Vertical directionality of low-frequency ambient noise in the ocean

Abstract

Using a 532-m-long vertical array suspended near the sound channel axis, the vertical directionality of ambient noise in the 10–50-Hz frequency band was measured at a location about 200 miles southwest of San Diego. The outputs from 20 hydrophones, configured as a nonuniform spaced array, were digitized for later beamforming. The frequency domain beamforming produced beam noise level versus direction for 200 discrete frequencies, separated by about 0.2 Hz, covering the band from about 10–50 Hz. At times when there were no ships in the immediate vicinity of the array, the noise in the 20–50-Hz interval was concentrated in a near horizontal direction with most of the noise included in the angular segment from −14° to +14° with respect to the horizontal. Below about 20 Hz this strong, near-horizontal lobe of high noise appears to diminish with decreasing frequency. In many instances in the 10–15-Hz band, the noise no longer shows the strong concentration near the horizontal but instead appears to be almost isotropic. [Work supported by ONR.]