strong dependence on frequency due to the modal nature of the propagation in the shallow-water channel 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 changes in the frequency content.
17. Ambient noise originates from a range of noise sources, both natural and anthropogenic and spans a large frequency range from below 1Hz, to well over 100kHz. It is most commonly expressed as spectral density levels in TOB in units of dB re 1 μPa2Hz-1, where the values have been divided by the bandwidth of each TOB. This is different from TOB power spectra (dB re 1 μPa2) more appropriate for radiated noise, where the total energy in the signal is of interest. In general, ambient noise measurements in the UK coastal waters indicate that maximum TOB
spectral noise density levels are typically between around 95 and 120dB re 1 μPa2Hz- 1 with these peak band levels occurring between frequencies of a few tens of hertz to a few hundred hertz, depending on location and time (Nedwell et al. 2007a; Theobald et al. 2010; Robinson et al. 2011).
18. Underwater sound can potentially have a negative impact on marine mammals and fish which can include death, and physical injury, and effects such as behavioural disturbance and masking (refers to reduced ability of an animal to hear a sound due to increased noise).
19. A number of sounds with various characteristics have been associated with the construction, operation and decommissioning of offshore windfarm developments. Noise from piling during the construction phase will be the prevalent source with the potential to significantly impact marine life. This entails driving a wind turbine foundation into the ground using an impact hammer and usually involves a gradual ramp-up in hammer energy (soft-start) until the maximum applied hammer strike energy is reached, which may then continue for several hours. Underwater noise from impact piling is known to generate considerably high peak pressure levels and SEL and may be expected to be distinguishable above ambient noise over distances of several tens of km from the source (Thomsen et al. 2006 and Nedwell et al. 2007a). The likely impact would depend on a number of factors which include the foundation type, the installation method, noise propagation conditions in the area and the noise sensitive marine species present in the area and their activities. Other sounds sources would also be present that would be lower in sound level, but may be present for extended periods of time. Examples include surface vessel noise or noise radiated from operational wind turbines.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 5
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