234 BELL SYSTEM TECHNICAL JOURNAL 



on the rolati\e sensitivity of the car at different frequencies- we 

 have multiplied the acoustic amplitude at each frequency by the 

 corresponding ear sensitivity factor and the results obtained are 

 taken to be the effective amplitude frequency relations which are 

 characteristic of these sounds. 



The data from the four male records and from the four female 

 records of each sound are separately composited and the resulting 

 curves are shown in the diagram (Fig. 3). This compositing process 

 was somewhat laborious because the analyses of the separate records 

 were made not with reference to predetermined frequency settings, 

 but rather for those critical frequencies which best determined the 

 shapes of the spectrum curves. The individual curves were there- 

 fore plotted, and the average ordinates were then read off for small 

 intervals of pitch. These ordinates were then averaged for each 

 group of four analyses. These average ordinates (after being cor- 

 rected for the calibration of the recording apparatus) were then 

 multiplied by the ear sensitivity factors for the corresponding fre- 

 quencies, and the curves so obtained were plotted on the musical 

 pitch scale according to the usual practice. The final spectrum 

 diagram thus shows the relative importance of the amplitudes of 

 all the components of each vowel for male and female speakers. 



The amplitude units are entirely arbitrary; it is only the shapes, 

 not the sizes of these curves which have any significance. The order 

 in which these curves are arranged is based upon the \<)\\el triangle 

 in Fig. 1. 



Characteristics ok the V'owel Sounds 



The results. of the analyses, as given in Fig. 3 show the essential 

 dynamical properties of these sounds. Consider first the sounds 

 numbered I to VI, which include those vowels usually designated 

 as having single regions of resonance. Progressing through the 

 sequence from I to VI this region of resonance rises in average fre- 

 quency and becomes narrower in range. The rise in average fre- 

 quency is of course a well known characteristic. There is also, at 

 least with the male voices, a somewhat scattered and less well defined 

 high frequency range of resonance, perhaps not essential in speech 

 but more highly developed in wcll-traint>d singing voices. 



The sound a (No. VI) is as it were the center of gravity of the 

 vowel diagram and occupies the key position in the phonetics of 



•See this Journal Vol. II, No. 4, October, 192.V The paper on audition, by H. 

 Fletcher shows a cut of the "Threshold of .^udibihty" curve from which these data 

 wer« obtained. 



