336 



BELL SYSTEM TECHNICAL JOURNAL 



given a rating of zero. This reference point was continued essentially 

 unchanged long after the time when this significance of the zero had 

 been lost by the introduction of improved facilities, and after the 

 Standard Reference System was replaced by the Master Reference 



% 20 



5 



AVERAGE FACTORY 

 NOISY OFFICE 



AVERAGE OFFICE 



NOISY RESIDENCE 



O 



LU -10 



QUIET OFFICE 

 AVERAGE RESIDENCE 



QUIET RESIDENCE 



Fig. 2 — Room noise in familiar locations relative to typical magnitude. 



System.' Because of the long use of this reference point both for 

 rating circuits and for specifying standards of transmission, the numeri- 

 cal values of the volume equivalents of the circuit and the volume 

 losses of the component parts, expressed by this system, have become 

 associated with transmission performance and it is considered de- 

 sirable, at least for the present, to retain this significance of the num- 

 bers as far as possible with the new method of describing circuit 

 characteristics. This has been accomplished, first, by selecting typical 

 limiting conditions for the working reference system and, second, by 

 making the effective equivalent of this system numerically equal to 

 the volume equivalent obtained from the volume loss data. This 

 numerical equality holds for the working reference system with any 

 adjustment of the line provided that the trunk contains enough atten- 

 uation to prevent material effects due to the interaction between the 



^"The Transmission Unit and Telephone Transmission Reference System," 

 W. H. Martin, B. S. T. J., Vol. Ill, p. 400. "Master Reference System for Tele- 

 phone Transmission," W. H. Martin and C. H. G. Gray, B. S. T. J., Vol. VIII, 

 p. 536. 



