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SOUND EQUIPMENT AND SOUND 417
than the string as a whole, thus creating a secondary sound to accompany the fundamental.
Such secondary sounds are commonly called harmonics, while the chief sound or frequency is called the fundamental. Pure sounds, without any harmonics, can be created by means of a tuning fork or by electrical means. "Pure" sounds are commonly recorded on test films or test records, but the sounds of ordinary speech and music are normally very rich in harmonics or "overtones."
Harmonics and Clarity
(12) If a violin and a piano both play the same note, both are creating fundamental vibrations of the same frequency, and the ear could not tell one instrument from the other were it not for the overtones that accompany the fundamental. When two people sing the same song in the same key of music, only the overtones make it possible to distinguish between their voices.
Recording and reproducing equipment must deal faithfully with the harmonics as well as with the fundamental vibrations of sound.
Resonance
(13) If the edge of a moving card vibrates with the frequency of, say, 200 cycles per second, and a violin is held near-by, the G string of the violin may vibrate in resonance. Each time the edge of the card moves toward the string, the string is driven backward by a wave of air pressure; when the vibrating edge moves away from the violin the string is sucked forward by a reduction of air pressure. If the string is so "tuned" that its inherent elasticity permits it to vibrate at 200 cycles per second it will resonate, or keep in step, with the motion of the card. But if the tuning, the elasticity, of the string is not such as to reverse its direction of motion at precisely the same time that the air pressure changes, the motion of the string will get out of step with the changes of air pressure, and resonance will not exist.