E. W. ScrvpUire — Nature of Vowels. 307 



in the vowels arises partly from finding different resonance 

 tones. 



The curves in fig. 1 furnish data concerning the physical 

 characteristics of vowels that seem to justify the following 

 conclusions : 



1. The movement of the air in the mouth cavity is a free 

 vibration and not a forced one. This is the theory first stated 

 by Willis in 1830.* It was criticised by Wheats'tone,f whose 

 overtone theory developed by Helmholtz^: has been almost 

 universally accepted. This latter theory asserts that the mouth 

 cavity acts as a resonator reinforcing one of the overtones of 

 the vocal cords. The mouth tone must adjust itself constantly 

 to one of the harmonics of the cord tone. The careful meas- 

 urements of Hermann§ show clearly that the mouth tone 

 remains constant for the same vowel sung on different notes. 

 Hermann's curves were obtained with great care ; they give 

 results for sung vowels that are consistent only with the earlier 

 theory. The curves of spoken vowels given in fig. 1 show 

 that the mouth tone is constant even while the cord tone is 

 steadily changing. It follows from these facts that the period 

 of the mouth tone is independent of the period of the cord 

 tone and that there is no necessary relation between the adjust- 

 ment of the size of the mouth cavity and the tension of the 

 vocal cords. If the period of the mouth vibration is inde- 

 pendent, it must be the period of the free or natural vibration. 



2. The cord movements in the vowels are of the nature of 

 explosive openings and not of the usual vibratory form found 

 in most musical instruments. According to Hermann, it is an 

 essential of the vowel character that the cords should emit a 

 series of puffs separated by intervals of silence. Such a series 

 would be similar to that emitted by a siren with a series of 

 holes passing before an air jet. This series of puffs is very 

 evident in the first part of the / in fig. 1 ; there is no sug- 

 gestion of a vibratory movement of the cords. This peculiarity 

 has led to the queer statement that talking machines are deaf 

 to the cord tones. The failure of vowel machines, like that of 

 Helmholtz, to produce perfect vowels by adding simple tones 

 together, and of harmonic curve tracers, like that of Preece and 

 Stroh, to produce more than distant resemblances to vowel 

 curves by compounding sine waves, is a natural one, if the cords 

 do not make vibratory movements. 



This view is also supported by the following facts. A vibra- 

 tory body, whatever its natural period, when acted upon by a 

 force varying harmonically, must itself vibrate with the period 



* Trans. Camb. Phil. Soc. f London and Westminster Review, 1837. 



\ Lehre v. d. Tonempfindungen. 



§ Archiv f. d. ges. Physiol., vols, xlv, xlvii, xlviii, li'ii, lviii, lxi. 



