THRESHOLD AND IMPEDANCE 



141 



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REC LINE 



1 35": 0034"- 



osc f-3C J BALANCED "f 



8-13S " 1 H P I COUPLING I i 



lZ_l^ii»< 1 AMPLIFIER \ ± 



REC LINE 

 «=l35<-> 



B 



Figure 7. Circuits used in obtaining coupling loss for 

 3A89 hydrophone. (A) For measuring input voltage (see 

 run No. 4. Figure 4). (B) For measuring output voltage 

 (see run No. 3, Figure I). 



higher. Thus the result obtained is that the projector 

 at 25 kc has a receiving response of —80.8 db vs 1 volt 

 per dyne per sq cm. 



The preceding computation, which was carried 

 through at one frequency only, is repeated at selected 

 frequencies throughout the entire frequency range. 

 From these data, a response characteristic is plotted as 

 shown in Figure 9. 



In these particular tests a 3A hydrophone, which is 

 a pressure-actuated device, was employed as a stand- 

 ard. If the hydrophone standard is of the pressure- 

 gradient type, such as the 1A or 2A hydrophone, it 

 generates a gain in the voltage, especially at low fre- 

 quencies, due to the curvature of the wave front. Fig- 



ure 13 in Chapter 5 shows the magnitude of the spher- 

 ical wave correction for different testing distances, 

 plotted against frequency. The indicated response of 

 the test hydrophone, because of this gain in the hydro- 

 phone standard (if the latter is of the pressure-gradient 

 type), is lower than it would be if the tests were made 

 in a plane-wave sound held. To refer to plane-wave 

 conditions, therefore, the relative response must be 

 correspondingly increased. 



7.2 



THRESHOLD AND IMPEDANCE 



After the receiving response characteristic has been 

 determined, it is possible to compute the threshold 

 characteristic. Equation (18) in Chapter 4 gives an ex- 

 pression for the threshold pressure. It can be seen that 

 this pressure depends on the resistance of the hydro- 

 phone as well as on its response. It is therefore neces- 

 sary to compute this resistance. The chart in Figure 

 10 shows readings for the projector taken at the test 

 station by means of the 5A impedance bridge. This 

 bridge gives the admittance in terms of parallel resist- 

 ance and capacity values. Since the bridge can meas- 

 ure directly only impedances below 1,000 ohms, it is 

 necessary to shunt the unknown impedance whenever 

 it exceeds that value. The bridge shunt resistance used 

 is given in Figure 10 as R„ and the bridge resistance 

 reading as R b . 



4 5 6 7 8 9 10 20 30 



KILOCYCLES PER SECOND 



40 



50 



Figure 8. Chart of coupling loss for 3A89 hydrophone. 



