ill. 



USRL TEST STATIONS 



tank consists of a rigid closed cylinder, 10 incites in 

 diameter and 20 inches high, in which the test hydro- 

 phone is hung. A coil-driven diaphragm from an 

 NDRC IK projector is mounted in the chamber wall 

 and produces high sound pressures of known magni- 

 tude in the water-filled chamber. The sound pressure 

 is dependent mainly on the force factor of the sound 

 source and on the mass and stiffness reactances of the 

 diaphragm and the enclosed water. The chamber and 

 the circular sound source and mounting are shown 

 in Figure 51. The chamber and source together meet 

 the requirements of a stiffness-controlled system op- 

 erating over a fairly wide range of low frequency. The 

 limitations of the chamber and the associated elec- 

 trical circuit are such that the effective range over 

 which accurate calibrations of "hard" (essentially in- 

 compressible) hydrophones may be made, lies be- 

 tween 2 and 100 c. Calibrations can be made only on 

 hydrophones which are sufficiently hard not to lower 

 the system stillness appreciably. Reduction of the sys- 

 tem stiffness lowers the resonant frequency and, con- 

 sequently, the upper limit of the frequency range 

 through which calibrations may be made. 



Facilities are incorporated to obtain measurements 

 at temperatures between 35 and 100 F and at pres- 

 sures up to 100 pounds per sq in. The use of a closed 

 testing chamber permits measurements of the de- 

 pendence of a hydrophone upon temperature and 

 pressure to be made more conveniently than under 

 free field conditions. Approximately six hours are re- 

 quired (o obtain a characteristic frequency response 

 over a complete temperature cycle at each hydrostatic 

 pressure. 



Hydraulic System 



A schematic diagram of the hydraulic system ap- 

 pears in Figure 52. 



Calibration Chamber. The chamber consists of two 

 bronze castings 1 inch thick, together weighing about 

 500 pounds. The dimensions and construction are 

 shown in Figure 53. The strength and size are such 

 that no chamber or wall resonances occur below 200 

 c. The resonant frequency of the sound source is well 

 above 250 c. 



Since even small quantities of air greatly reduce the 

 chamber stiffness, provisions have been made for de- 

 aerating the water by reduced pressure and also for 

 air venting of the chamber. The shape of the chamber 

 was designed to facilitate the removal of air, and the 

 direction of the water flow is such that air is carried 



Figure 50. Electrical equipment of low-frequency system. 



HOOKS FOR SUSPENDING 

 HYDROPHONE. 



RUBBER SEAL 



HYDROPHONE CABLE 

 GLAND 



COMPRESSION RING 

 RUBBER SEAl/ 



HYDROPHONE UNDER TEST 



CIRCULATING LINE OUTLET 



RUBBER SEAL 



SOUND SOURCE 



DIAPHRAGM 

 EALING FLANGE 

 MAGNET 

 BACK COVER 



PRESSURE 

 COMPENSATING LINE 



HYDROSTATIC PRESSURE 

 LINE 



CIRCULATING LINE INLET 

 A INCH PIPE 



Figure 51. Calibration chamber section of low-frequency 



svstem tank, showing sound source and other details. 



