D. Ambient Noise 



In order to calculate the masked threshold it is necessary to know 

 the ambient noise level at the frequency of interest, as well as the animals' 

 critical ratio. This is necessary, since a tonal signal of frequency, f, is 

 detectable when its sound pressure level equals or exceeds a level which is 

 the sum of the 'pectrum level (level in a one Hz band) of the ambient noise at 

 f and the critical ratio at f. 



liie fact that ambient noise varies widely in level and spectrum 

 shape, depending on the nature of noise sources in the areas, makes it 

 necessary to specify certain ambient noise level'", to ho introduced into the 

 sonar equation for calculation of the detection range of the oil platform 

 sounds. 



Comprehensive data on ambient noise levels and spectra have been 

 published by Wenz (1962) for various conditions of wind, sea state, and ship 

 traffic. These and other data have been combined by Urick (1975) to give sets 

 of curves for obtaining sound spectrum level for deep (Figure 5) and coastal 

 (Figure 5) waters. The curves from Figure 5 are used for data inputs in the 

 calculations in the following section. 



G-25 



