A ship's radiated noise monitoring system includes a plurality of arrays of spaced vibration-responsive detectors, the arrays being adapted for location adjacent to the ship's hull in the region of possible noise sources; summing amplifiers for receiving the detectors output signals from respective arrays and providing output signals representative of the average vibration signals received by each array; an analyzer connected to the output of each summing amplifier to perform a spectrum analysis; and computer apparatus to receive the noise spectrum from each array, apply respective weighting factors to the noise spectra and combine the weighted noise spectra to produce a predicted radiated noise spectrum. Preferably each array comprises a regular distritution of nine accelerometers attached to hull frames in groups of three. The system is used in conjunction with sonar equipment to enable self-noise to be monitored to optimise the sonar performance.