waveform contains noise. For the 100% value of SEL, it would be necessary to add 0.45dB to the 90% value.
160. The SEL for each impulsive noise event can also be aggregated by summation to calculate the total SEL (or SEL dose) for the entire piling sequence (Southall et al. 2007; Theobald et al. 2009). The concept of SEL dose is entirely analogous to the use in air acoustics to quantify the total noise dose for a subject receiver. The pulse duration is defined as the time occupied by the central portion of the pulse, where 90% of the pulse energy resides.
161. The calculation of the pulse duration and SEL are described graphically in Plate 9.16. Image A shows a typical pulse waveform, and image B shows a plot of the normalised energy in the pulse waveform against time. Indicated on the plot are the 5% and 95% energy levels and the t5 and t95 times that define the pulse duration.
Plate 9.16. Example of pulse time waveform for analysis, and B: Calculation of SEL over pulse duration.
9.9.1.5 Source Level 162. A metric used frequently in underwater acoustics to describe the source output amplitude is that of Source Level (SL), a term not commonly seen in air acoustics where the acoustic power is more commonly used. This term originates from sonar engineering, and as with acoustic power, the Source Level may be considered as a characteristic of the source itself. The decibel units for this quantity may be written as dB re 1 μPa·m, however the unit is much more commonly seen expressed as dB re 1 μPa at 1m. It should be noted that Source Level is an idealised acoustic far-field parameter and is not necessarily equal to the acoustic pressure or received level measured at a distance of 1 metre from the source. Instead, it may be considered as the sound pressure level that would exist at a nominal range of 1m from the acoustic centre of an equivalent simple monopole source, which radiates the same acoustic
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 83
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