TRANSMITTING RESPONSE 



143 



5 6 7 8 9 10 20 30 



KILOCYCLES PER SECOND 



50 



eo 



Figure 12. Transmitting chart, QB No. 1 1 1 transducer vs 3A89 hydrophone (see run No. 5. Figure 4). 



7.3 



TRANSMITTING RESPONSE 



Transmitting tests are similar to the receiving tests 

 described above. Figure 12 shows the transmitting 

 chart of the projector, using for the sound receiver 

 the same 3A hydrophone standard as in the receiving 

 tests. Reference should be made again to the log sheet 

 (Figure 4) for the details of this run. 



This log sheet shows that the available power sup- 

 plied to the projector was 1(50 db vs lO -10 watt (i.e., 1 

 available watt) from 135 ohms. The log sheet also re- 

 fers to the test circuit used in the transmitting tests. 

 This circuit is reproduced in Figure 1 3. The receiving 

 amplifier gain given on the log sheet is 4 db, and the 

 testing distance is given as 250 cm. 



From this information it is possible to compute the 

 transmitting response. This computation is carried 

 through numerically at 25 kc as follows: 



Chart reading, vs 10 — 16 watt in 135-ohm circuit 

 Correction for receiving amplifier gain .... 

 Correction for testing distance: 20 log 2.5/1 

 Correction for available power referred to 1 available 



+ 118 db 



-4db 

 + 8db 



Odb 



Level at hydrophone terminals, vs 10—10 wa tt in 135- 

 ohm circuit +122db 



In order to obtain the pressure in the sound field, it 

 is now necessary to refer to the calibration of the 3A 

 hydrophone shown in Figure 1. This calibration is in 



terms of db vs 1 volt. The computed level at the hydro- 

 phone must, therefore, be changed into these terms. 

 This correction is as follows: 



When the power dissipated in a 135-ohm resistance 

 is 10~ 16 watt, the voltage e across that resistance is 

 given by the relation 



135 



= 10- 18 



so that 



e 2 = 135 x lO" 16 . 



On a decibel basis relative to 1 volt, this gives 

 20 log e = 101og(135) - 160 

 = -138.7 db vs 1 volt. 



,w /-AV P0WER*I60 VS10 WATT 



QB*I11 PROJECTOR I* - *! V. 



® 



Figure 13. Circuit used in obtaining transmitting chart, 

 QB No. 1 1 1 transducer. 



