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MANAGERS AND PROJECTIONISTS 967
From Sound to Electric Current
JET us next discuss sound. Let us see just what -*' it is, how it is produced and how it is changed from its original form into an electric current capable of operating the sound recording apparatus.
The vocal cords of the human throat, or of an animal's throat, the reeds of an organ or its pipes, the base or snare drum or anything else capable of producing the thing we call "sound," does so by setting up vibrations in the atmosphere, which travel through the air until they encounter, for example, the drum of our ear, causing it to vibrate, and thus transmit to our brain the sensation we call "sound." Briefly that is how sound is produced and how it passes through the air in the form of air vibrations called sound waves.
In recording sound for the purpose of reproduction in synchronization with motion pictures, an instrument of enormous sensitivity is used, called a "microphone." Its operation is essentially as follows:
The sound waves striking the microphone diaphgram, made of a metal called duralumin from one to two thousandths of an inch thick, and therefore highly sensitive to vibratory impulses, cause it to vibrate with rapidity exactly equal to the rapidity or "frequency" of the sound vibrations or waves themselves. If the sound be low pitched, as that of a piano bass note, then the vibrations (frequency) per second will be low. If the sound be high pitched, as, for example, a shrill whistle, then the frequency (vibrations per second of or in the