PART IV.

MECHANISM OF SPEECH.

CHAPTER XVIII.

PHYSICS OF SOUND AND PHYSIOLOGY OF THE VOICE.

ARTICULATE speech is the result of vocal sounds, single or combined, continuous or interrupted, varied by pitch, volume, and intensity. It is based upon the physics of sound, a knowledge of the latter being essential to a comprehension of the mechanism of the former. All sounds, whether harmonious or discordant, are caused by the vibrations of matter. Musical sounds and noises are equally the result of vibrations; the first being the result of periodic or regularrecurring vibrations, and the second of those which are irregular, confused, non-periodic. Wherever there is sound there is motion; and, conversely, wherever there is motion. there might be sound, if there were a medium by which the vibrations could be conducted to our ears, and our ears were capable of apprehending the vibrations.

As our ears are constituted, they will not appreciate a sound which is the result of less than 16 vibrations to the second, nor will they take cognizance of sound when the vibrations exceed 40,000 per second. The lowest note of the double bass in an orchestra has 403 vibrations to the second, while occasionally an organ-pipe is introduced the note of which has but 16 to the second. The lowest C in a piano of 7 octaves has 32 vibrations, and the highest C 4,096. At 10,000 vibrations, sound becomes piercing and loses its mu

ALL SOUNDS COMPOSED OF DIFFERENT TONES.

413

sical character. At 32,000 to 40,000, the ear fails to take cognizance of the successive shocks. The vibrations which reach our ears may arise from a multitude of causes. They may originate in the air itself, or in some substance far removed, and be conveyed by the air to us.

The same number of vibrations always produce a sound of the same pitch, without reference to the substance or cause of motion; the duration, intensity, and quality may differ, but the pitch, with the same number of vibrations, is invariably the same. The intensity, strength, or loudness of sound is due to the amplitude of vibrations, or the distance the vibrating body traverses and the length of time the vibrations continue.

No sound, emanating from whatever cause, and heard by the ear as a single sound, is a simple sound; it is always a compound or composite one, made up of a number of tones of different intensity and pitch, together with a possible admixture of noises, which combined constitute the sound as heard. For example, if a string drawn to the proper tension to emit a sound be plucked, it will not vibrate throughout the entire length as one vibration, but will be divided and subdivided into vibrating segments of varying lengths, each of which will emit a different tone. The longest segment, producing the most ample vibration, gives the loudest tone, and also the pitch to the clang, and its sound is called the fundamental tone. The other tones made by the shorter segments mix with the fundamental tone, the higher in pitch being the overtones, and the lower in pitch the undertones; and the whole constitutes the clang which is heard as one sound. The fundamental tone is always associated with tones higher or lower in pitch, or with both, which modify it. The difference in the quality or timbre of the same musical note as emanating from different instruments, such as the violin, clarionet, and organ, is due to the different admixture of overtones and undertones with the fundamental note.

414

THE REENFORCEMENT OF SOUND BY RESONANCE.

The medium through which sonorous vibrations are generally received by the ear being the air, we find such vibrations affected by the substances with which they come in contact. Soft substances will absorb sound, while hard ones, on the contrary, will reflect sound. A room hung with massive drapery will muffle and absorb sound, while one with polished walls will reflect and echo it.

One of the most interesting phenomena of the physics of sound is its augmentation or reënforcement by secondary causes. A sound emitted by one body may cause another sounding body, capable of the same number of vibrations, to become sonorous by vibration, and the sound will then be repeated, augmented, and prolonged; but the two sounding bodies must be attuned in unison. For example, a musical sound made near the strings of a piano will cause the strings in unison with it to vibrate and emit the same sound, which may be continued even after the first has ceased. A hollow body or a short tube closed at one end, containing a column of air of a certain length, can be made to resound by bringing its open end near a sounding body. The sound emitted throws the column of air into motion, and these vibrations, being identical with those of the sounding body, augment or reënforce the original note. If the column of air in such a resonating tube is not in unison with any tone of the combination or clang, no response will be heard; but, if in unison with any tone of the clang, such tone will be augmented or reënforced. Thus, a resonating body may augment the fundamental tone, or, by unison with one of the overtones or the undertones, will reënforce that to the exclusion of all others. The shape of such resonating cavities may be various; the material of which they are constituted is not important; wood, glass, metal, rubber, or putty can be formed. into a resonating cavity. Resonance depends more upon the form and dimensions of the cavity than upon the substance of its walls.

The recognition of this phenomenon is the key to the

ORIGIN OF THE HUMAN VOICE.

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mechanism of speech. Man speaks with his voice, and voice is simply sound caused by vibrations, and subject to all the laws which govern sound arising from any other source. The apparatus which produces voice is both simple and complex-simple in its action, but complicated in its structure, and still more complicated and wonderful in its results. The combined power, the compass, and the richness of tone of the human voice are unapproached by strains from any other instrument.

Voice is intimately connected with one of the vital functions of life. Every breath we breathe can, with slight effort, be made to utter a sound. The vocal apparatus depends upon the breathing apparatus for its action and its power. The same current of air driven from the lungs in natural respiration, will produce sound by the slightest alteration in the form of the air-passages.

"The principal organ concerned in the production of the voice is the larynx (see Fig. 294, A A). The accessory organs are the lungs, trachea, the expiratory muscles, and the mouth and resonant cavities about the face. The lungs furnish the air by which the vocal chords are thrown into vibration, and the mechanism of this action is merely a modification of the process of expiration. By the action of the expiratory muscles the intensity of vocal sounds is regulated. The trachea not only conducts the air to the larynx, but, by certain variations in its length and caliber, it may assist in modifying the pitch of the voice. Most of the variations in the tone and quality, however, are affected by the action of the larynx itself, and of the parts situated above it.

"It is impossible to give a complete account of the structure of the larynx, without going more fully than is desirable into purely anatomical details. We propose here only to refer to the situation of the vocal chords, and to indicate the modifications which they can be made to undergo, in their relations and tension, by the action of certain muscles.

"The vocal chords are stretched across the superior open

416

ANATOMY OF THE VOCAL ORGANS.

ing of the larynx from before backward (Fig. 294, B and C). They consist of two pairs. The superior (B), called the false vocal chords, are not concerned in the production of the voice. They are less prominent than the inferior chords (C), although they have nearly the same direction. They are covered by an excessively thin mucous membrane, which is closely ad

herent to the subjacent tissue. The chords themselves are composed of fibers of the white inelastic variety, mixed with a few elastic fibers. The true vocal chords (C) are situated just below the superior chords. The anterior attachments are near together at the middle of the thyroid cartilage, and are immovable. Posteriorly they are attached to the movable arytenoid cartilages; and, by the action of certain mus