RECIPROCITY CALIBRATION OF STANDARDS 



147 



3 



TO 



2 3 456789 10 



KILOCYCLES PER SECOND 



Figure 20. Receiving chart. 3A7t hydrophone \s 1K13 

 projector as the sound source. 



2 3 456789 10 



KILOCYCLES PER SECOND 



Figure 21. Receiving chart. 3A74 hydrophone 

 projector as the sound source. 



1 K25 



merits had been made at 30.5 cm, the level would 

 have been increased by 



20 log 45.7 

 30.5 



+3.5 db. 



2. The available power into the projector was 150 

 db vs 10- 1G watt. With 160 db available power, 

 the level would be increased by + 10.0 db. 



Making these two corrections gives 



L t = 91.7 db vs 10-" watt. 



The current chart reading in Figure 19 at 1,000 c is 

 127 db vs 10 -16 watt. From this reading must be sub- 

 tracted the receiving amplifier gain, in this case 40 db, 

 and the correction factor which converts the reading 

 into db vs 1 ampere, 1 12 db, as stated above. In addi- 

 tion, the reading must be increased by 10 db, on the 

 basis that the standard test condition uses an avail- 

 able power level of 160 db rather than 150 db. Thus, 



20 log?: = (127 - 40 - 112 + 10) 

 = — 15 db vs 1 ampere. 



Thus the constant k is obtained: 



k = -164.3-91.7 +15- 138.7 = -379.7 db. 



The reciprocity calibration of any desired hydro- 

 phone can now be obtained from two receiving level 

 charts, L,, and L h ', taken of that instrument with the 

 1K13 projector and with the 1K.25 projector. These 

 data must be for the test condition for which k was 

 computed, that is, a testing distance of 30.5 cm, and 

 an available power into the projector ol 160 db vs 

 10 -10 watt, or else corrections must be made to reduce 

 the data to these conditions. 



Figure 20 shows a receiving chart for the 3A74 hy- 

 drophone taken with the 1 K 1 3 projector as the source. 

 Figure 21 shows a receiving chart for the same instru- 

 ment taken with the 1K25 projector as the source. 

 Both charts were taken at a testing distance of 30.5 

 cm, with an available power of 160 db applied to the 

 projector. Consequently, no corrections for testing 

 distance or available power need be made. A correc- 

 tion, however, must be made for the receiving ampli- 

 fier gain, which was 20 db in both cases. Since the 

 hydrophone in both tests was across a 600-ohm line, 

 no coupling correction is required if the receiving re- 

 sponse is desired in terms of the output voltage from 

 the preamplifier across 600 ohms. 



Reference to the chart in Figure 20 shows the level 



