MINIATURE CONDENSER MICROPHONE SYSTEM 



457 



The miniature microphone and the miniature vacuum tube with neces- 

 sary coupHng resistors and a stoppage condenser are contained in the 

 metal tube shown in the right foreground of Fig. 4. The dimensions 

 of the container tube are about 7.5 inches long and 0.8 inch in di- 

 ameter. The carrying case houses the second stage ampUfier together 

 with other accessories. 



CONCLUSION 



For the same frequency range, the efficiency of this miniature 

 microphone as determined by a thermophone calibration is about 

 2.5 db greater than that of the 394W type, because of a lower pro- 

 portion of dead to active capacity. In combination with a well 

 designed amplifier, the efficiencies as determined by the voltages on 

 the grid of the tube are about equal, most of the proportional contribu- 

 tion to dead capacity in the one case coming from the amplifier, 

 and in the other case from the microphone. 



In this miniature condenser microphone, the diaphragm is tuned 

 higher than is the 394W, so that the efficiency in association with the 

 amplifier is about 3 db lower than that of the 394W. The resonance 

 of the external cavity gives a maximum lift in response of about 3 db. 

 The microphone as a whole responds uniformly up to 10,000 c.p.s. 

 The diffraction and phase-difference effects are negligible up to that 

 frequency. 



40 

 45 



50 

 m 

 " 65 



z 



t^ 60 



MEASURED 



CALCULATED 



60 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



5 000 10 000 



Fig. 5 — Pressure and field calibrations of miniature condenser microphone, showing 



cavity effect. 



Curves showing the constant pressure and constant field ^ calibra- 

 tions of this microphone are given in Fig. 5. 



The work on this miniature microphone system has so far been 

 essentially of a research nature, and its use has been directed primarily 



^ Loc. cit. 



