Particular vessels produce unique acoustic signatures associated with noise source levels and frequency
bands. Sharp tonal peaks produced by rotating and reciprocating machinery such as diesel engines, diesel
generators, pumps, fans, blowers, hydraulic power plants, and other auxiliaries often are seen in these
acoustic signatures. Hydrodynamic flow over the ship’s hull and hull appendages is an important
broadband noise-generating mechanism, especially with increased ship speed. At relatively short ranges
and in isolated environments, the spectral characteristics of individual ships can be discerned. At distant
ranges in the open ocean, multiple ships contribute to the background, and the sum of many distant sources
creates broad spectral peaks of noise in the 5 to 500-Hz band.