Measurement of radiated noise from surface ships - influence of the sea surface reflection coefficient on the Lloyd's mirror effect
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The assessment of ship underwater noise signature is highly important not only to a range of naval applications but for the assessment and mitigation of shipping related noise impact on marine animals. One of the main phenomena affecting ship radiated noise measurement is Lloyd’s mirror effect, which describes the interference between the direct sound path and the path reflected on sea surface. Having published a first standard for radiated noise measurement in deep waters for comparison purposes, the ISO committee on underwater acoustics is working towards an objective of correcting the Lloyd’s mirror effect. While it is assumed that the surface of the ocean acts as a perfect mirror, this is not often the case due to sea surface deformation. The aim of the present study is to investigate the influence of a non-perfect reflection coefficient on the Lloyd’s mirror effect. This is achieved through the use of different models taken from the literature that provide an effective reflection coefficient depending on frequency, grazing angle and sea state. Results are presented in the form of acoustic pressure maps at short propagation distances and of frequency responses at different observation points, for sea states up to 3. Simulations show that the effect is small at low frequencies, and at high frequencies a deviation of up to 3 dB appears. However, these results are dependent on the reflection coefficient model used.