. The Bell System technical journal . to that of thereceiver just described, but with the cap omitted in order to exposethe diaphragm to the action of sound waves. The dimensions of thevarious elements were changed so that the impedance of the diaphragmwith its associated network should have a substantially constant valuethroughout a wide frequency range. The response as computed isshown in Fig. 8. The moving coil microphone was calibrated experimentally by compari -son with a calibrated condenser microphone. For this comparison eachtransmitter was mounted with its face outward in an opening i
. The Bell System technical journal . to that of thereceiver just described, but with the cap omitted in order to exposethe diaphragm to the action of sound waves. The dimensions of thevarious elements were changed so that the impedance of the diaphragmwith its associated network should have a substantially constant valuethroughout a wide frequency range. The response as computed isshown in Fig. 8. The moving coil microphone was calibrated experimentally by compari -son with a calibrated condenser microphone. For this comparison eachtransmitter was mounted with its face outward in an opening in the end RECEIVERS AND MICROPHONES 573 wall of a cylindrical drum 30 cm. in diameter and 7 cm. deep. Thetwo openings were spaced 180° with respect to the axis of the drumand on radii of cm. Cracks between the microphones and thewall were carefully sealed. The wall thus formed a baffle of the samegeneral character for each microphone. The drum was mounted on ashaft passing through its axis, about which it was rotated at a speed. 40 50 100 500 1000 5000 10,000 FREQUENCY IN CYCLES PER SECOND Fig. 8—Response of moving coil microphone. of 100 Slip rings were provided for making electrical con-nections to the microphones. The drum was placed in a sound fieldset up by a moving coil loud speaker supplied with current from avacuum tube oscillator. The voltage generated by each microphonewas then measured with an amplifier and this arrangement each microphone passed through practicallythe same sound field. By virtue of the symmetrical character of thedrum its rotation has very little influence on any standing wavepatterns in the room. A check on the reliability of the measurementswas the fact that, if the position of the loud speaker was changed verylittle difterence was observed in the ratio of the voltages generatedeven at the higher frequencies. Likewise, no change was observedwhen the electrodynamic microphone was moved a small distanceaxially
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Keywords: ., bookcentury1900, bookdecade1920, booksubjecttechnology, bookyear1
