624 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



80 82 84 



t06 



86 88 90 92 94 96 98 100 102 104 



NOISE LEVEL AT LISTENING END IN DECIBELS 



(above 10-'6 WATTS PER SQ Cm) 



Fig. 10 — Effect of noise at listening end on trunk loss. 

 This improvement is attributed to the following: 



1 . The better sidetone balance of the 500-set circuit reduces the level 

 of sidetone noise picked up by the transmitter and reproduced by the 

 receiver, thereby providing a gain in signal to noise ratio. 



2. The acoustic impedance looking from the ear canal into the Ul 

 receiver of the 500-type set is lower than the acoustic impedance of 

 the HAl receiver of the 302 set and, therefore, the acoustic noise pressure 

 built up in the ear canal by leakage around the receiver cap is lower 

 than when the HAl receiver is used. 



3. The output from the transmitter of the 500-type set under noise 

 conditions contains less extraneous distortion products, giving it a char- 

 acter which is less disturbing than with the 302 set. 



Included in the laboratory tests were experiments in which the out- 

 put of the transmitter was intentionally lowered. It is known that a 

 speaker in a noisy location instinctively raises his talking level. Since 

 it is desirable to keep the sidetone level low to improve the signal to 

 noise ratio, the output level of the transmitter at the listening end was 

 reduced about 10 db by shunting a resistance of approximately 40 ohms 

 across the transmitter. The sensitivity of the transmitter is reduced 



