Acoustic Source-Level Measurements for a Variety of Merchant Ships

This report describes a set of 50 source spectra obtained from ships of opportunity near Genova, Italy. The spectra were obtained from radiated-noise spectra measured on a towed array and a transmission-loss spectrum computed from a parabolic equation model. Ship characteristics are presented for the 32 spectra where the ship name was determined. The sub-sample of 36 spectra with known ship type is characterized in terms of 3 ship classes - passenger/ferries, cargo ships and tanker ships. The ship class percentages for this sub-sample differ significantly from ship class percentages for either a Mediterranean or world population. The source spectra are characterized in terms of the mean source spectrum and source-level histograms computed over 3 frequency bands. The histograms for the low frequency and the medium frequency bands are approximately gaussian with standard deviations of 5 dB and 5.5 dB respectively. The histogram for the high frequency band is skewed somewhat towards the low source-level values with a S.D. of 6.8 dB. The source spectra for the 3 ship classes have comparable means and comparable S.D.S. We conjecture that both the mean spectrum and the source-level histograms are representative of shipping in other regions with different ship class percentages. Original contains color plates: All DTIC and NTIS reproductions will be in black and white.