ABSOLUTE CALIBRATION OF CONDENSER TRANSMITTERS 109 



directivity of the transmitter is always desirable for pickup systems of 

 highest quality. 



A complete description of the performance of the microphone as an 

 electro-acoustic converter is extremely complex. It involves the 

 microphone, the sound source, their relative positions, and the sur- 

 rounding acoustic configuration. Furthermore, it is limited to sound 

 sustained long enough to allow the reflection pattern to attain a steady 

 state. Therefore, in order to obtain a reasonably simple and useful 

 statement of the transmitter response, the field calibration is made 

 under the ideal acoustic conditions stated in part A. Even then the 

 field calibration (including, of course, the azimuth measurements) is far 

 more difficult and laborious than the corresponding pressure cali- 

 bration. For some important purposes the pressure calibration is 

 sufficient, even though the transmitter be intended for use in an 

 "open" sound field. An instance is the specification and comparison 

 of instruments having similar contours. The difference between the 

 field and pressure calibrations, once determined for an individual 

 instrument, applies to all others. That is, provided the acoustic 

 impedances of their diaphragms are not too widely different, which 

 usually is the case. Therefore the response of any instrument, as a 

 function of frequency, age, barometer pressure, temperature, etc., is 

 given by the pressure calibration. The thermophone method (Method 

 1) is particularly suitable for rapid and reproducible determinations of 

 the pressure calibration. That is the method employed for the 

 specification of No. 394-Type Transmitters, and of others having 

 similar contours, in the Master Reference Systems ^^ for Telephone 

 Transmission in Europe and in this country. 



I am indebted to Messrs. R. T. Jenkins, H. T. O'Neil and E. M. 

 Little of Bell Telephone Laboratories for much of the material used in 



this paper. 



Appendix I 



The pressure generated by the thermophone is slightly reduced by 

 the heat conductivity of the chamber walls. That conductivity is so 

 great as compared with that of the gas, that zero temperature variation 

 at the walls may be taken as one of the boundary conditions of the 

 problem. This results in a solution nearly identical with that of eq. 

 (7), p. 336, in the original derivation.^ The correction factor given 

 there on p. 340, which takes care of the wall conductivity, is now 

 found to be more nearly unity. The difference between the two 

 solutions is shown in Fig. 12 for a special case typical of condenser 

 transmitter calibrations. As might be expected, it is greater the lower 

 the frequency. 



