152 



PRODUCTION TESTING OF SONAR TRANSDUCERS 



PROJECTOR 



HYDROPHONE 



-H 



Figure 1. Optimum test geometry in a simple rectangular 

 tank without baffles designed for a given test distance and 

 reflection path length. 



a 



PROJECTOR 



OPAQUE 

 BAFFLE 



Figure 2. Arrangement of baffles in a rectangular tank 

 for the interception of principal reflections. 



placed in such a way as to intercept the reflection 

 from the side of the tank. One possible arrangement 

 of baffles is shown in Figure 2. This procedure cannot 

 be used to eliminate reflections, but it will increase 

 the effective path length between them. 



»■* CHOICE OF SECOND TRANSDUCER 



The choice of the second transducer to be used in 

 the acoustic production test measurement depends in 

 general on the instrument under test. For receiving 

 response measurements the second transducer must 

 be a sound source, and vice versa. Its response should 

 be fairly uniform over the frequency range being in- 

 vestigated, so that it responds rapidly to acoustic and 

 electric transients; that is, the time constant of the 

 second transducer should be small if it is to be used 

 in pulse testing. Also, if its response is uniform, 

 slight inaccuracies in frequency will not cause appre- 

 ciable errors. The transducer should be fairly stable 

 and show only small changes in response with tem- 

 perature. 



For continuous-wave noise or warble measure- 

 ments, the transducer should be directional. This will 

 help to discriminate against reflections coming from 

 directions other than that of maximum sensitivity. 

 However, since at a given frequency greater directiv- 

 ity can be obtained only by increasing the size of the 

 transducer, there will be a certain maximum direc- 

 tivity beyond which it will be impossible to go. Fur- 

 thermore, the proximity effects increase with trans- 

 ducer size. 



The NDRC 6B standard projector will be a suit- 

 able transducer for most of the cases encountered. 

 Any other transducer with similar properties will also 

 be suitable. 



85 NATURE OF PRODUCTION TEST 



The degree of refinement of a production test de- 

 pends on the information desired. A calibration test 

 system such as that described in Chapter 6 is capable 

 of giving a permanent record of any and all charac- 

 teristics of a transducer. If the response of a trans- 

 ducer at only a few discrete frequencies is needed, the 

 test system may be a tank, oscillator, standard trans- 

 ducer, and a suitable a-c voltmeter. The response in 

 this case is given simply as a meter reading recorded 

 by the operator. Likewise, if only a few features of a 

 directivity pattern are required, no complicated polar 

 pulse recording system is needed. A simple way of 

 measuring relative magnitudes of pulses is to use an 

 attenuator in the circuit to keep the magnitude of the 

 observed pulses constant on a cathode-ray tube screen. 

 The settings of the attenuator give the relative magni- 

 tudes of the pulses. In this way, a directivity pattern 

 may be constructed from point by point observations. 

 While this procedure is laborious, the number of tests 

 to be made may not justify a more complicated system. 



The pulse method is recommended for determin- 

 ing the directivity patterns of large echo-ranging pro- 

 jectors. The pulse length used depends on the re- 

 sponse characteristic, in the manner described above. 

 The tank must be large enough to satisfy proximity 

 effect and reflection path length requirements. 



For measuring the response of small transducers, 

 testing distances may be short, and some sort of tank 

 with absorbing walls may be practical. While the 

 pulse technique is still applicable, continuous-wave 

 or noise and warble methods may be simpler and 

 yield results within the accuracy desired. 



The absorbent tank described above will also be 

 adequate for continuous-wave measurements of the 

 electric impedance of most devices. 



