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not less than 100 megohms with less than 30 pF of capaci- 

 tance. The high input impedance is necessary to have good 

 low -frequency response. This combination of source 

 capacitance, sensitivity, and input impedance resulted in 

 system input noise as shown by the curve of figure 3, at the 

 preamplifier input referred to an equivalent SPSL in water. 

 The empirical ambient-noise minimum 3 and the sea state 

 curve 4 are given for comparison. 



The actual receiving responses of two hydrophones 

 as determined at an acoustic calibration facility (NUWC: 

 SDL-TRANSDEC) are shown in figures 4 and 5. 



10 



100 



1000 



10,000 



FREQUENCY (HZ) 



Figure 3. Comparison of system noise at hydrophone-preamplifier input to the 

 sea state curve (Knudsen) and to the empirical ambient noise minimum (Wenz). 



