WAV£ AiSTALYSIS IN REFERENCE TO VOCAL ACTION. Il7 



even resembling a tuning-fork vibration. The period from one puff to 

 the next determines the pitch of the voice ; the form of the puff determines 

 the musical timbre. Some purely hypothetical curves to indicate puffs 

 of different forms are given in figure 110; they all have the same amplitude, 

 phase, and period, but the upper one is sharpest and the lower one is 

 smoothest. 



These puffs act on the vocal cavity, that is, on a complicated system 

 of cavities (trachea, larjmx, pharynx, mouth, nose) with variable shapes, 

 sizes and openings. The effect of the puff on each element of the vocal 

 cavity is double : first, to arouse in it a vibration with a period depending 

 on the cavity; second, to force on it a vibration of the same period as 



that of the set of puffs. The preva- 

 « n lence of one of the factors over the 



-i[ 'I other depends on the form of the 



puff, the walls of the cavities, etc. 



_f\ f \ Some vowels include the puff element 



as an important component, others 

 ^^'^x /'"N consist almost entirely of the cavity 



^-^ ^^~ vibrations. 



« T » The vowel curve — according to 



Fig. 109.— Slant fibers of the M. vocaiis. this theor>' — Contains the record of 



Fio.nO.-Hypothetical curves of pufif3. ^^^ ^^^^^^^ ^^^^ ^^^ ^f ^j^g gg^ of 



cavity tones aroused by it. The glottal puff is of the form of a frictional 

 sinusoid (p. 101) with very large amplitude and very large coefficient of 

 friction; the cavity vibrations are also of the frictional sinusoid form. In 

 a single wave there is not only a record of one glottal puff and of the 

 cavity tones for one vibration, but also of what is left over from the 

 fading vibrations of the preceding wave. 



Curves of vibration resulting from a single puff are found when the 

 interval between the puffs is so large that the vibrations die away before 

 the next puff occurs, as in the curve at the beginning of [ai] " I" in line 110 

 of the Depew plate. In speech the pitch of the glottal tone changes con- 

 tinually; the effects may be seen in the same vowel curve. As the pitch 

 of the tone from the glottis rises, the group of cavity vibrations come 

 closer and finally overlap. This produces very complicated forms, but 

 when the period of the puff becomes an even multiple of that of the cavity 

 the waves sum up in hke phases and strong, smooth vibrations result. 

 For example, when the puffs occur at exactly twice the period of the 

 cavity, each arouses a vibratory movement whose phase coincides with 

 that of the others; the vibratory movement going on in the cavity may 



