MICROPHONIC NOISE IN VACUUM TUBES 



619 



terized by very much less scattering than any normal tube mounting, 

 as may be seen in the correlation chart of Fig. 3, which is typical of all 

 of the tests made with these light suspensions. The maximum scatter- 

 ing here is only about 1 db. 



Going to the opposite extreme in tube mounting, similar tests have 

 been made with the tube base held tightly in a split metal clamp, 



26 26 30 



SECOND TEST : 



32 34 36 38 40 42 44 



MICROPHONIC LEVEL IN DECIBELS BELOW I VOLT 



Fig. 3 — Reproducibility of microphonic measurements using a rubber clamp tube 

 mounting with a constant, artificial, mechanical stimulus (37 No. 102F Tubes). 



which itself is bolted rigidly to a heavy base. As is to be expected, the 

 observed levels vary widely and erratically for successive insertions of 

 the tube, and the mere tightening or loosening of the thumb-screw 

 controlling the pressure of the clamp on the base in some cases changes 

 the level by as much as 10 db. 



As for the nature of the applied agitation and the vibrational char- 

 acteristics of the tube mounting, a countless number of combinations 



