on shore, by distant ships, by natural seismic activity, by ice activity in 

 frigid areas, and by various soniferous marine life, such as snapping shrimp, 

 croakers, etc. The total sum of these is termed "ambient sea noise," and is 

 generally at such a level that the audibility of a sound, such as that of a 

 drilling platform, is limited by interference or "masking" by this ambient sea 

 noise (Myrberg, 1978). Therefore, in order to predict the audibility of a 

 sound, one needs to know the "masked threshold" for the animal under the 

 environmental sea conditions at the time. This masked threshold for a given 

 animal is dependent on (1) the noise discrimination capability of the animal 

 (aural critical ratio, or bandwidth), (2) frequency component to be detected, 

 (3) background noise spectrum, which in turn depends on sea state, amount of 

 shipping in the general area, local noise-making animals, etc. The various 

 assumptions in this report relating to these are discussed below. 



Frequency of the sound is a critical factor in each of the elements 

 of the sonar equation: source, propagation, and rpceiver; therefore, each of 

 these will be considered as a function of frequency. The source is described 

 by its frequency spectrum at a known distance, sound transmission loss over 

 the sound path is considered as a frequency-dependent quantity; and receiver 

 minimum-detectable signal is approached in terms of a frequency-dependent 

 threshold based on ambient noise spectrum. 



G-11 



