RESULTS OF FIELD TESTS 

 Self-Noise 



The magnitude of the self-noise of the various sus- 

 pension and cable systems is illustrated in figure 19. 

 These results were obtained by a one -third octave band 

 analysis of the recorded noise from the simplified cable 

 system used on the Davis (fig. 17). The figure shows the 

 sound-pressure spectrum level of the noise at frequencies 

 from 10 to 1000 Hz at three different times during the 

 same run. One sample was taken before the run had 

 started and shows the measured level while the cable was 

 taut and the hydrophone was being towed at a speed of about 

 1 knot. The next sample was taken a few minutes later 

 with the cable in a slack condition. The difference in the 

 two samples is about 30 dB at 10 Hz and decreases with 

 increasing frequency until at about 40 to 50 Hz there is 

 practically no difference in the measured level. The third 

 sample was taken when the cable was again taut. This 

 high low-frequency noise level during towing was also 

 observed in all of the other cable systems tested and is 

 believed to be due to flow noise and cable vibration. 



Figure 20 shows four strip -chart records of the 

 noise sample. Three of them are one-third octave band 

 levels which are centered at 10, 50, and 1000 Hz. The 

 fourth is a broadband record of the measured ambient 

 noise. At 10 Hz the noise level is high at the start and 

 begins to drop as soon as the cable becomes slack and 

 then rises again as the cable becomes taut toward the end 

 of the run. Note that the character of the noise in the 50- 

 Hz and the 1000 -Hz bands during the taut condition is dif- 

 ferent from that in the slack-cable condition; however the 

 level remains about the same. 



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