CONDENSER AND CARBON MICROPHONES 



59 



One of the receivers was the conventional form of moving- coil direct 

 radiator and was used to provide sound in the lower frequency range. 

 The other was a special moving coil receiver with a short horn so 

 designed as to serve as an efficient source of sound up to 10,000 

 cycles.i^ To reduce the effect of standing waves the mounting for the 

 receivers was so constructed that they could be rotated through a 

 circle approximately five feet in diameter and always face the micro- 

 phone under test. Before starting the test of the carbon microphone 

 the receivers were calibrated by placing a calibrated condenser micro- 



pig_ 10 — Apparatus employed in calibrating the 387-type carbon microphone. 



phone at the point where the test instrument was to be located and 



determining the receiver current required to produce a pressure of one 



bar (one dyne per square centimeter) on the microphone diaphragm. 



The condenser microphone was then removed and the test microphone 



substituted. The open circuit voltage developed by the microphone 



when supplied with a direct current of .025 ampere per button was then 



measured. The data obtained in this way are essentially a "pressure 



calibration" of the microphone and in interpreting them in terms of 



"field" performance the same factors must be taken into account 



17 "An Efficient Loud Speaker at the Higher Audible Frequencies," L. G.Bost- 

 wick, Journal of the Acoustical Society, Oct. 1930, pp. 242-250. 



