TRANSMITTING 



19 



same auxiliary equipment, such as tuning coil or con- 

 denser and polarizer, should also be used throughout 

 the testing program. For example, in the measure- 

 ment of electric power, the current and impedance, or 

 current, voltage, and phase angle must be measured 

 from the same set of terminals. In more complicated 

 tests the matter of measuring from a single pair of 

 terminals, however, is sometimes overlooked. For in- 

 stance, in order to determine the efficiency of a pro- 

 jector, the transmitting response must be obtained, 

 the directivity measured, and the impedance of the 

 projector and of the source determined. To obtain 

 the correct answer, all electric measurements that 

 enter into these tests must be made from the same ter- 

 minals. Frequently this is inconvenient. The driving 

 amplifier, for instance, is at one place in the labora- 

 tory, while the impedance bridge is at another. If an 

 extra lead is added in either connection, an error 

 results. 



It may be noted in passing that the efficiency thus 

 determined includes any losses in the system beyond 

 the point where the measurements are made. If the 

 efficiency of the projector exclusive of these losses is 

 desired, the measurements must be made directly at 

 its terminals or the losses must be eliminated from the 

 data by computation. The latter method is usually 

 more time-consuming and less accurate. 



A general principle of testing is that it is easier to 

 make relative measurements than absolute measure- 

 ments, and that the precision of relative tests, for the 

 same amount of effort, is much greater. The easiest 

 tests to make are the so-called A-B comparisons which 

 involve immediate switching between two conditions. 

 Usually one of the conditions, say A, which is well- 

 known, serves as the reference condition, and the 

 other, B, which includes the unknown, is the test con- 

 dition. Many a testing difficulty can be avoided by re- 

 ducing the program to a number of such comparison 

 tests, and, if at all possible, tests should always be set 

 up on that basis. 



With these principles in mind, many specific prac- 

 tices have been established by USRL. Some of these 

 are described in Section 6.1.4. The rest of this section 

 is concerned with the physical characteristics to be 

 measured in calibration tests on underwater sound 

 equipment. 



Most acoustic devices are reversible, that is, they 

 can do two things depending on how they are used: 

 (1) When an electric voltage is applied at their ter- 

 minals, they generate acoustic power, and (2) when 



acoustic power is supplied to them, they generate an 

 electric voltage. A device which has these properties 

 in the underwater sound field is called a transducer. 

 The first action is called transmitting and the second 

 receiving. Certain conventions 3 have been set up by 

 agreement among the different groups interested in 

 the underwater sound field for the measurement of 

 transmitting and receiving performance. These con- 

 ventions are of value in making the meaning of meas- 

 urement results precise to all people in the group. 

 Also, by expressing all results on the same basis, dif- 

 ferent measurements can be more easily compared. 

 The most important use of test data is usually to 

 determine which instrument is best from among a 

 number that are available for a particular application. 



4.1 



TRANSMITTING 



Transmitting measurements usually involve three 

 factors: (1) the acoustic pressure delivered by the de- 

 vice in the desired direction for a known electric in- 

 put, (2) the distribution of the acoustic pressure in 

 other directions, and (3) the variation of the pressure 

 with frequency. 



It is noted that these items correspond respectively 

 to (3), (1), and (2) at the beginning of this chapter. 



These quantities must be expressed in such a way 

 that their meaning is unambiguous and that they af- 

 ford a ready means of comparing different designs. 

 For example, a statement of the pressure delivered 

 does not of itself tell much about the performance of 

 a device, since it is possible to change the pressure by 

 increasing or decreasing the electric input. Conse- 

 quently, the electric supply conditions must be speci- 

 fied as well. 



In most practical cases the projector is fed from an 

 amplifier. The most definite way to tie down the elec- 

 trical system in a practical way, therefore, is to specify 

 the amplifier. For calibration purposes it is desirable 

 that a class A amplifier be used, because the perform- 

 ance of such an amplifier can be accurately specified 

 and controlled. From the standpoint of circuit analy- 

 sis, a class A amplifier can be replaced by a generated 

 voltage e g and an internal resistance r g . The same 

 analysis also applies to class B and C amplifiers, but 

 the values then are a function of the power delivered, 

 whereas in the case of the class A amplifier these 



a Conference on underwater sound projectors in the Office 

 of the Coordinator of Research and Development of the Navy. 

 July 19. 1944. 



