366 BELL SYSTEM TECHNICAL JOURNAL 



In other words /3 is the intensity level per octave expressed in bels. 

 For example, the octave containing the most energy in men's voices 

 is — 1.75 to — .75 and it contains about lO"-^ or 31 per cent. The 

 octave below — 3 contains about 4 per cent and the octave above 

 + 1 about 5 per cent. For women's voices these figures are 31 per 

 cent for the most intense region, which is the octave from — .85 to 

 + .15, and .2 per cent and 7 per cent, respectively, for the other two 

 octaves. 



Audible Pitch Limits 



The audible pitch limits for conversational speech received at 

 various intensities are determined in the following way. It is seen 

 from Table III that the peak power exceeds the average power by 

 17 db 10 per cent of the time. The loudness of speech near the 

 threshold is probably determined by these louder components. For 

 convenience the term "effective intensity level" will be used when 

 speaking of these components only. With this nomenclature the 

 effective intensity level is 17 db above the average intensity level. 

 Using these figures and assuming that three-fourths of the speech 

 power is radiated through the hemisphere in front of the speaker, 

 then one can calculate that the effective intensity at one meter's 

 distance will be 6 X 10"^ microwatts per square centimeter or at an 

 effective intensity level of 22 db below one microwatt. 



To determine the sensation level the pitches and intensities of the 

 components in the vowels must be considered. A study of the fre- 

 quency spectra of these vowels indicates that the loudest component 

 contains from 1/2 to 1/5 of the total power of the vowel. From this 

 it is concluded that the components determining the threshold are 

 from 3 to 7 db below the effective level of the speech. The threshold 

 of hearing for pure tones in the pitch region between — 1 and + 1 

 octaves is from — 85 to — 95 db with an average value of — 91 db. 

 Consequently, it is concluded that at the threshold the effective 

 intensity level for the speech is approximately — 86 db and the average 

 level approximately — 103 db. Since the effective level of the speech 

 at one meter's distance was shown to be — 22 db, it is seen that the 

 sensation level at one meter's distance is 64 db. If the speech wave is 

 uninterrupted by reflections then this level decreases 6 db when the 

 distance between the speaker and the listener is doubled. This level 

 will be raised or lowered in accordance with the intensity of the speak- 

 ing, the variation for different speakers being in accordance with the 

 data in Table I. 



For example, using these relations one finds that the most probable 



