“20.log(r)”), or cylindrical spreading, (corresponding to a reduction in received level with range of “10.log(r)”). In practice, the spreading may lie somewhere between these two geometries and be described by “N.log(r)” where N typically has a value between 10 and 20. Such simple models do not include the effect of absorption in the medium. This may be included in a simplified manner by introducing a term which describes the reduction due to absorption with range (leading to a term of the type “α.r” where α is the absorption in dB per meter). A composite model of this kind would then be used to calculate the received level (RL) from the source level (SL) by: RL = SL – N.log(r) – α.r (Nedwell et al. 2007a). This type of model can also be adapted to include frequency dependent attenuation (Thiele 2002; Thomsen et al. 2006).
9.9.2.4 Comparison of models 183. Simple “lumped parameter” spreading models which incorporate simplified absorption, and conform to the general type “RL = SL –N.log(r) – α.r”, have been used in previous UK studies which attempt to estimate the likely noise levels generated by windfarm construction (Nedwell et al. 2007a). These models have the advantage that they do not require a large amount of input data (only values of N and α), are simple to compute for measured values of received level versus range, and may be set up to replicate the apparent transmission loss of the sound measured during piling operations at other windfarm sites. However, the limitations of these models should be considered carefully. Such a model does not account for transmission loss effects due to changes in bathymetry, and so cannot (for example) predict the extra reductions in level caused by sand banks and shallow coastal areas (for example due to the effect of mode stripping). In addition, such models do not include reverberation or consider the sound transmitted through the sediment, except in a highly simplistic way (e.g., by use of a composite value of α). Such a model is also frequency independent if it is applied to a time-domain parameter such as peak-to-peak sound pressure. This means it will depend only on range from the source. In practice, the transmission of sound in shallow water will show a strong dependence on frequency due to the modal nature of the propagation and the frequency-dependent absorption in the water and in the sediment. These phenomena will cause the time waveform to distort during propagation away from the source, typically causing a dilation of the acoustic pulse (an increase in pulse duration) and a reduction in high frequency content.
184. For the very shallow water environments, the normal mode and Parabolic Equation approach outlined above has the potential to provide good accuracy. This method can be made to incorporate the effects of variable bathymetry, sound speed profiles and frequency dependent absorption. However, such models do require a large
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 89
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