Contributions to VLF noise by merchant ships

Modified SQS-57A (XN-5) Sohobuoys with hydrophone sensors at 305 m depth were deployed in line array patterns to obtain estimates of the sound pressure levels corresponding to 22 assorted merchant ships in the frequency band 1–128 Hz. These were ships of opportunity enroute over the deep ocean basins of the Northeast Atlantic and the Mediterranean Sea. The data was taken on two field trips, August 1975 and January 1977. The ships ranged in size from 326 down to 12 thousand dead weight tons. These procedures yield an adequate representation of the contribution of merchant ships to the lower-frequency spectra whenever the sea state was three or less, and the range to the closest point of approach was less than 600 m. Besides the line components that can be identified with the shaft and blade rate lines, and an associated sideband structure that is easily discernible, there is a broadband infrasonic structure attributable to the ship. The source of this broadband noise has as yet not been ascertained. Possible mechanisms are discussed in terms of measured horizontal and vertical directionality patterns. [Work supported by Naval Material Command.]