Page:Popular Science Monthly Volume 9.djvu/417

Rh by their timbre; we hear some one speaking, and recognize who it is without seeing him; or we hear a strain of music, with several sounds of the same pitch, but we readily by the timbre distinguish from one another the violin, the flute, the clarionet. The differences are the result of the different forms of the vibrations; this may be demonstrated by experiment. Whether we consider the movements of a pendulum or of a tuning-fork, in every case the vibration, when traced automatically, gives a characteristic line for each variety of timbre. If by means of the ear, rendered highly sensitive by long practice, we study to distinguish the different forms of the waves, we recognize in addition to the fundamental sound other higher sounds, the harmonics. Helmholtz's resonators aid the analysis by the ear. The resonator is a little hollow sphere with two open tubes, one of them conical, so as to fit into the auditory passage. The fundamental sound, which is much deeper than the other sounds, is thus considerably reënforced. In like manner, with the aid of proper resonators, it is easy to hear the harmonics of the human voice. Helmholtz ascribes the diversity of timbres to the intensity of the harmonics. Physiologists hold that there exist other causes, as yet not ascertainable.

In the state of rest, when respiration is performed without effort and with regularity, the vocal cords are almost motionless; during the alternations of inspiration and expiration, the orifice of the glottis does not alter its form. After a cry has been uttered, there occurs a deep inspiration, and then the vocal cords diverge, widening the aperture. When expiration is suspended or performed slowly, the orifice closes more or less. At the moment of emitting a sound, the lateral cartilages of the larynx are brought near to one another, and the vocal cords are suddenly made tense and applied closely to each other in their anterior portion, or even throughout their entire length; the passage of air is thus intercepted. Instantly the orifice opens again, and the air in passing between the vibrating vocal cords is itself thrown into vibrations, and sound is the result. These operations are performed gently or forcibly according to circumstances. Here we have the "glottic sound," as it is called by Mandl; isolated, it is inaudible, but it reaches our ear only after it has traversed the pharynx and the mouth; the vibrations of the air modify it. Every one has remarked the change produced in sounds by their passing through a tube or the like; for instance, when we hear a voice coming from the bottom of a well. Hence the voice is formed by the combination of the sounds of the glottis and of the cavities lying above the larynx: when these cavities are passive, the voice is inarticulate; when they undergo certain changes of form, the voice becomes articulate.

The pharynx and the mouth, which serve as a resonating box, produce sounds whenever the air they contain is made to vibrate by the