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SOUND EQUIPMENT AND SOUND 415
vacuum, leaving a vacuum behind themselves into which air still further to the right must be drawn. The chain of events is now precisely the reverse of that which existed a fraction of a second previously. At the end of the process that particle of air which an instant before had been driven against the eardrum is sucked backward by the sudden appearance of a vacuum behind it, and the eardrum which had just been bent is now sucked outward. An instant more and the buzzer armature moves to the right again and the cycle of events just described is repeated. It may be repeated several hundred times, or several thousand times, in one second, according to the construction of the buzzer.
Pitch of Sound; Frequency of Virbation
If a calling card is held against the teeth of a revolving gear, the edge of the card will act in much the same way as does the vibrator of an electric buzzer. As each tooth strikes against the card its edge is urged forward. When the tooth has passed, the elasticity of the material causes the edge of the cardboard to spring back. As long as the gear continues to revolve the edge of the card will vibrate, just as the armature of the buzzer did.
(8) Now the frequency with which the edge of the cardboard moves can be altered very easily by changing the speed of the gear. Moreover the speed of vibration can be calculated. If the number of teeth in the gear are counted, and its speed of rotation measured, rinding out how many times the card moved backward and forward in one second is a matter of simple arithmetic. Thus the frequency of vibration of any sound — the number of cycles of changes of air pressure per second — might be determined by changing the speed of a revolving gear until a similar sound is created by a vibrating card or reed.
If the speed of the gear be increased the pitch of the sound created by the card becomes higher. If the gear slows down, the sound created by the card becomes lower in pitch.