Pleasant Ways in Science

by Richard A. Proctor

Now if we consider the meaning of such results as these, we shall begin to recognize the perplexing but also instructive character of the evidence derived from the telephone, and applied to the construction of the phonograph. It appears that when a disc is vibrating under such special conditions as to give forth a particular series of tones (the so-called fundamental tone of the disc and other tones combined with it which belong to its series of overtones), the various parts of the disc are vibrating to and fro in a direction square to the face of the disc, except certain points at which there is no vibration, these points together forming curves of special forms along the substance of the disc.

When, on the other hand, tones of various kinds are sounded in the neighbourhood of a disc or of a stretched circular membrane, we may assume that the different parts of the disc are set in vibration after a manner at least equally complicated. If the tones belong to the series which could be emitted by the diaphragm when struck, we can understand that the vibrations of the diaphragm would resemble those which would result from a blow struck under special conditions. When other tones are sounded, it may be assumed that the sound-waves which reach the diaphragm cause it to vibrate as though not the circumference (only) but a circle in the substance of the diaphragm—concentric, of course, with the circumference, and corresponding in dimensions with the tone of the sounds—were fixed. If a drum of given size is struck, we hear a note of particular tone. If we heard, as the result of a blow on the same drum, a much higher tone, we should know that in some way or other the effective dimensions of the drum-skin had been reduced—as for instance, by a ring firmly pressed against the inside of the skin. So when a diaphragm is responding to tones other than those corresponding to its size, tension, etc., we infer that the sound-waves reaching it cause it to behave, so far as its effective vibrating portion is concerned, as though its conformation had altered. When several tones are responded to by such a diaphragm, we may infer that the vibrations of the diaphragm are remarkably complicated.

Now the varieties of vibratory motion to which the diaphragm of the telephone has been made to respond have been multitudinous. Not only have all orders of sound singly and together been responded to, but vocal sounds which in many respects differ widely from ordinary tones are repeated, and the peculiarities of intonation which distinguish one voice from another have been faithfully reproduced.

Let us consider in what respects vocal sounds, and especially the sounds employed in speech, differ from mere combinations of ordinary tones.

It has been said, and with some justice, that the organ of voice is of the nature of a reed instrument. A reed instrument, as most persons know, is one in which musical sounds are produced by the action of a vibrating reed in breaking up a current of air into a series of short puffs. The harmonium, accordion, concertina, etc., are reed instruments, the reed for each note being a fine strip of metal vibrating in a slit. The vocal organ of man is at the top of the windpipe, along which a continuous current of air can be forced by the lungs. Certain elastic bands are attached to the head of the windpipe, almost closing the aperture. These vocal chords are thrown into vibration by the current of air from the lungs; and as the rate of their vibration is made to vary by varying their tension, the sound changes in tone. So far, we have what corresponds to a reed instrument admitting of being altered in pitch so as to emit different notes. The mouth, however, affects the character of the sound uttered from the throat. The character of a tone emitted by the throat cannot be altered by any change in the configuration of the mouth; so that if a single tone were in reality produced by the vocal chords, the resonance of the mouth would only strengthen that tone more or less according to the figure given to the cavity of the mouth at the will of the singer or speaker. But in reality, besides the fundamental tone uttered by the vocal chords, a series of overtones are produced. Overtones are tones corresponding to vibration at twice, three times, four times, etc., the rate of the vibration producing the fundamental tone. Now the cavity of the mouth can be so modified in shape as to strengthen either the fundamental tone or any one of these overtones. And according as special tones are strengthened in this way various vocal sounds are produced, without changing the pitch or intensity of the sound actually uttered. Calling the fundamental tone the first tone, the overtones just mentioned the second, third, fourth, etc., tones respectively (after Tyndall), we find that the following relations exist between the combinations of these tones and the various vowel sounds:—

If the lips are pushed forward so as to make the cavity of the mouth deep and the orifice of the mouth small, we get the deepest resonance of which the mouth is capable, the fundamental tone is reinforced, while the higher tones are as far as possible thrown into the shade. The resulting vowel sound is that of deep U (“oo” in “hoop”).

If the mouth is so far opened that the fundamental tone is accompanied by a strong second tone (the next higher octave to the fundamental tone), we get the vowel sound O (as in “hole”). The third and fourth tones feebly accompanying the first and second make the sound more perfect, but are not necessary.

If the orifice of the mouth is so widened, and the volume of the cavity so reduced, that the fundamental tone is lost, the second somewhat weakened, and the third given as the chief tone, with very weak fourth and fifth tones, we have the vowel sound A.

To produce the vowel sound E, the resonant cavity of the mouth must be considerably reduced. The fourth tone is the characteristic of this vowel. Yet the second tone also must be given with moderate strength. The first and third tones must be weak, and the fifth tone should be added with moderate strength.

To produce the vowel sound A, as in “far,” the higher overtones are chiefly used, the second is wanting altogether, the third feeble, the higher tones—especially the fifth and seventh—strong.