MELLEN: SOUND PROPAGATION IN A RANDOM MEDIUM 



Central Atlantic, the North Central Pacific waters show no measurable 

 scatter. The Thorp coefficient is estimated to be 0.5 in both cases. 



Another experiment, shown in Figure 12, was done much earlier in 

 the Gulf of Maine* and shows excess absorption similar to that for 

 Hudson Bay. In fact, the two are almost identical except for the 

 lower frequency points which are lacking in Hudson Bay. The dif- 

 fraction curve was based on infinitely lossy bottom as before and, 

 while the attenuation increases with decreasing frequency, the rate 

 is slower than predicted, probably because of finite bottom loss. 

 Any fall-off of diffusion loss at lower frequencies is obscured by 

 diffraction, however. 



The latest experiment was done in Baffin Bay (Browning et al . , 

 1974) in 1974. The results in Figure 13 also show a constant loss 

 of 0.02 dB/kyd above 200 Hz with a rapid fall-off below that fre- 

 quency. Since Baffin Bay is much deeper than Hudson Bay or the 



Gulf of Maine, the low frequency values are not obscured by dif- 



4 

 fraction. The f dependence below 200 Hz suggests Rayleigh scatter 



2 -7 

 from globs of scale size a = 3 m with y = 2 x 10 



o 



From ray diffusion theory we have devised a simple formula, 



shown in Figure 14, for the sound-channel diffusion attenuation in- 



2 

 volving p , the variance of index of refraction, a , the scale size, 



and Az, the depth from the channel axis to the bottom. Using the 



2 -7 

 values n = 10 and a = 15 m obtained from analysis of the SVP m 



o 



Hudson Bay and the Mediterranean, we see that the values predicted 

 for shallow channels (Az = 100 m) and the deep channels (Az = 2,000 m) 

 are in reasonably good agreement with experimental values. If a is 



* Unpublished BBN data, 



402 



