LOAD RATING THEORY 



641 



bility that the equivalent volume will be between V and V -}- dV is 

 given, for an iV-channel system, by 



p(V) = p{l)Pi(V) + p{2)p2{V) + 



+ p{n)pn{v). 



The pn{V) are given by equivalent volume curves such as those in 

 Fig. 8 and the p{n) by equation (1). Examples of curves thus com- 

 puted are given in Fig. 9, which shows the equivalent volume distribu- 

 tions at a point of zero transmission level for 3, 12 and 240 channel 

 systems. The equivalent volume that is exceeded 1 per cent of the 

 time, read from such curves, is plotted as curve B of Fig. 6. 



1.0 



0.9 

 S 0.8 



Q 



UJ 



^ r\-r 

 O 0.7 



X 



UJ 



'n 0.6 



UJ 



i 0.5 



_j 

 O 

 > 0.4 



0.3 



0.2 



0.1 



-22 -20 



-18 -16 -14 -12 -10 -8 -6 -4 -2 

 EQUIVALENT VOLUME IN DECIBELS 



10 



Fig. 8 — Equivalent volume distributions for n active channels. 



This curve gives, for any number of channels having uncontrolled 

 volumes, the equivalent volume which will be exceeded just 1 per cent 

 of the busy hour. To obtain the necessary load capacity, this must be 

 corrected for the multi-channel peak factor. In the controlled 

 volume case, for a given number iVof channels in the system, there was 

 no difficulty in deciding the value of n, the number of simultaneously 

 active channels, for which the multi-channel peak factor should be 

 taken. Now, however, there is no unique relation between equivalent 

 volume and the number w; in addition, the multi-channel peak factors 

 were measured with all n channels at the same volume, which repre- 

 sents a condition rarely holding on a system without volume control. 

 It is apparent, however, that in the majority of cases in which the 

 equivalent volume approaches values on curve B of Fig. 6, the number 



