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BELL SYSTEM TECHNICAL JOURNAL 



ponents which are successively occurring as the speech proceeds is 

 very much greater in the higher pitch regions. 



As shown above, the threshold is determined for conversational 

 speech when the average speech level is at a — 103 db. For the same 

 reason that only 10 per cent of the peaks having the highest levels 

 determined the threshold for the speech as a whole, the curves labelled 

 90 per cent of this figure can be used as a basis for determining the 

 sensation level in each of the bands. When the ear of the listener 

 is 10 centimeters from the mouth of the speaker the sensation level 

 will be 84 db and the average intensity level will be — 19 db. If ao 



2 OCTAVES FROM I KILO-CYCLE 5 



O O 



o o 



2 -i. 



Fig. 17 — Speech audibility curve (male voices). 



is the average threshold level for tones in each of the half octave 

 bands, then, if we subtract ao — 19 from each ordinate of the curve 

 in Fig, 16, we will obtain the sensation level of each half octave band. 

 A curve constructed in this way will be called an audibility curve and 

 is given in Fig. 17. This curve is for the case when the lips of an 

 average male speaker are 10 centimeters from the ear of an average 

 listener. It will be seen that the half octave bands above 3.25 octaves 

 and below — 4.25 octaves are just audible. If the distance between 

 speaker and listener is increased to one meter, which is the most 

 commonly used distance, then the audibility curve would be one which 

 is lowered 20 db from that one shown in Fig. 15 and the audible limits 

 would be + 3 and — 3.5 octaves, corresponding to frequencies of 



