FL UCTUA TIONS IN MICROPHONES A NO O TIIER RES I ST A NCES 20 1 



a varied slightly from cell to cell, the extreme values being 1.75 and 1 .07 

 with an average of about 1.85. 



Figure 3 gives the results of alternate measurements of contact noise 

 and acoustic modulation performed on a particular telephone trans- 



0.01 0.1 1.0 



APPLIED POTENTIAL IN VOLTS 



Fig. 2 — The mean square contact noise voltage in a standard carbon trans- 

 mitter as a function of the d.-c. voltage on the instrument. The noise level of the 

 amplifier and the thermal noise level of the transmitter are indicated. 



mitter. The acoustic field was of constant frequency and was supplied 

 by an accurately controlled oscillator and "artificial mouth." The 

 sound field was of such intensity that when the transmitter output was 

 measured the background noise gave only an inappreciable part of ihe 



