CONDENSER AND CARBON .UICROPIIONES 



51 



inserted between the damping plate and this diaphragm. This valve 

 consists of a disc of silk clamped between two aluminum plates of 

 unequal diameters, (ias in passing from the damping plate to the 

 compensating diaphragm moves laterally from the edge of the smaller 

 plate through the silk to a hole in the center of the larger plate. The 

 impedance of this path is high at voice frequencies but low enough for 

 steadily applied pressure differences to permit compensation for changes 

 in barometric pressure. 



After the damping plate and diaphragm are assembled the space 

 between the clamping rings is tilled with beeswax to make the joints 

 gas-tight and exclude moisture. A hole is, however, provided for 

 filling the microphone with nitrogen. The purpose of the nitrogen is 

 to prevent corrosion of the damping plate and diaphragm surfaces 

 and eliminate any reduction in pressure due to oxidation of the sealing 

 compound. 



It has been customary for some time to determine the response 

 characteristics of a condenser microphone by the thermophone 

 method.*^ In making this measurement the diaphragm of the micro- 

 phone is coupled acoustically to the thermophone in the manner 

 shown in Fig. 3. The thermophone consists of two strips of gold foil 



CONDENSER 

 TRANSMITTER' 



TO VACUUM TUBE 

 VOLTMETER 



THERMOPHONE 

 OF GOLD FOIL 



TO VOLTAGE 

 SUPPLY FOR 

 RESISTOR 



INLET OUTLET 

 HYDROGEN 



Fig. 3 — Cross-sectional view of the thermophone and the condenser microphone. 



which are mounted on a plate and fit into the recess in the front of 



the microphone. Capillary tubes are provided for filling the space 



enclosed between the plate and the microphone diaphragm wnth 



•^ "The Thermophone as a Precision Source of Sound," H. D. Arnold and I. B. 

 Crandall, Physical Review, July 1917, pp. 22-38. "The Thermophone," E. C. 

 Wenle, Physical Review, April 1922, pp. 333-345. "Speech and Hearing," H. 

 Fletcher, 1929, Appendix A. 



