SURFACE SHIP NOISE REDUCTION

A hydrophone placed in the sea will receive a wide range of sounds generated by a variety of sources over a large sea area. These sources could be shipping, marine life, flow noise around the hydrophone, weather effects, etc., and as such are variable in nature. The noise produced by ships and submarines has to be identified from this background before underwater (UW) noise can be used as part of a detection and weapon system. The physics of sound wave propagation in the sea has been the subject of considerable study and is discussed in detail in many books and reports, including references 1 and 2. Under ideal conditions, sound waves can travel many hundreds of miles. Ideal conditions, however, seldom exist over large areas and so the path of a sound wave can be distorted and modified many times between the noise source and the receiver. Factors such as: (a) refraction of the sound waves due to thermal and pressure gradients; (b) reflections from the surface, seabed, or layer boundaries within the water; (c) different attenuations associated with various frequencies; (d) scatter due to foreign bodies in the sea; may all be relevant when considering the noise path to the receiver. The greater the length of the noise path, the more likely it is that the sound wave will be affected by at least one modifying effect. For naval surface ships, the two areas in which underwater noise is of particular interest are anti-submarine warfare (ASW) and mine countermeasures (MCM). This article describes the background to the need for UW noise reduction and some of the methods that can be used to achieve low signatures. The opinions expressed are those of the authors and do not necessarily represent official MOD policy.