MICROPHONIC NOISE IN VACUUM TUBES 



631 



30 



40 45 50 55 60 65 70 75 80 



MICROPHONIC LEVEL IN DECIBELS BELOW I VOLT 



Fig. 9 — Effect of cushion socket. 



the intensity of the sound wave which is finally allowed to reach the 

 tube, by some such means as enclosing the tube in a heavy, air-tight 

 container. 



Reduction of Sputter Noise 



The reduction of sputter noise in vacuum tubes is chiefly a problem 

 for the tube manufacturer. Where sputter noise exists in a tube, and 

 exists only with agitation, it is often eliminated by the same cushioning 

 measures which are applied to reduce microphonic noise, but in many 

 cases, satisfactory reduction of sputter would require prohibitive 

 amounts of cushioning. Fortunately, however, the known design 

 features and manufacturing methods, which are now generally applied 

 to tubes of good design, are for the most part quite effective in reducing 

 sputter noise to a negligible level. In the older types of filamentary 

 tubes, for example, sputter noise was often present due to the rattling 

 of the fi,lament at the hook supports at operating temperatures. This 

 source of sputter has been removed in most present day tubes by keep- 

 ing the filament under tension at all times by means of flexible canti- 

 lever spring supports. The effectiveness of this treatment is illustrated 

 in Fig. 10, which shows distributions of sputter noise levels for two 



