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BELL SYSTEM TECHNICAL JOURNAL 



Fig. 9, developed about ten years ago. Essentially it is a vacuum 

 tube rectifier with a rapid action d.c. meter in the plate circuit. 

 It is operated on a part of the characteristic such that the rectified 

 plate current is roughly proportional to the square of the speech 

 voltage. The rectifier is preceded by an amplifier of adjustable gain. 

 For the speech level under measurement the gain is adjusted to such 

 a value that the fluctuating meter deflections attain a prescribed 



40 



30 



20 



.02 .04 .06 



.08 JO .12 .14 .16 .18 .20 

 DURATION OF PULSE -SECONDS 



.22 .24 .26 .28 



Fig. 9 — -Volume indicator — -deflection as a function of a.c. pulse duration. 



maximum value on the average of once in about three seconds. That 

 value of gain, expressed in decibels with respect to a certain normal 

 value of the gain, gives the volume indicator measure of the speech 

 level. The meter, combined with the electric circuit, has a dynamic 

 characteristic as shown in the curve, which gives the maximum 

 deflection as a function of the duration of the a.-c. input. For inputs 

 lasting more than about 0.18 second the maximum deflection remains 

 the same. Since the average syllable duration is of the order of 

 0.2 second, it follows that the maximum deflection of the "volume 

 indicator" meter is approximately proportioned to the mean power in 

 the syllable. By comparison with oscillograms, or by equivalent 

 methods, the volume indicator readings can be correlated by absolute 

 quantities. Fig. 10 is an example showing the relation between 

 speech level as measured with the volume indicator and the average 



