52 BULL SYSTEM TECHNICAL JOURNAL 



hydrogen. Tliis is done in order to make the wave-length of the sound 

 developed in the recess as large as possible compared with dimensions 

 of the chamber. If this were not the case the sound pressure at dif- 

 ferent positions in the chamber would not be in phase and the condi- 

 tions on which the computations of the magnitude of the sound 

 pressure are based would not be met. A direct current of known 

 value is passed through the foil. Superimposed upon the direct current 

 is an alternating current of the desired frequency which causes fluctu- 

 ations in the temperature of the foil and in the gas immediately 

 surrounding it. These tluctuations in temperature in turn cause 

 changes in the pressure on the microphone diaphragm. The magni- 

 tude of the pressure developed on the diaphragm can be computed 

 from the constants of the thermophone and the coupling cavity, and 

 the voltage developed by the microphone for a given pressure deter- 

 mined with suitable measuring circuits.'^ Obviously, such a calibration 

 affords a measure of the response of the microphone in terms of the 

 actual pressure developed on the diaphragm and is independent of the 

 external dimensions of the instrument. Hence, it does not take into 

 account any effect which the microphone may have on the sound field 

 when used as a pick-up instrument for recording or broadcasting pur- 

 poses. The thermophone calibration is often referred to as a "pres- 

 sure" calibration and the response obtained by placing the instrument 

 in a sound field of constant pressure, a "field " calibration. A thermo- 

 phone calibration of a representative Western Electric 394-type con- 

 denser microphone is shown on Fig. 4. 



For many of the uses to which the condenser microphone is put, for 

 example the calibration of head type telephone receivers, the condi- 

 tions under which it operates agree with those under which the thermo- 

 phone calibration is made. There are, however, cases where this 

 agreement does not exist, for when a microphone is inserted in a 

 sound field of uniform intensity the pressure on the diaphragm may 

 depart rather widely from a constant value in certain frequency 

 ranges. Several articles ** have recently appeared calling attention to 

 this discrepancy between the pressure and field calibrations and 

 pointing out that a pressure calibration of a microphone may not be 

 entirely representative of its performance under the conditions which 

 exist in a studio. 



'• "Master Reference System for Telephone Transmission," W. H. Martin and 

 C. H. G. Gray, Bell System Technical Journal, July 1929, pp. 556-559. 



"^''The Use of a VVente Condenser Transmitter to JNleasure Sound Pressures in 

 Absolute Terms," A. J. Aldridge, 1\ O. E. E. Journal, Oct. 1928, pp. lU-US. 

 "Effect of the Diffraction Around the Microphone in Sound Measurements," S. Bal- 

 lantine. Physical Review, Dec. 192S, ])p. 988-992. " Measurements of Sound 

 Pressure on'an Obstacle," \V. West, hist. Elec. Eng. Journal, 1929, pp. 1137-1142. 



