628 BELL SYSTEM TECHNICAL JOURNAL 



Talking Volumes 

 A second source of variation in the load on a given channel is that 

 the impressed volume may have any value within rather wide limits 

 when a channel is active. By "volume" is meant the reading of a 

 volume indicator of a standard type. Its importance in the present 

 problem arises from the fact that the volume is an approximate mea- 

 sure of the average speech power being introduced into the channel. 

 Although some other instrument might give a better measurement of 

 the latter quantity, only the volume indicator has been used sufficiently 

 widely in the plant to give data on the distribution of average speech 

 power per call under commercial conditions. The average speech 

 power is dependent on the type of instruments, the character of the 

 speech, and the time interval over which the average is determined. 

 From an analysis of phonograph records of continuous speech it is 

 found that the average speech power of a reference volume talker may 

 be taken as 1.66 milliwatts, and the relationship between volume' 

 and average power may be expressed by the following equation : 



^^ , , „, 10 logio Average Speech Power in Milliwatts ,^. 



Volume (do) = ^ : -^— . (2} 



1.66 



This equation is based on the long average speech power. It will 

 be understood that for purposes other than load rating computations, 

 a different relation might be found more suitable. 



The use of equation (2) to relate volume to average speech power 

 is applicable to speech in a single channel. It is convenient to refer 

 to a quantity related in the same way to the total average power con- 

 tributed by a number of channels as the "equivalent volume." 



The single-channel volumes on commercial circuits are conveniently 

 measured at the transmitting toll test board, which will be taken as a 

 point of "zero transmission level." Henceforth it is assumed that 

 there is no gain or loss between this point and the output of the ampli- 

 fier, so that the latter is also a point of zero transmission level. While 

 this will seldom be the case in an actual system, the necessary change 

 in the load capacity is easily computed. The volumes at this point 

 are found to be distributed approximately according to the "normal" 

 law; that is, the probability that the volume will be between V and 

 V -\- dV \s given by 



1 



(V-Vo)^ 



p{V)dV^-^e 2.2 dV. (3) 



-v27ro- 



^ Subsequent to the preparation of this paper, a new volume indicator was stand- 

 ardized for use in the Bell System. With the new volume indicator, volume is 

 expressed in vu, + &vu being approximately equal to reference volume (0 db) as 

 used herein. 



