176 



MICROPHONES 



tortion, a microphone ^ with a stretched diaphragm and two carbon 

 buttons is used (Fig. 9.3). The mechanical impedance of a stretched 

 diaphragm below its resonance frequency is a stiffness mechanical reactance. 

 Therefore, a constant sound pressure on the diaphragm will produce sub- 

 stantially constant displacement. Since the change in resistance of the 

 carbon buttons and the resultant developed voltage is proportional to the 

 displacement, the voltage output will be independent of the frequency. 

 To provide damping at the resonance frequency of the diaphragm the 



INSULATOR 

 DIAPHRAGM 



STRETCHING^ 



RING 



MICROPHONE 



CIRCUIT DIAGRAM 



CROSS-SECTIONAL VIEW 



Fig. 9.3. Cross-sectional view and electrical circuit diagram of a double button, stretched 

 diaphragm, carbon microphone. The graph shows the open circuit voltage response fre- 

 quency characteristic for constant sound pressure on the diaphragm. 



damping plate is placed very close to the back of the diaphragm. As the 

 diaphragm moves air is forced through this small space. The high vis- 

 cosity loss in a small slit provides the damping. See Sec. 5.4. In order 

 to reduce the stiffness, in the small space, suitable grooves are provided 

 which reduce the length of the slit. The rear button is enclosed in the 

 damping plate while the front button is supported by the bridge. The 

 duraluminum diaphragm is gold plated over the area occupied by the car- 

 bon buttons to insure contact between the carbon granules and the dia- 

 phragm. The resonance frequency of the stretched diaphragm is usually 

 placed between 5000 and 8000 cycles. See Sec. 3.4. In the absence of 

 the damping plate the amplitude for a constant force at the resonance fre- 

 quency would be greater than that below the resonance frequency. By 

 means of the damping plate the amplitude at the resonance frequency can 

 be reduced to correspond to that of the remainder of the range. A response 

 frequency characteristic of this microphone is shown in Fig. 9.3. 



The circuit diagram for this microphone is shown in Fig. 9.3. For a 



2 Jones, W. C, Bell Syst. Tech. Jour., Vol. 10, No. 1, p. 46, 1931. 



