power into the medium as the source in question (Ainslie 2011). However, for real sources which are acoustically large (such as occurs for marine piling) the value of the Source Level will not be equivalent to the sound pressure level at the reference range of 1m.
163.
In practice, for real sources, the source level is calculated by measuring the received level at a distance from source which is in the acoustic far-field and propagating the acoustic pressure back to the reference distance of 1m from the acoustic centre of the source using an appropriate propagation model. This distance required to be in the far-field is related to both the dimensions of the source and the wavelength of the sound. Indeed, for large distributed sources, this reference distance of 1m may be in the acoustic near-field (or sometimes even inside the source). In the near-field region, the sound field amplitude fluctuates due to interference between the waves that radiate from different parts of the source.
164.
It should also be noted that propagation of sound in the ocean rarely corresponds to simple spreading laws. This is especially true in shallow water typical of offshore windfarms. In general, source level (SL) may be given by:
SL = R PL L +
where received level is the received level in the acoustic far-field and propagation loss is the propagation loss (dependent on frequency, seabed, bathymetry, etc).
165. Estimation of source level from sound pressure measurements in shallow reverberant channels is not straightforward since an estimate must be made of the true propagation loss (sometimes termed transmission loss) (Urick 1983), which is complicated by the interactions of sound with the seafloor and sea surface. An important fact to note is that the source levels for marine piling reported in previous windfarm studies, have almost exclusively been obtained by extrapolation back to the source using simple spreading formulae. This means that these reported values are not true Source Levels and are generally not consistent with the accepted definition of source level by Urick (1983) and others (Ainslie 2011). This means that comparisons may not be possible with other sources measured in deep water. To distinguish between formats, data derived from simple spreading formulae are referred to as “Effective” Source Level.
166. Source level might be expressed in a number of ways, for example in terms of sound pressure level (in units of dB re 1 μPa·m), or in terms of energy or sound exposure level (units: dB re 1 μPa2·s·m2).
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 84
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