Condenser and Carbon Microphones - Their Construction and Use

F the numerous microphones which have been developed since Bell's original work on the telephone, only two are used extensively in sound recording for motion pictures, namely, the condenser microphone and the carbon microphone. It has therefore been suggested that it would be fitting to review at this time the construction of these instruments and consider some of their transmission characteristics and the precautions which should be exercised in their use. CONDENSER 1 MICROPHONE In 1881, A. E. Dolbear proposed a telephone instrument which could be used either as an electrostatic microphone or receiver. This * Presented at Soc. of Motion Picture Engineers' Convention, Oct. 20, 1930; Journal, Soc. of Motion Picture Engineers, Jan., 1931. 1 "A New System of Telephony," A. E. Dolbear, Scientific American, J u n e 18, 1881, p. 388. 46 CONDENSER AND CARBON MICROPHONES 47 instrument consisted of two plates insulated from one another and clamped together at the periphery. The back plate was held in a fixed position whereas the front was free to vibrate and served as a diaphragm. It is obvious that, if the diaphragm were set in vibration by sound pressure, the electrical capacitance between the two plates would be changed in response to the sound waves, and if a source of electrical potential were connected in series with the instrument a charging current would flow which would be a fairly faithful copy of the pressure due to the sound wave. Apparently Dolbear realized (hat the current developed in this way would be minute, for in the telephone system which he proposed as a substitute for the one using Bell's magnetic instruments he employed the electrostatic, instrument only as a receiver and adopted the loose contact type of microphone.