MARTIN/PERRONE : GEOGRAPHICAL VARIATION OF AMBIENT NOISE IN THE OCEAN 

 FOR THE FREQUENCY RANGE FROM 1 HERTZ TO 5 KILOHERTZ 



this respect, so area delineation has to be subjectively chosen. 

 Figure 12 shows several of these curves for the areas discussed in 

 this paper. The frequency region which shows decorrelation times of 

 4 to 12 hours is associated with shipping dominated noise, and fre- 

 quencies with long decorrelation times (greater than 25 hours) are 

 identified with wind generated noise. In between, both influences 

 act on the noise and a transition region is obtained. The identifi- 

 cation with wind speed, as stated earlier, can be made rather directly 

 by consideration of the cross-correlation curves and by measuring the 

 wind speed autocorrelation function. The very low frequency data 

 obtained on the Grand Banks have time constants peculiar to wind 

 generated noise. 



Strong variations of ambient noise versus depth within the same 

 general area are also possible. Figures 13 and 14 show curves of 

 the depth dependence of ambient noise for, respectively, the center 

 of the Labrador Basin and for a site toward the Denmark Straits. The 

 strongest difference between these data sets is below 200 Hz and has 

 already been identified with shipping dependent noise. The levels at 

 the shallow depths agree quite well at these lower frequencies; the 

 differences are at the deeper depths. As shown in Figure 15, the 

 site near the Denmark Straits is more topographically confined than 

 the one in the center of the basin. It is hemmed in by nearby 

 Greenland from the northwest to Iceland in the northeast and by the 

 less confining Reyk janes Ridge from Iceland to the south-southwest. 

 The major shipping lanes are on the side of the ridge opposite this 

 site. It is suspected that the shallow critical depth and the 

 similarly shallow depth of the ridge permit excellent coupling of 

 the shipping lane noise to the shallow hydrophones, as well as an 

 effective proportional blocking considered to result from the increas- 

 ing depth of the ridge summit toward the southwest and the gaps in the 



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