NATURE OF SPEECH AND ITS INTERPRETATION 



143 



above or frequencies below 1550 cycles an articulation of 65 per 

 cent will be obtained. The two dotted curves necessarily intersect 

 at 50 per cent. 



The cur\es in Fig. 9 show how the articulation of some of the 

 fundamental speech sounds was affected by eliminating certain 

 frequency regions. The ordinate gives the number of times the 

 sound was written correctly per 100 times called. As in Fig. 8 the 



XMlJIl: I r 



lOCO MO MO '000 5000 



lOOC :000 XCC 4000 iOOO 



Cut-off frfq^e'^^/ 



1000 iCOO iOCO •JJ.jj 5000 



Cut'off Frequf^Cy 



lOOO 2000 300.' KOO iOOO 



Cufoff Frequency 



1000 iOSO XOO 4000 5000 



Cut -off f'equef<cy 



lOOO 2000 iOOG '000 5000 

 Cul-cff r,eciue"c. 



lOOO 2000 iCCO 4X0 5CO0 



Cufgff frequency 



Fig. 9. 



left hand curve shows the effect of eliminating all frequencies below 

 and the right hand curve the effect of eliminating all frequencies 

 above the frequency specified by the abscissa. 



These nine speech sounds were chosen as representing three im- 

 portant classes. It is seen that the long vowels e, 1 and i can be trans- 

 mitted with an error of less than 3 per cent when using either half 

 of the range of frequencies. When using either frequencies from 

 to 1700 or from 1700 to infinity e was interpreted correctly 98 per 

 cent of the time. Similarly 1 was interpreted correctly 97 per cent 

 of the time when using either the range from to 1000 or 1000 to 

 infinity, and i 96 per cent of the time when using either the range 

 from to 1350 or from 1350 to infinity. The short vowels, u, o and 

 e are seen to have important characteristics carried by frequencies 

 below 1000. More than a 20 per cent error is made on any of these 



