MEASUREMENT OF TELEPHONE NOISE 



153 



telephone subscriber's receiver, a second curve indicating the relative 

 interfering effects of different single-frequency currents in the telephone 

 line has been derived. This is also shown in Fig. 1. These two curves 

 have been incorporated in the indicating noise meters for use in measur- 

 ing noise in the subscriber's receiver and noise at the terminals of a toll 

 circuit, respectively. 



0.6 

 0.5 



0.05 



0.01 

 0.008 



0.006 

 0.005 



1500 2000 2500 3000 3500 4000 

 FREQUENCY IN CYCLES PER SECOND 



Fig. 1 — Relative interfering effects of telephone circuit noise currents. 



A second important factor considered was the manner in which 

 various single-frequency noises combine in the human ear. The rule 

 of combination adopted was that by which each single frequency 

 contributed to the total meter reading in proportion to its weighted 

 power. (This is the equivalent of the familiar root-sum-square rule 

 for summing up currents or voltages,) 



