146 



COMPUTATION FROM TEST DATA 



I 2 3 4 5 6 7 8 9 lOl 20 



KILOCYCLES PER SECOND 



Figure 18. Receiving chart. 1 R 13 projector acting as a 

 hydrophone vs 1K25 projector acting as a sound source. 



I 2 3456789 10 



KILOCYCLES PER SECOND 



Figure 19. Chart of current in the 1K2 

 as a sound source. 



projector acting 



In the following, the computation of the absolute 

 reciprocity calibration of a 3A hydrophone is carried 

 out at 1 ,000 c from test data which have been obtained 

 at the station. In these tests, the 1K13 projector was 

 used as the auxiliary reversible transducer and the 

 1K25 as the other sound source. 



Figure 18 shows a receiving chart for the 1 K13 pro- 

 jector, taken with the 1K25 projector acting as the 

 sound source. 



The pertinent test conditions are recorded as usual 

 on a log sheet. The following information at 1,000 c 

 is taken from that sheet: 



Transmitting conditions for 1K25 projector 



Source impedance = 4 ohms 



Available power = 150 db vs 10 — 1" watt 

 Testing distance = 45.7 cm 

 Receiving conditions for 1KI3 projector 



Coupling loss = 0.6 db (as shown by chart readings- 

 see Section 7.1) 



Receiving circuit = 135 ohms 



Receiving amplifier gain = 30 db 



Figure 19 shows a chart of the current in the 1K25 

 projector when 150 db vs 10 -16 watt available power 

 is supplied to it from a 4-ohm source. In order to con- 

 vert the current chart reading into db vs 1 ampere, it 

 is necessary to use the current-measuring circuit cali- 

 bration. In this particular case, this calibration is as 

 follows: The level indicated by the recorder when 1 

 ampere flows in the current-measuring circuit is 112 



db vs 10- lfi watt. Thus 112 db must be subtracted 

 from the chart readings. 



In addition to the chart in Figure 17 giving the 

 value of the reciprocity constant /, the charts in Fig- 

 ures 18 and 19 are sufficient for the computation of 

 the constant k. 



It is desirable in this computation to reduce all 

 values to a standard test condition which will be used 

 in the subsequent tests on the hydrophones whose ab- 

 solute calibrations are to be obtained. This test condi- 

 tion will be based on (1) an available power level for 

 the projector of 160 db vs 10~ 1G watt, and (2) a testing 

 distance of 30.5 cm. 



The value for the reciprocity constant / can be read 

 from the chart in Figure 17. At 1,000 c for a testing 

 distance of 30.5 cm this figure is — 164.3 db. 



The level received in the 135-ohm circuit is ob- 

 tained as follows: 



Receiving chart reading (Figure 19) + 107.6 db vs 10 — 16 watt 

 Collection for receiving amplifier gain —30.0 

 Correction foi coupling loss +0.6 



Level received in the 135-ohm circuit 78.2 db vs 10 — 18 watt 



This level must be reduced to standard conditions by 

 two corrections: 



1 . The testing distance was 45.7 cm. If the measure- 



