Identification of Sources and Their Directivity in the Global Underwater Radiated Noise from a Merchant Ship
Abstract
The global marine traffic is intensifying and generates noise pollution, directly affecting marine biodiversity. To limit the effects of marine traffic on the environment, traffic noise must be reduced. Current standard (ANSI/ASA S12/64-2009) models a ship’s acoustic signature as a single, punctual and omnidirectional source. However, the noise sources under consideration, like engines and propellers, are actually spatially distributed over the ship’s dimensions. This project aims to analyse separately individual noise source and establish their directivity pattern, to better understand and model underwater noise radiated by ships.
The SASMAR station (Signatures Acoustiques Sous-MArines Rayonnées as Underwater Radiated Noise Signatures) was deployed in the maritime railway in the St-Lawrence Estuary to measure merchant ships acoustic signatures. Including four vertical three-hydrophone arrays, its unique design allows measuring underwater radiated noise of individual ships from starboard and port side at three vertical angles.
In this study, three passages of a merchant and passengers ship through the station are analysed. From the narrow band acoustic signatures, specific features like frequency peaks corresponding to machinery and wide band low frequency noise typical from cavitation are identified. Since underwater radiated noise is measured from multiple directions while the ship approaches then sails away from the measuring station, a directivity map is also built for each identified source.
Knowledge of sources directivity at various frequencies will allow the improvement of ships underwater radiated noise models and better assessment of the impact of shipping on the marine environment. Also, source directivity will improve the understanding of ship detection by cetaceans and therefore the prevision of collision risk.
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