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BELL SYSTEM TECHNICAL JOURNAL 



in the list. If now he should call a sentence 2 db low, the lamp to the 

 left will light. If he persists in calling 2 db low the illumination will 

 move farther to the left, but if he raises his voice to the correct value 



Fig. 1 — Visual syllable indicator and signal lamps of automatic volume indicator — in 



calling booth. 



the lights will remain unchanged. By raising his voice 2 db higher the 

 light can be brought back to the center. Thus changes in the position 

 of the lighted lamp show the departure of the sentence called from the 

 desired value and the position itself show's the algebraic sum of these 

 departures. With very little training the caller learns, under the 

 guidance of the signal lamps, to keep the individual deviations small 

 and at the same time to approach the desired average closely. No 

 difficulty is found in attaining an average value for the 66 syllables of 

 the list differing by less than 0.1 db from the desired value. 



Experience shows that this method of assisting a caller to maintain 

 a given intensity is capable of much greater precision than methods 

 which indicate the intensity for each sentence and rely on the caller 

 to make each sentence reach the desired value. It is particularly 

 difficult for the caller, when using calling intensities which are higher 



