HANDBOOK OF ACOUSTICS - online book

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FLUE-PIPES AND REEDS.
115
varying the tension of the vocal ligaments. This is effected by the contraction of certain muscles, which act on the cartilages to which the ligaments are attached in such a way as to stretch these latter to a greater or less extent. The density of the vocal ligaments also seems to be variable. According to Helmholtz " much soft watery inelastic tissue lies underneath the proper elastic fibres and muscular fibres of the vocal chords, and in the breast voice this probably acts to weight them and retard their vibrations. The head voice is probably produced by drawing aside the mucous coat below the chords, thus rendering the edge of the chords sharper and the weight of the vibrating part less, while the elasticity is unaltered."
As in other reed instruments, the tones of the human voice are very rich in overtones. In a sonorous bass voice, it is easy to detect the first seven or eight, and by the aid of resonators even more. When a body of voices are heard together, close at hand, singing forte, the shrill overtones are only too prominent. These overtones are very largely modified in intensity by the size and shape of the nasal cavity and the pharynx, also by the varying size and shape of the mouth and position of the tongue. Hence, when the vocal ligaments have originated a compound tone rich in partials, the varying features just mentioned, may reinforce now one set of partials, and now another, in very many different ways, thus producing the endless variety of qualities found in the human voice.
The following is a very instructive experiment in connection with this subject, showing how the reinforcement of particular partials by the resonance of the mouth cavity modifies the quality. Strike an ordinary C tuning-fork, and hold the ends of the vibrating prongs close to the open mouth, keeping the latter in the position required for singing " ah." Notice how the quality of the fork is affected. Now do the same again, but put the mouth in the position required for sounding " oo." Observe the change of quality. A looking glass will be required in order to see that the fork is in the right position. Once more repeat the experiment; but this time, while the vibrating fork is held in position, move the mouth from the position "ah" to the " oo" position, at first gradually, and then rapidly; the corresponding change in the quality of the tone of the fork is very striking, For a detailed account of the Larynx and of Voice production, the student is referred to Behnke's " Mechanism of the Human Voice."