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CONCEPTS OF PHONATION |
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and, being elastic, they may be set into vibratory activity by the pressure of the expiring breath stream. The internal contraction of these thyroarytenoid muscles tenses and stiffens them for phonation and the degree of this tension also influences the pitch of the phonated tone. Coinci-deatly with this tensing action, the vocal edges (cords) of the glottis are brought together by the approximation of the two arytenoid cartilages to which they are posteriorly attached. This increases the resistance of the vocal edges to the expiring breath stream and forces them into vibration, much as the lips of a bugler vibrate when he blows into his instrument. *T£ the air pressure remains constant . . . the vibratory action will recur at rhythmical intervals and an audible tone will be produced." [p. 459] 5) The hyoid bone lies horizontally above the entire phonating mechanism and completes the laryngeal framework. Its chief function is to hold the superior entrance into the larynx open where it joins the lower throat cavity (pharynx), just as the cricoid cartilage holds the inferior entrance to the larynx open where it joins the trachea tube. The hyoid bone also serves as an anchorage for the tongue and other muscles. Because it is always firmly attached to the thyroid cartilage (by means of thyro-hyoid muscles and ligaments), the hyoid bone usually moves as a unit with the latter, imparting its own movements to the thyroid cartilage and vice versa, [p. 16]
Extrinsic mechanisms. Also attached to each of the rigid structural parts of the larynx described above are numerous extrinsic muscles and ligaments that activate the phonating mechanism or eke hold the larynx in a steadying position in the throat that is favorable to phonation. These extrinsic attachments also increase the vibratory area of phonadon and thus augment the initial vibrations generated by the tiny vocal cords themselves. (The vocal cords of an adult measure from about 12.5 to 17 mm. in the female larynx and from 17 to 25 mm. in the male larynx, or from about one-half inch to less than one inch in length.) [p. 457] The process of phonation thus encompasses a much wider musculature than is represented by the laryngeal mechanism itself.
The vibratory area of phonation. The tones of phonation really consist of two types of vibratory energy: a) originating vibrations and b) sup plementary or enforced vibrations. Curry estimates that only twenty per cent of the energy applied to the vocal cords in the form of breath pressure is utilized at the glottis. [124, p. 49] The remaining eighty per cent is either dissipated in unused breath or it may be absorbed into contiguous areas in the form of sympathetic (enforced) vibrations of connecting muscles and surfaces. Such supplementary vibrations considerably augment the volume and also influence the quality of the phonated tone. |
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