IV. DATA AND CALCULATIONS 



The source-path-receiver model is used in this section to calculate the 

 range at which a hypothetical baleen whale may be expected to hear various oil 

 platform sounds under several stated environmental conditions. The analytic 

 expression used for these calculations is the passive sonar equation which 

 states mathematically that the source level (SL) minus the transmission loss 

 (TL) equals the minimum detectable signal (MDS), The MDS under the masking 

 limited conditions assumed is equal to the spectrum level of the ambient noise 

 (NL) plus the critical ratio (CR). The directivity index is omitted here as 

 we have assumed it to be zero (omnidirectional hearing). Therefore, 



SL - TL = NL + CR, or 



TL = SL - NL - CR (3) 



For known values of source level, ambient noise level, and critical 

 ratio, the transmission loss acceptable to enable tfic source sound to just be 

 heard is calculated. Then the distance at which this TL occurs is obtained by 

 calculation using the transmission loss expressions (equations (1) or (2)) or 

 graphically from Figure 3 or similar plots. 



G-27 



