368 



BELL SYSTEM TECHNICAL [JOURNAL 



conditions, and then only by a small amount. Furthermore, in spite 

 of the wider variations in sidetone of the anti-sidetone set, these varia- 

 tions are over a range such that the resultant variations in effective 

 losses are smaller than for the sidetone set. 



Considering Fig. 5a, it will be noted that for either sidetone or 

 anti-sidetone sets, the sidetone is louder than for natural speech 



RANGE OF 



SIDETONE SET u- 



I 

 AVERAGE I 



35 30 I 25 



TELEPHONE-SET SPEECH-SIDETONE 



20 

 IN DEC 



I 



_ RANGE OF , 



ANTI-SIDETONE SET 

 AVERAGE 



15 I 10 I 5 



IBELS (above air SPEECH-SIDETONE) 



15 10 5 -5 -10 -15 -20 



TELEPHONE-SET SIDETONE ROOM-NOISE IN DECIBELS 

 (above ROOM-NOISE IN FREE EAR) 



Fig. 5 — Effects of sidetone on user of telephone. 



sidetone, which, as noted before makes the user think he is talking 

 louder than he actually is. The average sidetone reduction of 10 db 

 for the anti-sidetone set results in less of this restraint on his talking 

 level, with a resultant net effective gain in transmitting of about 4 

 db compared with the sidetone set. 



In receiving, Fig. Sh, sidetone introduces an effective loss by the 

 reproduction in the telephone ear of room noise picked up by the 

 transmitter. It will be noted that for the anti-sidetone set, the 

 reproduced noise is in general appreciably lower than the room noise 



