50 



BELL SYSTEM TECHNICAL JOURNAL 



A duralumin spacing ring .001 inch in thickness separates the 

 damping plate from the diaphragm. It is essential that all dust and 

 dirt be excluded from this space. To prevent foreign material from 

 entering through the holes in the plate a piece of silk is fastened over 

 the outer surface. The asseml)l\' of the diaphragm and the damping 

 plate is made in a dust-proof glass cabinet. 



If the back wall of the condenser microphone were rigid, changes in 

 the separation between the damping plate and the diaphragm of 

 sufficient magnitude to affect not only the sensitivity of the instrument 

 but also its frequency response characteristic would result from vari- 

 ations in barometric pressure. Complete compensation for these 



COMPENSATING 

 DIAPHRAGM 



DIAPHRAGM 



DAMPING 

 PLATE GROOVE 



Fig. 2 — Cross-sectional view of the 394-type condenser microphone. 



changes in pressure can only be obtained by permitting free inter- 

 change of air between both sides of the microphone diaphragm. 

 This is, however, objectionable owing to the fact that sufficient 

 moisture is likely to be introduced to start corrosion and affect the 

 insulation between the damping plate and the diaphragm. A com- 

 pensating diaphragm of organic material has therefore been introduced 

 which prevents this undesirable effect of humidity but is sufficiently 

 low in stiffness to equalize the changes in pressure encountered in the 

 normal use of the microphone. 



In order to prevent transmission losses at voice frequencies due to 

 the presence of the compensating diaphragm, an acoustic valve is 



