Chapter 7 

 COMPUTATION FROM TEST DATA 



Bn 



Eginhard Dietze and L. Pauline Leighton 



The method of computing the calibration of an 

 instrument from the test data is described in this 

 chapter. Reference should be made to Chapter 6, 

 which outlines the procedure for taking test data, 

 and to Chapter 4, which gives the theoretical back- 

 ground for these computations. 



For a numerical example, the calibration of a trans- 

 ducer is given, since such a unit can be operated as 

 both a receiving and a sending device, affording the 

 opportunity of illustrating both types of computation. 



7.1 



RECEIVING RESPONSE 



To test the receiving response over a wide frequency 

 range, several sets of tests with a number of projectors 

 may have to be made. However, since the computa- 

 tion is the same at all frequencies, only one set of data 

 is discussed here, and the numerical computation is 

 limited to one frequency, 25 kc. 



It is assumed that calibrated standards are available 

 so that the comparison method may be used. This is 

 the usual test procedure as discussed in Chapter 6. 

 The computation of the calibration of the standards 

 themselves by reciprocity is discussed in Section 7.4. 



Usually several standards are used in one test to 

 provide a mutual check on their performance. In this 

 discussion, however, attention is confined to a single 

 standard, a type 3A hydrophone. The calibration of 



5 z 



£ z 

 < o 



10 100 



FREQUENCY KC 



Figure 1. Calibration of 3A89 crystal hydrophone. 



the 3A standard, serial No. 89, used in this particular 

 illustration is shown in Figure 1. 



The data furnished by the test station are illus- 

 trated by the receiving response charts, Figures 2 and 

 3, and by Figure 4, which shows a log sheet applicable 

 to these tests. Figure 2 shows a receiving chart for the 

 3A89 hydrophone and Figure 3 a receiving chart for 

 the transducer under test. The conditions of the tests 

 applicable to these two receiving charts are stated on 

 the log sheet, from which can be obtained: 



1. The testing distance between the source and the 

 hydrophone. 



2. The available power that was used for the pro- 

 jector. 



3. The receiving amplifier gain that was used in 

 the tests. 



4. A reference to the circuit sketch for this test. 



With these data on hand, the calibration can pro- 

 ceed. The first item to be checked is whether or not 

 the same testing distance was employed for the test 

 unit as for the hydrophone standard. If not, the read- 

 ings must be corrected to the same testing distance by 

 means of the formula given in Chapter 4, 



C = 20 1og- r , 



"u 



(1) 



where (/ and rf are the testing distances for the test 

 unit and for the hydrophone standard respectively. 



It will be recalled that the receiving response usu- 

 ally is expressed in terms of the generated voltage of a 

 hydrophone. One exception to this rule is in the cali- 

 bration of the 3A hydrophones, in which the response 

 is expressed in terms of the voltage across 135 ohms. 



The receiving charts give the level (in db vs 10 _lc 

 watt) impressed on the recorder. A number of correc- 

 tions have to be applied to obtain the generated volt- 

 age of the hydrophone from the chart readings. Cor- 

 rections must be made for (1) the receiving amplifier 

 gain, and (2) the coupling loss. By the latter is meant 

 the loss (or gain) in the voltage applied to the input to 

 the receiving amplifier as compared to the generated 

 voltage of the hydrophone. Since the loss in the line 



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