Chapter 4 

 TYPES OF ACOUSTIC MEASUREMENTS 



By Eginhard Dietze 



The choice of what should be measured is prob- 

 ably as important a part of a testing program as 

 any and requires a clear understanding of the nature 

 and purposes of the tests and of the character and ap- 

 plications of the device under test. Furthermore, the 

 conditions under which the tests are made must be 

 carefully controlled. The existence of controlled con- 

 ditions is one of the principal reasons for substituting 

 laboratory tests for field tests. 



The tester, furthermore, in order to carry out his 

 task intelligently, must possess a broad knowledge of 

 the applications of the device as well as of measure- 

 ment technique. Assume, for instance, that an echo- 

 ranging projector is to be tested. To set up a program 

 for such tests, it is necessary to know what factors are 

 important in echo ranging and how these factors de- 

 pend on the physical characteristics of the device. 

 Only then can tests be made that will throw light on 

 how the device will perform in service and how its 

 performance could be improved. 



A great deal of thought has been given by the 

 Underwater Sound Reference Laboratories [USRL] 

 to these questions. Based on these studies, the perti- 

 nent physical characteristics that should be measured 

 in a calibration test on an echo-ranging projector are 

 (1) directivity (directivity index, horizontal and verti- 

 cal beam widths, magnitude of largest side lobes), (2) 

 frequency-response characteristic, (3) power output 

 (efficiency), (4) selectivity, (5) threshold pressure, (6) 

 receiving response, and (7) impedance. 



It is necessary to devise proper tests for the precise 

 measurement of these characteristics. In acoustic tests, 

 this is not always simple, and even with the greatest 

 care it is usually not possible to equal the precision of 

 electric circuit tests. All factors should be of the opti- 

 mum design in order to achieve even a reasonable de- 

 gree of precision. A first requirement in this connec- 

 tion is one of testing equipment. Any effort expended 

 in obtaining the best possible laboratory equipment 

 will be well repaid. It is almost axiomatic that with- 

 out such equipment the situation is hopeless. Assum- 

 ing that such equipment is available, there are certain 

 fundamental rules of testing which must be observed. 



The most important one, although very simple, is fre- 

 quently violated, often with disastrous results. This 

 rule is as follows: /;; a test, as in any experiment, only 

 one factor may be varied at a time. All other factors 

 must be held constant throughout the tests. Usually 

 more than one characteristic is to be measured, so 

 that more than one test must be made. The above 

 rule, that all factors except the one under test must be 

 kept constant, applies to the entire testing program. 

 A few illustrations which apply to underwater sound 

 testing follow. 



It is essential that all characteristics of the medium 

 remain constant throughout the tests. For example, 

 temperature: unless it is a variable of the test, the 

 temperature must be uniform throughout the pro- 

 gram. In addition, the device itself must be in tem- 

 perature equilibrium with the medium. Depending 

 on its size and type of construction, this may require 

 waiting several hours or even a full day, while the 

 unit is immersed, before tests can be started. It has 

 also been found that results obtained at one tempera- 

 ture do not necessarily apply at other temperatures. 

 Thus the temperature of the water during the tests, 

 the temperature dependence of the device, and the 

 speed with which it reaches temperature equilibrium 

 are important factors. 



In any extended testing program, special care must 

 be taken that the signal levels, the transmission prop- 

 erties of the medium, and the noise background do 

 not change. Drifts in the amplifier or in the oscillator 

 characteristics affect the acoustic conditions by chang- 

 ing the level or the frequency of the sound in the 

 water. To avoid such drifts, it is necessary to check the 

 electric system frequently during the tests. Similar 

 precautions must be observed in using acoustic de- 

 vices. For instance, x-cut Rochelle salt crystals are 

 variable under some conditions and for this reason 

 are undesirable as standards. In many devices one side 

 is grounded, increasing the chance for noise pickup 

 and necessitating proper shielding of the leads. 



Another factor to be considered is that all measure- 

 ments must be made at the same point in the circuit, 

 that is, the leads should be the same for all tests. The 



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