MICROPHONIC NOISE IN VACUUM TUBES 



621 



of the same group of tubes on two different agitating systems. One 

 system in this case consists of a rectangular slate block vibrated by 

 repeated blows of an electrically operated hammer. The other system 

 consists of a steel panel carrying the tube under test, mounted on an 

 apparatus rack which is vibrated by a single blow from a steel ball 

 falling as a pendulum against the rack. The points on this chart 

 scatter about an ideal line over a band about twice as broad as that 

 in Fig. 2 where a test is made and repeated on the same testing unit. 

 It may also be observed that the mean noise levels produced by the 

 two systems are different, about 35 and 43 db below one volt respec- 

 tively. 



The effect of varying the intensity of agitation is shown in Fig. 5. 



O30 



I 35 



Q 

 Z 50 



O 65 



a. 



O 



2 70 



50 100 500 1000 5000 10,000 



IMPACT MOMENTUM IN GRAM CENTIMETERS PER SECOND 



Fig. 5 — Effect of intensity of agitation on 4 No. 264B Tubes. 



The four curves represent four No. 264B Tubes tested under the same 

 conditions. In making the measurements, the tube under test is 

 mounted in an ordinary socket on a heavy base, which is agitated by 

 means of a pendulum swinging against it and making one rebound. 

 From measurements of the initial swing of the pendulum, its rebound, 

 and its mass, the total momentum imparted to the tube mounting 

 during the impact can be calculated. This quantity is plotted as 

 abscissa in the figure and is proportional to the initial velocity imparted 

 to the tube mounting at the point of impact. Different values of 

 momentum are obtained by varying the initial swing and the mass of 

 the pendulum. At the lower values of momentum, the observed 



