116 BELL SYSTEM TECHNICAL JOURNAL 



similarity between the spectra of / and long e. In the semi-vowel / the 

 characteristic high frequency (if viewed as a transient) decays much 

 more rapidly than the corresponding vibration in the e sound ; this fact 

 we have from the records themselves, but not from the frequency 

 spectra. It may be that such phenomena as these will require a more 

 definite adherence to the "transient" point of view in dealing with the 

 vowel sounds, a matter previously discussed at some length. 



The transitory or unstable qualities in the actual speech sounds 

 almost defy imitation by mechanical means. There is, for example, 

 the variation in fundamental frequency during the course of a vowel 

 or semi-vowel sound which was pointed out in the paper "The Sounds of 

 Speech." There is also the lengthening of the fundamental period for 

 semi-vowels and voiced consonants as compared with vowel sounds; 

 also the shortening of the fundamental cycle at the beginning of a 

 voiced consonant. 



Finally there is the question of classification of the speech sounds. 

 We have already noted difficulties for some of the vowel sounds. It is 

 likely that the vowel triangle or the arrangement of the vowels in a 

 linear series will require modification. A satisfactory classification for 

 all the sounds, from the dynamical standpoint, is at present an un- 

 solved problem; but in conclusion one suggestion may be permissible. 

 We might limit the application of the term "vowel sound" to those 

 sounds which can be satisfactorily produced by the simple double 

 resonator system. The more complicated vowel-like sounds (/, ng, n, 

 m and possibly r) and some of the consonants can undoubtedly be 

 related to systems of three or more degrees of freedom. A study of 

 these systems is beyond the aims of the present paper; but it is to be 

 hoped that such a study can be carried out, for the sake of the aid that 

 mechanical theory offers in helping to visualize the mechanism of 

 speech. 



