Underwater radiated noise from small vessels in shallow water: propagation modelling and experimental measurements

Abstract

Underwater noise is a growing problem in many of the world's oceans and waterways, with a large body of research now showing the detrimental impact this has on marine life. Noise in shallow coastal waters is a particular problem because the concentration of both marine life and human activity is often much higher. Furthermore, noise attenuates more slowly in shallow water, and so its impact is greater. For naval operations, this can increase the detection distance of a vessel, which places more onerous requirements on the designer to reduce the levels of radiated noise, and potentially degrades the operability of the vessel when operating in shallow littoral waters. In this work, the impact of small vessels on the levels of underwater noise in a shallow water environment is considered. Acoustic recordings are obtained using a hydrophone and the data are analysed to determine the sound levels and frequency content from different vessels. The noise levels recorded are found to greatly exceed those caused by environmental factors and occur primarily over the frequency range 63 ≤ f ≤ 500 Hz, unless propeller singing or cavitation are present, in which case excess noise levels can occur for frequencies up to 20 kHz. Over a 25-minute period, the mean overall sound pressure level is found to be in excess of 90 dB, which is attributed to only a small number of boats operating in the test area. To provide more insight into noise propagation in shallow water, numerical modelling is carried out by numerically solving the acoustic wave equation in the time domain, which allows for phenomena such as reflection, absorption, and wave interactions to be captured. Propagation of sound from a source located at the sea surface is computed for deep and shallow water, with two different seabed materials also being considered. The propagation pattern in shallow water is shown to be more complex than for deep water, with the reflection/absorption properties of the seabed playing an important role along with the depth.