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 fiow which would be a fairly faithful copy ot 

 the pressure due to the sound wave. Apparently Dolbear realized 

 that 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. 

 At approximately the same time an article appeared in the French 

 press - calling attention to the use of a condenser as a microphone and 

 commenting on the fact that this type of microphone had been found 

 to be less sensitive than the loose contact type. 



Owing to the low sensitivity of the condenser microphone, the field 

 of usefulness of this instrument was extremely limited for a number 

 of years and it did not assume a position of importance among the 

 instruments used in acoustic measurements and sound reproduction 

 until suitable amplifiers had been developed. The development of 

 the vacuum tube amplifier, however, filled this need. In 1917 E. C. 

 Wente ^ published an account of the work which he had done on an 

 improved condenser microphone having a stretched diaphragm and a 

 back plate so located relative to the diaphragm that in addition to 

 serving as one plate of the condenser it added sufficient air damping 

 to reduce the eft'ect of diaphragm resonance to a minimum.* The 

 response of this instrument was sufficiently uniform over a wide range 

 of frequencies to make it not only useful in high quality sound repro- 

 duction but a valuable tool in acoustic measurements in general. 



The major portion of the condenser microphones used today in 

 sound recording embody the essential features of the Wente micro- 

 phone. Marked progress has, however, been made in the design and 

 construction of these instruments since the initial disclosure and it 

 will no doubt be of interest to many to consider briefly the nature of 

 this advance. 



2 "La Lumiere Electrique," 1881, p. 286. 



3 "A Condenser Transmitter as a Uniformly Sensitive Instrument for the Absolute 

 Measurement of Sound Intensity," E. C. Wente, Physical Review, July 1917, pp. 

 39-63. "Electrostatic Transmitter," E. C. Wente, Physical Review, May 1922, pp. 

 498-503. 



■» A discussion of the theory of air damping is given in "Theory ot \ ibratmg 

 Systems and Sound," 1. B. Crandall, pp. 28-39. 



