An outline of the existing state of knowledge regarding the acoustic principles of tone-production must be drawn mainly from one source. This is the latest authoritative work on the subject, The Study of Speech Curves, by E. W. Scripture (Washington, 1906). In this work Prof. Scripture overthrows several of the conclusions of Helmholtz which had hitherto furnished the basis of all the accepted theories of vocal acoustics. Considering the eminently scientific character of all Prof. Scripture's research work, his thorough acquaintance with every detail of the subject, and the exhaustive attention devoted to this series of experiments, we are fully justified in accepting his present statements as conclusively proved.
A first impression received from a careful reading of The Study of Speech Curves is that the subject is vastly more intricate than had formerly been believed. Helmholtz's theory of vocal acoustics was fairly simple: The vocal cords vibrate after the manner of membranous reeds; a tone thus produced consists of a fundamental and a series of overtones; vowel and tone quality are determined by the influence of the resonance cavities, which reinforce certain of the overtones with special prominence. This theory is discarded by Prof. Scripture. "The overtone theory of the vowels cannot be correct." In place of this simple theory, Prof. Scripture reaches conclusions too complicated to be given in detail here. A brief outline of the subject must suffice for the needs of the present work.
Prof. Scripture found that the nature of the walls of a resonating cavity is of more importance than either its size, shape, or opening. A flesh-lined cavity is capable of reinforcing tones covering a range of several notes. Further, the vowel sound, and presumably also the tone quality, are determined more by the action of the vocal cords than by the adjustment of the resonance cavities. "The glottal lips vibrate differently for the different vowels." This adjustment of the glottal lips "presumably occurs by nervously aroused contractions of the fibers of the muscles in the glottal lips." Continuing, Prof. Scripture says:
"Physiologically stated, the action for a vowel is as follows: Each glottal lip consists mainly of a mass of muscles supported at the ends and along the lateral side. It bears no resemblance to a membrane or a string. The two lips come together at their front ends, but diverge to the rear. The rear ends are attached to the arytenoid cartilages. When the ends are brought together by rotation of these arytenoid cartilages, the medial surfaces touch. At the same time they are stretched by the action of the crico-thyroid muscles, which pull apart the points of support at the ends.
"In this way the two masses of muscle close the air passage. To produce a vowel such a relation of air pressure and glottal tension is arranged that the air from the trachea bursts the muscles apart for a moment, after which they close again; the release of the puff of air reduces the pressure in the trachea and they remain closed until the pressure is again sufficient to burst them apart. With appropriate adjustments of the laryngeal muscles and air pressure this is kept up indefinitely, and a series of puffs from the larynx is produced. The glottal lips open partly by yielding sidewise,—that is, they are compressed,—and partly by being shoved upward and outward. The form of the puff, sharp or smooth, is determined by the way in which the glottal lips yield; the mode of yielding depends on the way in which the separate fibers of the muscles are contracted.
"These puffs act on the vocal cavity, that is, on a complicated system of cavities (trachea, larynx, pharynx, mouth, nose) with variable shapes, sizes, and openings. The effect of the puffs 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 prevalence of one of the factors over the other depends on the form of the puff, the walls of the cavities, etc."
Prof. Scripture does not undertake to point out a difference between the correct vocal action in tone-production, and any incorrect action. This difference in action does not seem capable of definition by any analysis of the acoustic principles involved.
Mechanical Principles of the Vocal Action
In Part II, Chapter II, it was seen that the outflow of the breath in tone-production is checked by the vocal cords, in accordance with Pascal's law of fluid pressures. Another law of mechanics bearing on this operation is now to be considered, viz., the law of the transformation and conservation of energy.
The application of the law of the transformation and conservation of energy to the operations of the voice is nicely illustrated by the well-known candle-flame test of (supposedly) breath-control. To perform this test the singer is instructed to practise the exercises for breath-control while holding a lighted candle with the flame an inch or two in front of the lips. According to the idea of the breath-control advocates, the expired breath should escape so slowly, and with so little force, that no current of air can be detected at the lips, the expiration therefore does not cause the candle flame to flicker.