396 BELL SYSTEM TECHNICAL JOURNAL 



Condenser Microphone 



One of the first so-called high-quality microphones developed for 

 use with amplifiers was of the condenser type. This is in principle one 

 of the simplest of all microphones. It consists essentially only of two 

 parallel insulated plates, one of them fixed and the other movable 

 under the action of the alternating pressure of the sound wave. When 

 these plates are connected in series with a resistance and a battery an 

 alternating current will flow in this circuit in accordance with the 

 variations in capacitance between the two plates. The resulting 

 potential variations across the resistance are impressed on the grid 

 of a vacuum tube. 



A different method of using the condenser microphone has been 

 described by Riegger.^^ The microphone is made a part of the capaci- 

 tance element of a high-frequency electric oscillator. The frequency 

 of the oscillations is thus modulated in accordance with the sound 

 pressure acting on the diaphragm. If the modulated current is 

 transmitted through a circuit, the transmission of which varies linearly 

 with the frequency, in series with a linear rectifier, the output current 

 of the rectifier will correspond to the sound pressure. 



The condenser microphone as commonly used is of a size such that 

 at the higher acoustic frequencies it will distort the sound field. The 

 pressure and free field calibrations begin to diverge from each other 

 at about 1000 c.p.s. To eliminate this distorting effect a number of 

 investigators ^^ have developed miniature condenser microphones for 

 laboratory use. Generally such instruments have been designed at a 

 sacrifice in sensitivity and uniformity of response. The small size 

 microphone developed by Harrison and Flanders, however, has a 

 remarkably flat response frequency characteristic and a sensitivity 

 comparable with that of the larger instrument. Still smaller condenser 

 microphones have been constructed but at a sacrifice in sensitivity. 



At this point it may be of interest to give an example which illus- 

 trates the great advantage that the vacuum tube amplifier has given 

 us in the design of sound measuring instruments. With an amplifier 

 having a uniform amplification from 50 to 10,000 cycles, it is possible 

 to measure, under favorable conditions, voltages as low as 1 micro- 

 volt. The amplitude of motion of the diaphragm of a common form 

 of condenser transmitter delivering this voltage is about 10~'^ cm., or 

 about 1/1000 of an Angstrom. This illustrates the extremely small 

 amount of motion that has to be imparted to the moving element of the 



2« Wiss. Ver. Siemens-Konz. 3, 2, 67-100 (1924). 



"K. Hall, Jour. Acous. Sec. Anier. 4, 83 (1932). Harrison and Flanders, Bell. 

 Sys. Tech. Jour. XI, 451 (1932). 



