Directivity Patterns of Ship Underwater Noise Emissions

The Underwater Radiated Noise (URN) from ships has gained growing attention in the last years. The evaluation of the acoustic impact of shipping activity (as well as of other anthropogenic noise types) on marine fauna has become an important issue for society and regulators. The normative framework on the subject is not yet in place, but it is in rapid evolution and in the process of being finalized soon, as reflected by the research efforts being deployed on the subject by the E.U. (see the completed SILENV project and the ongoing AQUO and SONIC projects). In parallel, at an international level, the same subject has been covered by the International Maritime Organisation (IMO) through the activity of a dedicated Working Group. A key aspect of the control problem for URN from ships is a proper characterization of the source represented by the vessel. This item presents peculiar aspects due to the environment in which radiation occurs, but it is also driven by the complex features and large dimensions of the ship source. An important issue is the directivity of the emission, which influences: the patterns of the radiation, the distance at which the presence of the ship is perceived and the acoustic impact of the vessel. In this paper, experimental surveys performed at sea within the SILENV project are analysed to derive the directivity patterns of different ships in different conditions at different frequencies. The work covers the measurement procedure adopted, as well as the post-processing techniques and the uncertainties affecting the results.