424 BELL SYSTEM TECHNICAL JOURNAL 



ready comparison, and yet not be subject to fatigue by prolonging 

 the stimulation without an adequate rest period. At high levels it 

 was found that a tone requires nearly 0.3 second to reach full loudness 

 and if sustained for longer periods than one second, there is danger of 

 fatiguing the ear.^'' 



To avoid the objectionable transients which occur when sounds are 

 interrupted suddenly at high levels, the controlling circuit was de- 

 signed to start and stop the sounds gradually. Relays operating in 

 the feedback circuits of the vacuum tube oscillators and in the grid 

 circuits of amplifiers performed this operation. The period of growth 

 and decay was approximately 0.1 second as shown on the typical 

 oscillogram in Fig. 19. With these devices the transient effects were 



^A"/vYvV," r",Wv' 



0.1 SEC; 



<i» n i» w. >wiViWj W »W>W»V 



Growth 



A ^ A A A A A A A A A 



, / 



V y * V V V V V V v' V* 



0.1 SEC. 



A 



VwWtWA WW WWW iw iw w wM 



Decay 

 Fig. 19 — Growth and decay of 1000-cycle reference tone. 



reduced and yet the sounds seemed to start and stop instantaneously 

 unless attention was called to the effect. A motor-driven commutator 

 operated the relays which started and stopped the sounds in proper 

 sequence, and switched the receivers from the reference tone circuit to 

 the sound under test. 



"G. V. Bekesy, "Theory of Hearing," Phys. Zeits. 30, 115 (1929). 



