Measurements of acoustic ambient noise in shallow water due to breaking surf

Abstract

Horizontal directionality of ambient noise was measured at ranges up to 15 km from the southeastern shore of Monterey Bay, California. Water depths at the sites ranged from 8–175 m. A steerable cardioid receiving pattern was formed using signals telemetered from dipole and omnidirectional hydrophones suspended from tethered buoys. With no nearby shipping, whenever the maximum of the cardioid pattern was directed toward the beach, noise levels in the frequency range 20–700 Hz were greater than those obtained when the maximum was directed seaward. This difference or anisotropy (seaward versus shoreward), which depends on range from the beach, on frequency, and on surf intensity, was 10 dB at 300 Hz at the 9-km site during very heavy surf. Surf beat was clearly audible when the cardioid maximum was steered shoreward at ranges as great as 2 km. The anisotropy effects diminish both in magnitude and frequency range with lower wave height but are still observable during light surf. During heavy surf, the omnidirectional ambient noise levels also increased significantly in the same frequency range at which the anisotropy is evident. The spectrum levels during heavy surf compare reasonably well with data reported by Knudsen et al. [J. Mar. Res. 7, 410–429 (1948)] and by Piggott [J. Acoust. Soc. Am. 36, 2152–2163 (1964)]. We conclude that intense breaking surf can contribute significantly to ambient noise in fairly deep continental shelf waters.