. Elements of acoustical engineering. Electro-acoustics; Sound. 176 MICROPHONES tortion, a microphone ^ with a stretched diaphragm and two carbon buttons is used (Fig. ). 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 a
. Elements of acoustical engineering. Electro-acoustics; Sound. 176 MICROPHONES tortion, a microphone ^ with a stretched diaphragm and two carbon buttons is used (Fig. ). 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 0 ^ ^5 ^li. <y^ 00 1000 10000 CROSS-SECTIONAL VIEW Fig. 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. 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. 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 rang
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