Acoustics and Sound: Beating

Beating is a very important concept in musical instruments. This tremolo-like variation in sound intensity occurs when two pure tones of slightly different frequencies are sounded simultaneously. An experiment can be performed with two tuning forks (one regular and one wrapped with tight rubber bands on the prongs) struck at the same time. The resultant sound intensity will vary, rising and falling periodically. When the sound wave arrives at the ear, the waves initially appear out of phase (destructive interference) then appear in phase (constructive interference). The superposition of the two waves is shown to have a pulsation effect. The frequency of the pulsations is determined by the beat frequency, which is the difference in frequency of the pure tones.

beating

The pitch perceived by the ear is the average of the two pure tones. A demonstration can be done with two pipes in an organ (one adjustable pipe). The adjustable pipe is varied within a certain range of the other pipe’s frequency. If the second pure tone is greater than approximately 15 Hz away from the first tone, beating is no longer heard.

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