turbine will generate a continuous type noise signal but the resulting broadband noise levels are generally considered to be relatively low in level, and would hence be expected to be restricted to a small area around the turbine. For harbour porpoise specifically they are believed to be too low in level and frequency to cause masking problems (Tougaard and Henriksen 2009). This is further discussed in Section 9.6.2.
9.5.2.5 Audibility 56.
The audible distance or the physical range over which marine species can hear the construction activity will extend to the distance that the sound either falls below the ambient perceived sea noise level or the auditory threshold of the animal. Whether the sound is audible to an animal is not usually a consideration used for impact assessment, since impact is usually judged in terms of physical or behavioural effects triggered at levels that exceed mere audibility thresholds which may already be within the ambient noise level. There may be no consequence, negative or otherwise, of the animal hearing the sound.
9.5.3 Review of Available Criteria Data for the Impact of Underwater Noise on Marine Mammals
57.
The US Marine Mammal Criteria Group of the NMFS (National Marine Fisheries Service part of NOAA) have proposed the 'M-weighting' model (Southall et al. 2007), as part of the Marine Mammal Noise Exposure Criteria. They classify marine mammals into one of five bands: three for cetaceans: low, mid and high-frequency and two for pinnipeds: water and air (see Table 9.2). Harbour porpoise was considered as the only high frequency cetacean expected in the general area in and around the East Anglia Zone, as other species do not occupy UK waters or are generally rare and found in deeper water (e.g. Kogia) (Reid et al. 2003). The M- weighting is applied in much the same way as the ‘A-weighting’ is applied in airborne acoustics when considering the perceived response of a human receptor. The marine mammal noise exposure criteria were developed through consensus of an expert committee and peer-reviewed. They are perhaps the most developed and recognised exposure criteria for marine mammals, compiling the findings of much of the published literature, including key work on the effects of noise on marine mammals by Finneran et al. (2000; 2002a and 2002b) and Lucke et al. (2007). In the development of the criteria published audiograms shown were considered. The criteria are rapidly finding acceptance internationally and are now being recommended in the UK for use in environmental impact assessments.
58.
In the case of the SEL, a series of filters have been developed analogous to human hearing response weightings (M-weighting is actually more analogous to the human
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 19
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