between the peak of the pulse and the RMS value (the RMS value for a pulse depends on the pulse length, which depends on the pulse shape, the decay time, etc.) (Madsen 2005; Ainslie 2011). This can cause confusion and make comparisons between pulse type sounds and continuous type sounds meaningless even though they appear to be described using the same units.
156. For this assessment, the root mean square of the sound pressure is used when considering continuous type noise sources(e.g. turbine operational noise) and can be expressed in units of dB re 1 µPa and is calculated from:
R SPLMS = 2 log0
PRMS P0
where P0 is the RMS reference pressure of 1 µPa.
9.9.1.4 Sound Exposure Level 157. For a piling pulse, SEL is related to the sound energy in the pulse and is calculated by integrating the square of the pressure waveform over the duration of the pulse. The duration of the pulse is defined as the region of the waveform containing the central 90% of the energy of the pulse. The calculation is given by:
t95 E 90
= ∫ p t dt . t5
2 ( )
158. The value is then expressed in dB re 1 µPa2·s and is calculated from:
SEL where E0 is the reference value of 1 µPa2·s.
= 1 log0
E E
90 0
159. Note that for a plane-wave in a free-field environment (an unbounded medium), the pulse pressure squared integral in µPa2·s can be converted to units of energy flux density in J/m2 (joules per square metre) by dividing the cumulative squared acoustic pressure by the specific acoustic impedance, Z, of the medium, the specific acoustic impedance being the product of medium density and sound speed in the medium (ρc). When expressed in decibel notation, this means that 0dB re 1 Jm-2 is equivalent to 182dB re 1 µPa2·s in water. Note also that the definition above uses the central 90% of the energy in the pulse (i.e., the pulse duration is defined as the time occupied by the central portion of the pulse) where 90% of the pulse energy resides. This is because it can be difficult to determine the exact start of the pulse when the
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 82
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