Elements of acoustical engineering (1940) Elements of acoustical engineering elementsofacoust00olso Year: 1940 PRESSURE MICROPHONES 173 the carbon granules between the diaphragm and carbon cup without impeding the motion of the diaphragm. A displacement of the diaphragm produces a change in the pressure between the carbon granules which changes the resistance from granule to granule. The net result is a change MICROPHONE I-H|.|.h-i' ELECTRICAL CIRCUIT CASE CROSS-SECTIONAL VIEW -'I r V ::::^j^z'' r ' ^-s Fig. Cross-sectional view and the electrical circuit diagram of a single button ca
Elements of acoustical engineering (1940) Elements of acoustical engineering elementsofacoust00olso Year: 1940 PRESSURE MICROPHONES 173 the carbon granules between the diaphragm and carbon cup without impeding the motion of the diaphragm. A displacement of the diaphragm produces a change in the pressure between the carbon granules which changes the resistance from granule to granule. The net result is a change MICROPHONE I-H|.|.h-i' ELECTRICAL CIRCUIT CASE CROSS-SECTIONAL VIEW -'I r V ::::^j^z'' r ' ^-s Fig. Cross-sectional view and the electrical circuit diagram of a single button carbon microphone. The graph shows the free space open circuit voltage response frequency characteristic. in the resistance between the diaphragm and the carbon cup. For small displacements the change in resistance is proportional to the displacement. Consider the circuit of Fig. , for sinusoidal motion of the diaphragm, the current, in amperes, in the circuit is given by reo + hx sin co/ where e = voltage of the battery, in volts, Veo = total resistance of the circuit when ^ = 0, in ohms, X = amplitude of the diaphragm, in centimeters, h = constant of the carbon element, in ohms per centimeter, CO = 2Tf, and â / = frequency, in cycles per second. Equation may be expanded as follows, rEo\ hx . h'^x^ . â sm CO/ + sm^ co/ ,â) e hx . AV. h^x^ â sm CO/ -f -â^ â -â- cos 2co/ i'eq 2r£o' 2r£:o .â) Equation shows that there is a steady current, an alternating current of the frequency of the diaphragm vibration and harmonics of this vibra-
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