Modelling the vertical directivity of noise from underwater drilling

To the novice, quantifying acoustic propagation from an underwater source is often limited to spherical spreading. This approach is justified when considering a high-frequency, omni-directional source located far from a boundary however there are other scenarios where this is insufficiently rigorous. Attention is drawn to the emerging marine renewables sector where acoustic propagation from sources of sound, in particular underwater drilling, cannot be modelled accurately using such a simple representation. For these, the variation in sound level radiating directionally from the source is a key parameter. It is noted that the sound directivity arising from the drilling site has, hitherto, received scant attention in the published literature with little data available. A pragmatic approach to modelling radiated noise from underwater drilling is therefore required. A simple frequency-dependent radiation pattern of sound from the source is proposed. The radiation pattern is used as an input parameter to calculate acoustic propagation losses determined using otherwise standard computer programs. The applicability of the directivity model is demonstrated by comparisons with real data acquired during the summer of 2011 when underwater noise was recorded during the drilling of a foundation socket for a wave energy device at the EMEC range, Billia Croo, Orkney.