The effect of water surface roughness on the measurement of radiated ship noise

As a result of the increased interest in the potential environmental impact of noise from shipping several international standards have been developed for determining the radiated noise level and equivalent monopole source level of vessels. It is important to understand the systematic errors and uncertainties associated with such techniques. One potential issue is the effect of water surface roughness on the measured radiated noise. The roughness reduces the amplitude of the coherent reflection from the sea surface and therefore reduces the impact of Lloyd’s Mirror effect at higher frequencies. In order to investigate this further two cases are considered: a deep water case with translationally symmetric surface waves and a shallow water case with a two-dimensional rough sea surface. The deep and shallow water cases are simulated using an equivalent source method based model and a three-dimensional Helmholtz-Kirchhoff approximation, respectively. The results indicate that the incoherent scattering component almost completely compensates for the coherent component loss. The variation in the results for different sea surface implementations indicates how the sensitivity of the measurement varies with frequency.