15.
The metrics used during this assessment are peak pressure level and SEL which are suitable descriptors for impulsive sounds such as impact pile-driving. The use of these metrics maintains consistency with the Marine Strategy Framework Directive (FHWG 2008), and are also consistent with the metrics described in the UK, German and Dutch guidance documents (Robinson 2014; Mueller and Zerbs 2011; De Jong et al. 2011).
16.
Another important characteristic of sound is its frequency, described as the number of oscillations per second. The unit of frequency is the hertz (Hz). The frequency range of applications in underwater acoustics is very large, with seismic exploration involving frequencies of less than 1Hz, and acoustic current profilers operating at frequencies of millions of hertz. Marine piling tends to generate noise with most of the energy between around 100 and 400Hz, with the noise levels outside of this frequency range significantly reduced. It is common to see the frequency range divided up into one-third octave bands. One-third octave bands are also commonly used in underwater acoustics as a convenient way of expressing the sound level as a function of frequency, where each band is one-third of an octave, an octave representing a doubling of frequency.
17.
To assess the impact on marine fauna of underwater noise resulting from windfarm construction requires:
Modelling of piling noise to establish the received levels as a function of distance from the source; and
The use of generic criteria against these received levels to assess the ranges at which injury and behavioural impacts are expected to occur.
9.4.3.2 Modelling of Piling Noise 18.
To predict the received level as a function of range from the source requires both the source level and the propagation or transmission loss to be known. If these are known then the received level (RL) is simply calculated by:
RL SL PL
19. Where SL is the source level which describes the sound radiated into the acoustic far-field, and PL is the propagation loss expressed as a positive number in dB (dependent on frequency, sea bed, bathymetry, etc.).
20.
The primary model used for the long range propagation employs an NPL implementation of the energy flux solution by Weston (1976) which is capable of propagation over large distances whilst accounting for range-dependent bathymetry,
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 9 Underwater Noise Page 5
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