MICROPHONIC NOISE IN VACUUM TUBES 



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TABLE I 

 Microphonic Levels in db Below 1 Volt 



indicator are used in both sets of measurements. Table I gives the 

 mean noise levels for each of the three types of tubes and the differences 

 between the values obtained for each type with the two amplifiers. 

 The results represent about ten tubes of each type. The weighted 

 amplifier, of course, gives the lower levels for all tubes since the noise 

 components at all frequencies except 1000 c.p.s. are amplified less by 

 this amplifier than by the more uniform amplifier. The magnitude 

 of the difference in level depends on the frequency spectrum of the 

 microphonic noise being measured and in general is different for differ- 

 ent types of tubes as in this illustration. 



Still another important factor which affects the microphonic response 

 of a given system is the relation between the rate of variation of the 

 noise intensity and the time-response characteristic of the system as a 

 whole, usually determined by the indicator. The indicator may be a 

 meter, oscillograph, or other device, or it may be the ear of a listener. 

 A slow moving indicator would respond less to a pulse of noise, such as 

 might be produced by a single shock to a tube, than a more rapidly 

 responding indicator having the same sensitivity to a steady signal. 

 The time required for the ear to reach its maximum response to a 

 suddenly applied sound is about 0.2 second.^ 



The degree of importance of the time-response characteristic of the 

 indicator in measuring transient pulses may be inferred from Table II. 

 This table gives the results of two sets of measurements made on the 

 same three groups of tubes with a single impact type of agitator. The 



TABLE II 

 Microphonic Levels in db Below 1 Volt 



^"Theory of Hearing; Vibration of Basilar Membrane; Fatigue Effect," G. V. 

 Bekesy, Physikalische Zeilschrift, v. 30, p. 115, March, 1929. 



