Chapter 8 

 PRODUCTION TESTING OF SONAR TRANSDUCERS 



By Erwin F. Shrader 



8.1 



GENERAL CONSIDERATIONS 



Production testing may be distinguished from type 

 testing. Before a design is adopted, very careful 

 calibrations must be made on a number of samples or 

 pre-production models. The models are then taken 

 out to sea for performance tests. Often several modi- 

 fications are made in the original design before it is 

 acceptable. When a satisfactory type is finally evolved, 

 the manufacturer proceeds with the production of 

 the device. Specifications are set up to make the prod- 

 uct as much like the samples as possible. To insure 

 this, tests must be made on each unit; thus production 

 testing involves calibration, albeit in simplified form. 



A production testing program serves two purposes: 

 (1) It insures that each product meets certain specifi- 

 cations, and (2) it permits quality control, that is, a 

 running check on the quality of the manufactured 

 products which notes and corrects any deviations 

 from the accepted standard. Without regard to lite 

 detail of the nature of the test or of the device under 

 consideration, certain basic requirements must be 

 met in order to have a successful production test. The 

 procedure must involve a relatively simple technique 

 not requiring highly trained personnel. The time re- 

 quired for each test must be short. The test should 

 not be affected by conditions beyond the operator's 

 control, such as phenomena of noise interference, 

 water temperature, etc. 



Production testing of a sonar transducer falls into 

 two parts: (1) tests of physical strength, watertight- 

 ness, and polarity of electrical elements, and (2) acous- 

 tic measurements of directivity, response, and impe- 

 dance. This chapter deals only with tests falling in 

 the second category. 



In connection with acoustic production tests, lakes 

 and rivers are, in general, eliminated from considera- 

 tion as testing sites on the following counts: (1) They 

 are, as a rule, separated from the factory; (2) the con- 

 ditions there may not be sufficiently well controlled 

 for routine testing; (.3) they may be subject to noise 

 from water traffic. An indoor tank in the factory offers 

 the best chance of circumventing these difficulties 



while still meeting the requirements of simplicity, 

 speed, and sufficient accuracy. It is true that, in a con- 

 fined body of water, proximity effects and reflections 

 are present and will affect the measurements. On the 

 other hand, there are various methods of eliminating 

 the effects of these reflections and of correcting for 

 proximity effects. (See Chapter 5 for complete discus- 

 sion.) These methods will be considered here with 

 particular application to the problem of acoustic pro- 

 duction testing procedure. 



82 PRODUCTION TEST MEASUREMENTS 



The measurement of the acoustic properties of a 

 sonar transducer has been discussed in Chapters 4, 5, 

 and 6. For a complete description of a sonar trans- 

 ducer, it is necessary to know the receiving or trans- 

 mitting response as a function of frequency, the im- 

 pedance as a function of the frequency," and the direc- 

 tivity patterns at several frequencies in one or more 

 planes, depending on the symmetry of the device. 

 Since the test requirements for each of these measure- 

 ments are by no means identical, it is necessary to dis- 

 cuss the requirements for each measurement sepa- 

 rately and to evolve a test procedure which satisfies 

 the maximum demand of each test. It should be kept 

 in mind, however, that each type of transducer will be 

 a special problem and that, in many cases, certain 

 measurements may be considerably simplified and 

 sometimes even eliminated. 



8.2.1 



Response Measurements 



For a production test of response, an absolute meas- 

 urement is not necessary. The response of the trans- 

 ducer can be compared directly with that of a second- 

 ary standard, which may be a transducer of the same 

 type meeting manufacturing specifications. The cali- 

 bration of the secondary standard should be obtained 

 from a complete free-field calibration. A relative 



■i Impedance measurements have been included in this group, 

 since they depend on (lie acoustic terminating impedance of the 

 transducer. 



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