Depth dependence of ambient noise in the northeastern Pacific Ocean

Omnidirectional ambient noise levels were measured at two deep-water locations in the northeastern Pacific Ocean. Hydrophones were positioned throughout the water column at depths ranging from about 200 m below the surface to about 150 m above the sea bottom. Analyses of the data over the frequencies from 15 to 800 Hz show that at low frequencies the noise levels decrease with increasing depth. The decrease with depth is greater below the critical depth than it is in the sound channel. These low-frequency noise levels, and their depth dependence, are independent of the wind speed. At higher frequencies the noise levels and the depth dependence are controlled by the wind-generated noise. At low wind speeds there is a decrease in levels below the critical depth, but above this depth both increases and decreases in levels with depth were noted. At these high frequencies during high wind speeds the noise levels not only rise, but also fill the water column to the extent that there is little decrease in level with increasing depth, even for the region below the critical depth.