B. Path 



The sound propagation path is a one-way water path between source 

 and receiver. Such paths are generally quite complex, involving vertical 

 curvature due to sound velocity gradients in the water, and multiple 

 reflections from the surface and bottom. In order to carry out the 

 calculations of transmission loss in a reasonably tractable manner, a number 

 of simplifying assumptions relating to the path and its boundaries are made. 

 These have been validated by many years of use in naval applications related 

 to detection of submarine and ship noises by passive sonar (Urick, 1975). The 

 literature contains a large body of both theoretical and experimental data on 

 underwater sound propagation (Urick, 1975). The sound propagation assumptions 

 used in this report are described in Section III below. 



C. Receiver 



The receiver in the OCS model is the animal whose behavior is 

 possibly subject to modification by hearing the sound. In order to estimate 

 the greatest range at which a sound may be detected by the animal, it is 

 necessary to determine the weakest sound that is detectable. This is called 

 the "threshold of hearing," and is generally dependent on the frequency of the 

 sound. If the animal is listening in an environment free of interfering 

 noise, the threshold is termed the "absolute threshold." Ordinarily, however, 

 the animal is in an environment in which certain normal sounds of the sea are 

 present. These are caused by wind and waves at the sea surface, by breakers 



G-10 



