels in shallow water at frequencies above 1 kHz or so are the same as in deep water; the so-called ``Knudsen'' curves have long been used in deep water. At lower frequencies, many shallow locations are often more quiet than deep water because of the absence of deep-going favorable transmission paths. When environmental conditions can be estimated, one's knowledge of shallow-water noise should be good enough to permit a prediction of the expectable noise level at an arbitrary location---at least as good as one's prediction capability of transmission loss out to useful ranges. [Work supported by the Applied Research Laboratory, The Pennsylvania State University, and the Office of Naval Research.]