Low-Frequency Acoustic Source Levels of Large Merchant Ships

: Acoustic source-level spectra have been measured for 14 merchant ships, including eight large oil tankers, for the frequency range 2 to 128 Hz. Results are compared with semiempirical cavitation models: a discrete-line model (Gray and Greeley, 'Source Level Model for Propeller Blade Rate for the World's Merchant Fleet', Technical Memorandum 458) (Bolt, Beranek, and Newman, Inc., Cambridge, MA), and a model for broadband levels (Ross, Mechanics of Underwater Noise, Pergamon Press (1976)). The distribution of measured monopole source levels for the second blade harmonics of these ships peaks at about 175 dB re 1 microPa, consistent with the Gray-Greeley model, and individual levels agree within the 6 to 8 dB variance by implied by the model. Measured levels in the continuum, at 50 and 100 Hz, average 167 and 161 dB re 1 microPa, respectively, about 6 dB below the levels predicted by the Ross model. Below 15 Hz, measured line and broadband levels increase at about 8 dB per octave of decreasing frequency, a systematic difference from cavitation model predictions. This is possibly caused by hull reradiation effects, suggesting a low-frequency limit for the validity of a point-source model for these large ships. This report includes a set of broadside source-level spectra at several ranges for each ship, and tables of ship characteristics and engineering data. These acoustic measurements and the associated data comprise a data bank suitable for further study of ship-generated noise.