118 



USRL TEST STATIONS 



Equipment for Cooling, Heating, and Circulating 

 the Water. The water line from the test chamber 

 leads to a 13x25-inch pressure tank in which are 

 sealed the impeller of a circulating pump and the 

 evaporating coils of a cooling system. A 2,000-watt, 

 110-volt immersion heater is located in the line re- 

 turning from the tank, to the test chamber. Control 

 facilities enable the system to be held within 1 degree 

 of any desired temperature. 



Temperature reduction is accomplished by an air- 

 cooled Freon compressor with 1 1/9-hp motor. A close 

 control of temperature in cooling is obtained by us- 

 ing the heater in conjunction with the refrigerating 

 unit. Safety controls are incorporated which prevent 

 the system from either freezing or rising above 105 F. 



Circulation is maintained by a rotary pump of 14 

 lip while the temperature is being changed, but when 

 the desired value is reached the circulation is stopped 

 and the test chamber closed off by valves. 



The water is deaerated by a vacuum pump with a 

 %-hp motor. 



Electrical System 



The apparatus consists of an oscillator, power am- 

 plifier, variable attenuator, and receiving amplifier 

 and level indicator. Figure 54 shows the connections 

 in a typical testing setup. The units which are per- 

 manently installed are mounted in bays adjacent to 

 the hydraulic system. For flexibility and ease of op- 

 eration, the equipment is terminated in several jack 

 strips. 



The signal generator is a Hewlett Packard audio 

 oscillator, Model 202DR. It imposes the lower limit 

 of 2 c on the system. Sound pressures up to 10"' dynes 

 per sq cm may be used at any point in the frequency 

 range. 



The power amplifier was designed especially for 

 low frequency. It operates on alternating current and 

 has a maximum output of 6 watts. The gain may be 

 controlled over 45 db in 5-db steps with continuous 

 adjustment through each step. The circuit has been 

 equalized to give a frequency response which is flat 

 to 0.1 db over the working range. The input and out- 

 put impedances are 600 ohms and 50 ohms, respec- 

 tively. 



The attenuator provides 45 db in 1-db steps giving 

 a dial which reads hydrophone responses directly 

 from —80 to —125 db vs 1 volt per dyne per sq cm 

 when the chamber stiffness is 10 8 dynes per cm. 



The component parts of the receiving amplifier 



POWER 

 AMPLIFIER 



*l*@vw\ 



"TEST" -C4C 

 W 



1 -r rnturvir 



HYDROPHONE^ 



ATTENUATOR 

 0-45 DB 



TEST CHAMBER 



=A 



RECEIVING 

 AMPLIFIER 



AND 

 INDICATOR 



Figure 54. Electrical measuring circuit used with low- 

 frequency system. 



and indicator are the amplifier, a copper oxide recti- 

 fier, and a meter reading from — 10 to + 10 db. Input 

 impedances of 80,000 ohms unbalanced, and 600 

 ohms balanced or unbalanced, are available. The 

 gain is essentially flat over the range of 2 to 100 c. 

 The dial of the amplifier-gain control covers 50 db in 

 5-db steps so that its setting plus the meter reading 

 gives the input signal level into the 600 ohms in db 

 vs 10- 1C watt. With a fixed 20-db gain which may be 

 added, a range from 50 to 1 20 db can be read directly. 

 Two combinations of capacitors and resistors are 

 available for insertion in the meter circuit. For fre- 

 quencies up to 10 c, the one with the larger time con- 

 stant is selected to give a fairly steady meter deflection. 

 At higher frequencies, the lower time constant is 

 chosen to increase the rate of meter response. 



Procedure 



The stiffness of the chamber must be measured 

 over the whole range of both pressure and tempera- 

 ture since it enters as a correction in all determina- 

 tions of hydrophone sensitivity. The procedure is to 

 close key 1 (Figure 54), thus putting a small direct 

 current through the projector coil and producing a 

 pressure indicated on the manometer. From this 

 measurement and known diaphragm constants, the 

 stiffness may be computed. Extensive computations 

 are obviated by the use of a chart relating these fac- 

 tors. 



The system is thus calibrated so that the sound 

 pressure in the chamber is known for any projector 

 current at any point in the range of stiffness. Thus 

 the sound pressure which produces the measured out- 

 put voltage of the hydrophone could be calculated 



