Ambient-Noise-Spectrum Levels as a Function of Water Depth

Abstract

Recent measurements have indicated that the level of ambient sea noise is a function of water depth. The results presented in this paper were obtained from measurements made simultaneously with four hydrophones located in the open ocean, on the slope of a sea mount, in an area of the Northwest Atlantic. The four hydrophones were located in water depths of 400, 1100, 2400, and 2500 fathoms (f). All hydrophones were bottom-mounted except for the hydrophone at 2400 f, which was suspended 400 ft above the bottom. The ambient-noise levels are compared with wind-speed measurements recorded simultaneously from an anemometer system located on a fixed platform in the area of the hydrophones. The data were obtained during the month of January. Broad-band ambient-noise levels from the four hydrophone outputs were sampled simultaneously every 2 h for a 2-min period. An average value of ambient noise in logit filter bands of frequencies ranging from 11 to 1414 Hz was estimated for each 2-min sample. The results show a dependence of ambient-noise levels as a function of water depth, frequency, and wind speed. The spectrum slope as a function of water depth is also observed to be dependent on wind speed. The variations in ambient-noise levels as a function of depth produce a triangularly shaped pattern. This pattern illustrates the changing-spectrum slope as a function of depth. The changing-spectrum slope and the ambient-noise-level depth dependence are probably caused by the two major directional noise sources existing in the open ocean. One noise source is sea surface and is generated by local wind-speed conditions; the second source is generated by long-distance shipping. The surface-generated noise signal is primarily locally generated and arrives at the hydrophones principally via direct paths. Distant shipping contributions arrive at the receiver via refracted, surface reflected (RSR) and wholly refracted (RRR) paths.