A methodology is presented for measuring the radiated noise from a ship in shallow water, and deriving the source level spectrum. The method is applied to the measurement of dredgers in UK waters, vessels which are restricted to shallow water during aggregate extraction. Estimation of source level requires an estimate of the transmission loss which accounts for the effect of both the surface and seabed. The measurement method used involves the simultaneous measurement of the radiated noise at a number of measurement stations, each consisting of hydrophones which are either deployed from a stationary survey vessel, or from an autonomous recording buoy. The measurements at up to four ranges from the source vessel allow for empirical estimation of the source level using appropriate transmission loss models, with the data analysed in third-octave frequency bands. Measurement results presented are for trailing suction hopper dredgers, which lower a drag head and suction pipe to the sea floor to extract the sand or gravel, whilst returning unwanted material and water over the side.. Noise levels are shown for the same dredger under different operational modes illustrating that, for these vessels, the noise output level is partially dependent upon the aggregate type being extracted.

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