260 BELL SYSTEM TECHNICAL JOURNAL 



during each one-eighth-second-interval throughout a typical conversa- 

 tion, we should find that for seventeen per cent of them the peak 

 power would lie between eight to sixteen times the average over a 

 long interval. It is seen that the most frequently occurring value of 

 the peak power is about ten times the average. 



Although a typical voice of a man and a typical voice of a woman 

 are alike in that they use the same average power and variations of 

 power from this average, they are different in other respects which we 

 shall now consider. It is well known that the pitch of the voice of a 

 woman is about one octave higher than that of a man. It was not 

 known, however, until our experiments revealed it, that the intensity 

 of the components having vibration rates above three thousand 

 cycles per second was definitely greater for voices from women than 

 from men. The following investigation shows the extent of this 

 difference. 



An apparatus has been devised in our laboratory which will receive 

 the speech during a conversation and then sort out the components 

 into groups depending upon their intensity and pitch. Those lying in 

 each half-octave band on the pitch scale are automatically grouped 

 together and the group power measured. Also, by means of another 

 automatic device, a sorting process is accomplished within the group 

 placing together all the components having powers between certain 

 power boundaries so that they operate a particular recording meter. 

 It was by means of an apparatus of this latter type that the results in 

 Table II were obtained. It was found that the powers were dis- 

 tributed in each of these pitch bands in approximately the same 

 manner as indicated in Table II for speech as a whole. 



The relative values of the average speech power in each of the half- 

 octave bands are shown in Fig. 4. The horizontal positions give the 

 pitch in octaves above or below a tone having a vibration rate of one 

 thousand cycles per second. The vertical positions give the fraction 

 of the total power which comes into each half-octave band. For 

 example, consider the half-octave from — 2.25 to — 1.75, which is 

 the octave with its midpoint at middle "C" on the musical scale. 

 The fraction of the power coming into this half-octave is about one 

 quarter. It will be noted that for both types of voices the maximum 

 power occurs in the second octave below one thousand cycles. This 

 particular octave contains about one half of the total speech power. 

 The octaves on either side of this one containing the maximum power 

 contain slightly less than one quarter of the total power. No other 

 octave contains more than about three per cent of the total power. 

 It is seen that for the band of lowest pitch the voices from men contain 



