9.9.1.6 Propagation/Transmission loss 167. Propagation Loss (PL) or Transmission Loss (TL) is the term used to describe the reduction of the sound level as a function of distance from an acoustics source. The mechanisms by which the sound intensity reduces are primarily geometrical spreading, sound absorption in the water and losses into the seabed or other boundaries. In shallow water, particularly with varying bathymetry, this can be quite complicated due to multiple interactions with the surface and seabed and is described further in Section 9.9.2). In shallow water, the depth can also restrict the propagation of lower frequency (also described further in Section 9.9.2).
168.
It is normal for propagation/transmission loss to be stated as a positive number in dB representing the loss for the total range between the reference distance (1m for Source Level) and the receiver location. The quantity is a function of frequency, and depends on seabed type, bathymetry, surface roughness, sound speed profile etc.
9.9.1.7 Received level 169. The received level (RL) is the acoustic pressure measured by a hydrophone at some distance away from a sound source. It is also considered to be the sound pressure which arrives at any acoustic receptor which is exposed to a sound.
170. The received level might be expressed in a number of ways, for example as a sound pressure level (dB re 1 μPa) or a sound exposure level (dB re 1 μPa2·s).
171. When predicting received levels from estimated source levels for zones of impact, the received level is simply determined by subtracting the transmission loss in dB from the source level in dB, RL = SL – TL, where the TL is estimated using a transmission loss model (see below). When the source level is estimated from measured received levels then the source level is simply found by addition of received level and transmission loss, SL = RL + TL. To calculate TL accurately requires an accurate model for the propagation of the sound and its interaction with the seabed and sea surface. Sometimes, the transmission loss is empirically estimated from the measured received level data as a function of range. Ideally the transmission loss should still be estimated by fitting an appropriate transmission loss model capable of accurately modelling propagation for a complex environment.
9.9.2 Sound Propagation Modelling
9.9.2.1 Environmental dependence 172. Perhaps even more so than for airborne sound, noise levels in the ocean produced by human activities are determined not only by the acoustic power output of the source, but equally importantly by the local sound transmission conditions (Urick
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 85
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