576 BELL SYSTEM TECHNICAL JOURNAL 



the diaphragm. Acoustical resonance in this cavity increases the 

 pressure on the diapiiragm, which in the case of the W. E. Co.'s 394 

 Transmitter may, under certain circumstances, amount to 5 db at a 

 frequency of 3500 c.p.s. The microphone here described is beUeved 

 to be relatively free from this effect, as the cavity in front of the trans- 

 mitter is conical and quite shallow. The diaphragm is also smaller, 

 so that the response is uniform over a wider angle of sound incidence. 

 This microphone has important practical advantages over the con- 

 denser microphone in that the amplifier may be at some distance from 

 the microphone without loss in efficiency and in that no polarizing 

 voltage is required. The sensitivity of this microphone is about 10 db 

 higher. It is therefore better adapted for use in cases where the 

 source of sound is at some distance from the microphone, since, with 

 the smaller amplification required, mechanical and electrical dis- 

 turbances, and amplifier noises in general, may be kept at a relatively 

 lower level. 



