On the possibility of estimating ship parameters using acoustical Lloyd's mirror effect

The Lloyd's mirror effect is an acoustic effect taking place when a source of noise moves parallel to a reflecting surface. In the case of ships, the underwater sources of noise, mainly the propeller, are at constant depth, i.e. a constant distance, from the free surface that can be considered a perfectly reflecting surface. In such a configuration, a destructive-constructive interference pattern is generated in the time-frequency domain by the combination of the direct and surface-reflected sound waves. As the shape of the acoustic patterns depends on the speed of the ship, the relative distance between the ship and the hydrophones and the sound celerity, it is in principle possible (it has been successively done for aircrafts) to derive the above mentioned characteristics simply analysing the corresponding Lloyd's mirror effect. This is particularly interesting when monitoring shipping noise traffic when data regarding ships cannot directly derived by the Automatic Identification System (AIS). In the paper a set of measurements carried out in the framework of two EU FP7 European Projects are analysed to discuss about the possibility of estimating ship parameters using acoustical Lloyd's mirror effect. During the measurements the main ship parameters have been monitored by GPS therefore representing a good reference test.