BUCK: ARCTIC ENVIRONMENTAL LF ACOUSTICS MEASUREMENTS, 

 MODELS AND PLANS 



installed and certainly the largest ever installed at or close to the 

 effective sound channel axis. The systems were tested using CW 

 projectors in the far field. Attained array gain was measured over a 

 reasonably long period by monitoring the S/N outputs of a single 

 hydrophone in the array and of the processed beam containing the projec- 

 tor. The background noise was found to be anisotropic and, at certain 

 times, highly so. Occasionally the array gain equalled the direc- 

 tivity index (DI) , but on the average it was a few dB below, and at 

 times it dipped to 7 dB below. 



Figures 17 and 18 present three dimensional pictures of the 

 broadband output of the T3 DIMUS under a couple of background noise 

 conditions. 350° of relative bearing (128 beams) are shown on the 

 X axis, beam output on the y axis, and time on the z axis. 



An info2rmative way to view Arctic low- frequency environmental 

 acoustic data is to present the data in terms of signal-to-noise 

 ratios instead of transmission loss and ambient noise independently. 

 The next few figures make this type of comparison. In all cases 

 one parameter is compared with another so the end result is a ratio 

 of signal-to-noise ratios. In every case we have used shot energy 

 for the "signal" and median ambient noise as "noise." 



Figure 19 presents S/N as a function of frequency for three 

 source ranges. Here source depth is 400 feet and hydrophone depth 

 is 100 feet. For all three ranges the S/N is nojrmalized to dB at 

 10 Hz. The importance of very low frequencies to detection at extreme 

 ranges in the Arctic is apparent from the two bottom curves. 



Figure 20 presents relative S/N ratios for three hydrophones at 

 different depths below 12-foot thick pack ice. The 200-foot sensor 

 is the reference and the 100-foot and 30-foot units are plotted relative 



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