The amount of vibration. --The extent of movement of the cords will vary with the force of the outgoing current of air, and thus the loudness of the sound will increase with greater force of expiration.
The rapidity of the vibrations.—The pitch of sound depends, as we have seen, on the rapidity of the vibrations, and that is deter mined by two circumstances—the length of the cords and their tightness, for the shorter and tighter a string is the higher the note which its vibration produces. The vocal cords of women are about one-third shorter than those of men, hence the higher pitch of the notes they produce. The cords of tenor singers are also shorter than those of basses and bari tones. In children the vocal cords are shorter than in adults. In boys, at the of puberty, the larynx enlarges. This is accom panied by an elongation of the cords, and a consequent change in the pitch of the voice, which becomes lowered. In popular language the voice is said to crack. Thus age and physi cal growth determine the pitch of the voice. Voluntary variations in pitch are due to the fact that the tension or tightness of the cords can be varied at will by muscular movements. It has been pointed out (p. 354) that the thy roid cartilage can be pulled downwards and forwards over the cricoid by the action of the crico-thyroid muscle. Now the front ends of the vocal cords are attached to the inner surface of the thyroid, and when it is pulled forwards they are stretched, provided the arytenoid car tilages to which their other ends are connected are fixed. The more strongly the thyroid car tilage is pulled on, the higher will be the pitch of the note produced by the vibration of the cords, because the greater will be their degree of tightness. It is believed by some, also, that the vocal cords may be shortened by muscular action causing the cords to overlap one another, so that part of their length is "stopped ", and so prevented vibrating. This would also in crease pitch, just as a violinist varies the pitch of a note produced by his violin string by " stopping " it at varying distances. It has been suggested that it is such a "stopping" action that determines the production of the falsetto voice.
In opposition to muscles whose contraction tightens the cords are others which pull up the thyroid cartilage, and otherwise produce relaxation and shortening of the cords, and consequently lowering of the pitch of the voice.
The quality of the voice depends on phy sical conditions of the cords, their degree of smoothness, elasticity, thickness, &c. More over, the form of the windpipe, larynx, throat, mouth, &c., takes part in determining quality. In fact, the air-passages both below and above the vibrating cords act as resonators, or re sounding chambers, and intensify and alter the sounds produced by the cords. Indeed, the sounds emitted by the cords would be feeble but for the intensifying action of the air-passages. By their form, however, they are fitted to intensify sonic notes more than others.
The quality of au individual's voice is as much determined, therefore, by the shape and general structure of his throat, mouth, &c., as by his vocal cords. It may be remarked that a knowledge of this fact shows the neces sity of the mouth being properly opened during speech, and especially during singing. Each person has the power to alter the shape of his mouth, and so is able to adapt it to a great extent to intensify any particular sound be wishes to utter.
In the ordinary production of voice, there fore, there is a great variety of muscular move ment of a very delicate character; how com plicated and rapid must be the movements executed by a singer ! It is impossible to conceive the delicacy and complexity of the movements voluntarily executed by the well trained and skilful singer, who adapts vocal cords, larynx, throat, mouth, &c., with speed and precision, so as to produce accurately the desired sounds. The muscular movements in- ' volved are all produced in response to ner vous stimuli reaching the muscles from the brain.
Various points worthy of notice in connection with the care of the voice will be considered in the succeeding section.
Voice is, then, produced in the larynx, and is altered by the rest of the respiratory pas sages.