144 



COMPUTATION FROM TEST DATA 



3 4 5 6 7 8 9 10 20 30 



KILOCYCLES PER SECOND 



40 



60 



Figure 14. Response characteristics of transmitting and receiving systems during calibrations. 



The voltage delivered by the 3A hydrophone conse- 

 quently is 



122.0 - 138.7 



16.7 db vs 1 volt. 



The calibration chart (Figure 1) shows that, if the 

 sound pressure is 1 dyne per sq cm, the hydrophone 

 delivers —98.2 db vs 1 volt. For the voltage to be 

 — 16.7 db, the sound pressure must have been 81.5 db 

 above 1 dyne per sq cm. Thus, the transmitting re- 

 sponse of the projector at 25 kc is found to be +81.5 

 db vs 1 dyne per sq cm. 



^3 



tr lj 



Zi z 

 IT) >• 

 If) Q 



0.1 



FREQUENCY KC 



Figure 15. Transmitting response of QB No. Ill 

 transducer. 



The electrical system is usually adjusted at one fre- 

 quency, and the sensitivity may vary somewhat with 

 frequency. When this is the case, a compensating cor- 

 rection must be made in the transmitting response. 

 Figure 14 shows response characteristics of the trans- 

 mitting and receiving systems used in the present 

 tests. At 25 kc this correction amounts to about 0.5 db. 

 The transmitting response must therefore be in- 

 creased by this amount, i.e., the transmitting response 

 is actually 82.0 db vs 1 dyne per sq cm. 



This computation when carried out over the fre- 

 quency range gives the transmitting response charac- 

 teristic of the projector, shown in Figure 15. 



After the transmitting response has been obtained, 

 the projector efficiency is computed. This requires a 

 knowledge of the directivity at the frequency at which 

 the computation is made. Figure 16 shows a directiv- 

 ity pattern of the projector taken at 25 kc. Chapter 4 

 contains descriptions of graphical charts which facili- 

 tate the computation of the directivity index from the 

 measured directivity pattern. Since the side lobes of 

 this particular pattern are low, it is possible to base 

 the computation on the relation between the beam 

 width and the directivity index for a circular piston 

 (see Figure 7 in Chapter 4). The beam width in this 

 case is 19.8 degrees, giving a directivity index of —24.2 

 db by the chart. 



Equation (8) in Chapter 4 gives the expression for 



