HANDBOOK OF ACOUSTICS - online book

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

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32                      HAND-BOOK OF ACOUSTICS.
amount of friction on the lower one, which is stationary, and forms the upper side of a wind chest. When air is forced into this chest by means of the tube below, and the disc is made to revolve, a puff will pass through each hole, every time the holes coincide. Thus, supposing there are 15 holes, they will coincide 15 times for every revolution, and therefore one revolution will give rise to 15 sound waves. By an ingenious device, which, however, in the long run is a disadvantage, the current of air itself moves the upper disc. This is brought about by boring the holes in the upper disc in a slanting direction, while those in the lower one are bored in the opposite direction, as shown in fig. 21. On the upper part of the axis of the revolving disc, a screw is cut, in the threads of which work the teeth of the wheel shown on the left in fig. 21. Further, the right hand wheel in fig. 21 is so connected with the left hand one, that while the latter makes one revolution the former moves only one tooth. Each of these wheels has 100 teeth. On the front of the instrument (fig. 20) are two dials, each divided into 100 parts corresponding to the 100 teeth on each of the wheels, the axles of which passing through the centre of the dials, carry the hands seen in the figure. Thus the right hand dial records the number of single revolutions up to 100, and the left hand one the number of hundreds. Finally, this registering apparatus can be instantly thrown in and out of gear, by pushing the nuts seen on both sides.
Suppose now we wish to obtain, by means of this instrument, the vibration number of a certain sound on a harmonium or organ. "We first see that the registering apparatus is out of gear, and then placing the syren in connection with an acoustical bellows, gradually blow air into the instrument. This causes the disc to revolve, and at the same time a sound, at first low, but gradually rising in pitch, is heard. As we continue to increase the wind pressure, the pitch gradually rises until at last it is in unison with the sound we are testing. Having now got the two into exact unison, we throw the registering apparatus into gear, and maintain the same rate of rotation, that is, keep the two sounds at the same pitch for a measured interval of time—say one minute. At the expiration of this time the recording apparatus is thrown out of gear again. We now read off the number of revolutions: suppose the dial on the left stands at 21, and that on the right 36, then we know the disc has made 2,136 revolutions. Multiply this by 15 (for we have seen that each revolution gives rise to 15 waves), and we get 32,040 as the number of waves given off in one minute. Divide this by