# HANDBOOK OF ACOUSTICS - online book

### A complete view of Acoustical Science & its bearings on music, for musicians & music students.

 30 HAND-BOOK OF ACOUSTICS. causing a condensation on the other side. The current of air is immediately cut off by the revolu­tion of the disc, and consequently a rarefaction follows the previous condensation—in short, a complete sound wave is formed. Another hole appears opposite the jet, and another wave follows the first, and so on, one wave for each hole. While the disc is being slowly revolved, the waves do not follow one another quickly enough to give rise to a musical sound,— each puff is heard separately; but on gradually increasing the speed, they succeed each other more and more quickly, till at last a low musical sound is heard. If we now still further increase the speed, the pitch of the sound will gradually rise in proportion. Savart's toothed wheel (fig. 4, p. 6 ) is another instrument which proves the same fact. While the wheel is being slowly turned, each tap on the card is heard separately, but on increasing the speed, we get a musical sound, which rises in pitch as the rate of revolution increases, that is, as the rate of vibration of the card increases. We see, therefore, that the pitch of a sound depends upon the vibration rate of the body, which gives rise to it. By the vibration rate, is meant the number of complete vibrations—journeys to and fro—which it makes in a given time. In expressing the vibration rate, it is most convenient to take a second as the unit of time. The pitch of any sound may therefore be defined, by stating the number of vibrations per second required to produce it; and to avoid circumlocution, this number may be termed the " vibration number " of that sound. We shall now proceed to describe the principal methods which have been devised for ascertaining the vibration number of any given sound.