610 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



and 



20 log Tj:^ =9 dh (to the nearest decibel). 



Extensive tests have shown that a level of 17 db at the receiver where 

 db = 10~^^ watts will produce no noise impairment of transmission. 

 The corresponding level at the input to the noise meter in the measuring 

 condition is 17 + 9 or +26 db. 



Currents in talking circuits may vary from 0.025 amperes in many 

 central office circuits to approximately 0.150 amperes in short sub- 

 scriber's loops. At a current of 0.025 and assuming AR = 0.002 ohms 

 the noise voltage will be 



e = ^ X 0.025 X 0.002 = 35.4 X 10"' volts 



and the voltage at the input to the noise measuring set will be % e = 

 23.5 X 10~* volts. The power into the set will be 



23.5 X 10"' 



600 



or 1 X 10"^^ watts or db. The noise measuring set is equipped with a 

 weighting network which takes into account the relative interfering 

 effect of different noise frequencies on received speech. For random 

 noise, the effect of this network is to reduce the measured noise level 

 by about 8 db as compared with the reading that would be observed 

 without the network. Accordingly the noise produced by the above 

 23.5 X 10~^ volts would measure —8 db. 



