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

 FOR THE FREQUENCY RANGE FROM 1 HERTZ TO 5 KILOHERTZ 



measure of the wind speed and non-wind speed (presumably shipping) 

 contributions to the noise field and permit rapid assessment of the 

 overall importance of the two contributors with frequency. Note the 

 high correlation below 20 Hz and at 30 Hz. It is felt that a 

 correlation descriptor of this type would be useful for model develop- 

 ment purposes, especially when attempting to evaluate the model in 

 the transition regions which are noted here to be between 100 and 

 200 Hz for the Bermuda area. 



Figure 5 shows the time series of observed ambient noise level 

 at three frequencies, together with concurrently measured wind speed. 

 This particular data set was obtained in the Ionian Sea, over a period 

 of 9 days. The influence of wind speed on these levels is shown by 

 the cross-correlation curves of Figure 6. While it is evident from a 

 consideration of Figure 5 that 9 days of data represents a small data 

 set for this type of analysis, the resulting correlation curves are 

 remarkably well behaved. These curves represent results from hydro- 

 phones at different depths and have essentially the same shape. This 

 shape is quite different from that observed for the Bermuda area, 

 displaying a more gradual and higher frequency transition region 

 (200 to 1,000 Hz vice 100 to 200 Hz) and a lower correlation coeffi- 

 cient above 1,000 Hz. This longer transition region and lower 

 correlation for high frequencies results from a much stronger shipping 

 influence over the entire frequency range of measurement. 



This strong lack of dependence on wind speed for the Ionian data 

 is further observed in the spectra of Figure 7. Here, no wind speed 

 dependence is found at 10 Hz but this may be due to the small amount 

 of data at the higher wind speed condition occurring in the measure- 

 ment period. The levels between 10 and 20 Hz are comparable to those 

 observed in Bermuda but, at 50 Hz, levels in the Ionian Sea are at least 



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