Calibrate Boxes 



The purpose of the calibrate boxes is to provide 

 calibration signals to the hydrophone -preamplifier input 

 using the insert resistor method. The calibrate boxes 

 (fig. 9) contain a 1000-Hz sine-wave generator and a broad- 

 band, flat -spectrum, random-noise generator. It also 

 contains a monitoring circuit for measuring the current in 

 the calibrate pair leads and a meter provides a visual indica- 

 tion of the amount of current. Provision for inserting exter- 

 nal calibrate signals is included. 



The calibration signals are applied through cable 

 leads 4 and 5 to the 10-ohm resistor at the input of the 

 hydrophone-preamplifier. The calibration signal thus 

 appears in series with the hydrophone signal. This method 



HYDROPHONE 



AND 

 PREAMPLIFIER 



r 



CAL 

 PAIR 



CAL BOX 



SIGNAL PAIR 



CAL SIGNAL 

 CURRENT 

 MONITOR 



"0 



-#o- 



AMPLIFIER 



LEVEL 

 SET 



"1 



OSCILLATOR 

 1000 HZ 



Li 



WHITE-NOISE 

 GENERATOR 



SOUND- 

 LEVEL 

 METER 



EXTERNAL 



CAL 



SOURCE 



Figure 9. Simplified block diagram of the calibrate box and connections to 

 the hydrophone and sound-level meter. 



14 



