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

 FOR THE FREQUENCY RANGE FROM 1 HERTZ TO 5 KILOHERTZ 



methods used in editing the data. However, even the longer term 

 measurements provided by the Ambient Noise Buoy (ANB) and Moored 

 Acoustic Buoy (MABS) show large differences. 



Measurements made on the Grand Banks (Figure 9) show that 

 spectrum levels between 10 and 200 Hz are near the upper range of 

 values observed in the center of the basin. Measurements at this 

 site, however, were made down to 1 Hz. Between 1 and 2 Hz there 

 appears to be a strong wind dependence in the results. The strong 

 trend in the spectrum between 2 and 6 Hz does not have an apparent 

 wind dependence and neither can it be reasonably associated with 

 shipping. More investigations into this very low frequency region 

 are obviously warranted. 



Another way of measuring shipping and wind influence on ambient 

 noise is to use the autocorrelation of the ambient noise level time 

 series for each frequency to estimate a time constant or decorrelation 

 time of the noise as a function of frequency. This is shown for the 

 Moored Acoustic Buoy (MABS) data in Figure 10. This time-constant is 

 the value of x , in hours, that one obtains simply by extrapolating 

 the main lobe of the time series autocorrelation function to zero. 

 While this analysis provides some of the same qualitative information 

 as the wind speed cross correlation analysis, it also provides addi- 

 tional information which could be of value to system designers for 

 both concept formulation and design and ultimately to system analysts 

 for evaluating system performance. In this figure, the time-constant 

 coordinate is divided into three regions designated shipping, wind, 

 and a mixture of the two. The rationale for the wind speed region 

 is shown by the curve of Figure 11 which is a plot by month of the 

 wind speed time-constant in Bermuda. Unfortunately, there is no such 

 simple function which represents a direct measurement of shipping in 



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