Deep-Ocean Ambient-Noise Spectra in the Northwest Atlantic

Abstract

Ambient sea noise was measured from an omnidirectional hydrophone located in the deep-ocean environment. The hydrophone is installed in about 2400 f and is suspended 400 ft above the ocean bottom. The results illustrate the time variability and the correlation of deep open-ocean ambient-noise spectra with simultaneous wind-speed and wave-height measurements recorded during typical winter conditions. Broad-band ambient noise levels were automatically recorded on magnetic tape for 2 min every 2 h. The average value of ambient-noise in 25 logit-filter bands of frequencies ranging from 11 to 2816 Hz was measured for each 2-min sample. Autocorrelations of ambient-noise levels, wind speed, and wave height, as well as cross-correlation of ambient-noise levels with wind speed and wave heights, are presented. The ambient-noise spectra show the superposition of two major noise sources. One noise source is suspected to be distant shipping. This noise source predominates in the frequency range between 17 and 112 Hz and shows noise maxima occurring at 20 and 60 Hz. The other noise source is wind dependent and is observed below 17 and above 112 Hz. The variation in the standard deviation as a function of frequency at low wind speeds is suspected to be the result of the superposition of the two noise sources. The correlation coefficient of wind speed or wave height to sea noise varies as a function of frequency and wind-speed group.