Page:Popular Science Monthly Volume 13.djvu/57

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47
HOW SOUND AND WORDS ARE PRODUCED.

1,100 feet in a second. Now, our A-fork, vibrating 440 times a second, sets 440 sound-waves in motion in a second, so that, at the end of a second, there would be 440 air-waves afloat, and the first one would have reached a distance of 1,100 feet away. Now, there being 440 equal air-waves in 1,100 feet, how far apart are they—in other words, how long is each wave? Dividing the 1,100 feet by the 440 waves, we get two and a half feet, or 30 inches, as the length of the air-waves of the first A-tone above "middle C"—the A-string of a violin. In the same way we find that the first A of the bass of our piano produces air-waves about forty feet in length, while the waves of the last A of the treble are not quite four inches long. We find the length of the air-waves of any musical note—that is, the distance apart of the pushes in the air—by dividing 1,100 feet, the distance which the waves would cover in a second, by the number of the note-vibrations per second, which represents the number of air-waves it would make in that time.

One thing we notice in all sounds, and that is their character, or peculiarity. They may be as near alike as they can be made, but each different kind will have something about it which distinguishes it from every other, and it is by this means that we distinguish different instruments or voices. The cause of this is the peculiar shape in which the wave comes from different sources, a sort of individual stamp by which a sound carries the telltale mark of its maker. These different stamps or trimmings of air-waves may be illustrated in Fig. 4, and will

PSM V13 D057 Sound waves.jpg
Fig. 4.

be explained presently. The heavings represent the compressions, and the hollows the rarefactions, in the air. Let A represent the wave-form of the purely ideal tone of the note A with no stamp or quality given to it. B might represent the wave-form given it by a piano; and C, that given to it by a violin. In each case the wave-length, or distance between swells, and therefore pitch of tone, and the amplitude, or size of swells, and therefore loudness of tone, are the same; the only difference is, that the last two tone-waves seem to be trimmed with feather-waves, so to speak, the trimming varying with the source of the wave.