provide a low impedance path compatible with input requirements of most 

 existing reproducing devices, 



(f) Overall System Characteristics 



The system response versus frequency from sound pressure at 

 hydrophone to input of the terminal amplifier is plotted- in Figure 12. 

 It refers to the level in db below 1 volt at the amplifier input pro- 

 duced by a sound pressure of I microbar at the hydrophone. To obtain 

 the response, the hydrophone was replaced by an equivalent capacity 

 and a small series resistance „ A known voltage was introduced across 

 the resistance and the corresponding voltage measured at the terminal 

 amplifier input. The measurements were made just before installation 

 and the actual preamplifiers, cable lengths, transformers and terminal 

 amplifiers were used. The hydrophone calibration shown in Figure 2 

 was assumed to be valid to 10,000 cps. 



Figure 13 shows the relative system response including the 

 record-reproduce loss for a tape speed of 1 7/8 ips . It was obtained 

 by correcting the system response of Figure 12, for the loss as 

 measured with arbitrary gain settings of the record and reproduce 

 amplifiers . 



In order to complete the picture of system response it is 

 necessary to consider the record and playback characteristics of any 

 external device employed in playback or analysis. 



(g) Miscellaneous 



(1) Ambient Noise Monitoring 



Four Esterline Angus chart recorders, two for each hydro= 

 phone, are used to record ambient viotse levels on a 24 hour basis. 

 They are operated with a chart speed of three inches an hour and 

 a full scale deflection of one milliampere. Currently, noise levels 

 in the full frequency band of the system and in a 15 to 2,000 cps 

 band are being recorded for both hydrophones. 



(2) Filters 



Provision has been made to install one or more variable 

 frequency selective filters to be used during monitoring and play- 

 back. 



7. Installation Procedure. 



(a) Cable Assembly 



The usual installation procedure is to lay the hydrophones 

 at the desired location and pay out cable toward shore. Since the 

 points at which the hydrophones were to be placed were not critical, it was 



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