Acoustic source-level measurements for a variety of merchant ships

Abstract

This report describes a set of 50 source spectra obtained from merchant ships of opportunity near Genova, Italy. These spectra were obtained from radiated-noise spectra measured on a towed array and a transmission-loss spectrum computed from a parabolic equation model. The characteristics of the ships 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 three ship classes - passenger/ferries, cargo ships and tankers. It is seen that the ship class percentages for this sub-sample differ significantly from the ship class percentages for either a Mediterranean population or a World population. The source spectra are characterized in terms of the mean source spectrum and sourcelevel

histograms computed over three frequency bands. The mean spectrum is comparable in level and shape to a spectrum computed from a well-known empirical model. The source-level 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 standard deviation of 6.8 dB. Finally, it is seen that the source spectra for the three ship classes have comparable means and comparable standard deviations. From this result we conjecture that both the mean spectrum and the source-level histograms are representative of the shipping in other regions with different ship class percentages.