Chapter 5 

 TESTING TECHNIQUE 



By Leslie L. Foldy 



si THE TESTING PROBLEM IN 



GENERAL 



5I -' Calibration and Operational Testing 



The testing of underwater sound devices assumes 

 two forms, depending upon the type of informa- 

 tion desired. In one type of test, it is desired to obtain 

 information which characterizes the device independ- 

 ent of its environment to such an extent that its be- 

 havior in any particular environment can be pre- 

 dicted. Such a test is known as a calibration test. On 

 the other hand, when a device is to be used in a par- 

 ticular application, it is often desirable to obtain 

 directly information bearing on its efficacy in carry- 

 ing out an assigned task under the conditions which 

 prevail in the particular application. Such a test is re- 

 ferred to as an operational test. 



The difference in philosophy of the two types of 

 tests is essentially the following: A calibration test is 

 made under carefully controlled conditions, with the 

 object of eliminating all extraneous factors entering 

 into the measurement which represent characteristics 

 of the environment rather than those of the device it- 

 self. In an operational test, on the other hand, en- 

 vironmental factors are of prime importance, since 

 information is desired not on the intrinsic character- 

 istics of the device but on its behavior in an environ- 

 ment closely approximating actual operating condi- 

 tions. There is, of course, a relationship between the 

 operational performance of a device and its inherent 

 characteristics as determined by calibration measure- 

 ments. Operational tests in general are beyond the 

 scope of the activities of the Underwater Sound Ref- 

 erence Laboratories and the present discussion is 

 largely limited, therefore, to testing technique in 

 calibration measurements. 



5.1.2 The Characterization of Transducers 



Most calibration measurements on underwater 

 sound equipment consist of measurements on trans- 

 ducers, so that principal interest is attached to these, 

 although much of the discussion is applicable to 



measurements on domes, baffles, and similar auxiliary 

 equipment. An important consideration in the cali- 

 bration of a transducer is the information which is 

 required to characterize the device. A linear, passive, 

 electroacoustic transducer" is completely character- 

 ized when certain parameters and parametric func- 

 tions are known as functions of frequency, as is shown 

 in Chapter 3. When these relations are known, one 

 can in principle compute the behavior of the device 

 in any well-defined environment. However, neither 

 the determination of the characteristic quantities 

 nor the determination from these of the behavior of 

 the instrument in even relatively simple environ- 

 ments can actually be carried through because of the 

 complexities of the measurements and computations 

 necessary. Fortunately, however, such a complete 

 characterization is neither necessary nor desirable 

 under most circumstances. In the majority of cases, 

 most of the useful information about a transducer 

 can be obtained by relatively simple procedures, and 

 operational characteristics can be derived from these 

 data in a relatively direct and simple manner. 



The characterization of a transducer, as dictated by 

 practical considerations, is summarized in Chapter 4. 

 Therein are indicated the principal functional rela- 

 tionships whose measurement gives information 

 which, if not complete, is at least sufficient to charac- 

 terize a transducer and to allow its operational be- 

 havior to be evaluated for most cases of interest. 



The intent of the present chapter is to indicate the 

 means by which one may determine the true Tallies of 

 the quantities measured; in other words, it is to find 

 the means by which the measured values may be cor- 

 rected to make the results independent of the charac- 

 teristics of the equipment, the location of the tests, 

 etc. This problem presents two aspects: (1) to deter- 

 mine the conditions of the test so that local extra- 

 neous factors do not enter significantly into the 

 measurements and, (2) where the above is not pos- 

 sible under the conditions present, to correct for the 

 effects of local extraneous factors. 



a The definitions of these terms are found in Chapter 3. 



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