CONDENSER AND CARBON MICROPHONES 



49 



currents and that of the permanent magnet of the receiver set the 

 test diaphragm in motion. The resonant frequency is determined by- 

 noting the frequency at which the output from the condenser micro- 

 phone is a maximum. 



In the early Wente microphone the damping plate was a continuous 

 surface. Subsequent work by I. B. Crandall ^ showed that the re- 

 quired amount of damping at the resonant frequency could be obtained 

 without adding unduly to the impedance at other frequencies by cut- 

 ting grooves in the plate. This reduced the stiffness introduced by the 

 air film and decreased the irregularity in response at low frequencies 

 previously mentioned. The grooves in the damping plate of the 



Fig. 1 — Western Electric Company's 394-type condenser microphone. 



Western Electric Company's 394-type microphone are cut at right 



angles. Holes, tapered at the outer end to reduce resonant effects, 



are bored through the plate at the intersection of the grooves to form 



connecting passages between the air film at the front and the cavity 



at the back. In order to prevent the resonance which would result 



if the grooves extended into the portion of the chamber surrounding 



the damping plate, the outer ends are closed by an annular ring which 



is pressed over a shoulder on the plate. The surface of the damping 



plate is plane within 8 X 10"^ inch. The departure from a plane in 



any individual case is determined commercially by the interference 



pattern developed when an optically flat plate is placed over the 



damping plate under test. 



* "The Air Damped Vibratorv System: Theoretical Calibration of the Conilenser 

 Transmitter," I. B. Crandall, Physi'cal Rcvinc, June 191S, pp. 449-46U. 



