MICROPHONIC NOISE IN VACUUM TUBES 



617 



< 70 



O 50 



5 10 15 20 25 30 35 40 



MICROPHONIC LEVEL IN DECIBELS BELOW 1 VOLT 



Fig. 1. — Typical distribution of microphonic noise levels produced by a constant, 

 artificial, mechanical stimulus (Western Electric No. 102F Vacuum Tube). 



It is found experimentally that with no type of commercial socket 

 which has been tested can a tube be removed and reinserted, or even 

 be left in the socket for a period of time subject to incidental jars and 

 temperature fluctuations, with the expectation of perfectly reproducing 

 a previously measured microphonic level. The sort of random varia- 

 tion which is usually found is illustrated in the reproducibility chart of 

 Fig. 2. In this chart, each point represents two separate observations 

 of microphonic level made on the same tube in the same apparatus, the 

 tube having been removed and reinserted between the two observa- 

 tions. The two levels thus measured are represented by the abscissa 

 and ordinate, respectively, so that if the measurements were perfectly 

 reproducible, all of the points would lie on a straight line making an 

 angle of 45 degrees with the coordinate axes and passing through the 

 origin. The amount of maximum scattering here is about 5 db and 

 may be considered an average value. In some cases, with commercial 



