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Beats and interference of sound waves review

Review of how to produce beats through the constructive and destructive interference of sound waves.

Key terms

BeatRegularly alternating soft and loud sound heard from two sound waves of different frequencies interfering.
Beat frequency (f, start subscript, B, end subscript)The magnitude of the difference between the two interfering frequencies. Scalar with SI units of start text, H, z, end text.

How interference produces beats

When a musician needs their instrument to have the correct sound, they produce the desired frequency and try to match it with their instrument’s frequency. If the two frequencies don’t match, they interfere to produce alternating loud and soft sounds.
Figure 1 below shows two waves with different frequencies, f, start subscript, 1, end subscript (purple) and f, start subscript, 2, end subscript (green). The waves are interfering constructively in the green highlighted regions, leading to a larger amplitude when the waves combine in f, start subscript, B, end subscript. The white highlighted regions are where destructive interference occurs, and the amplitude of f, start subscript, B, end subscript is smaller than the original sound waves. The variation in amplitude results in the alternating volume (loudness) of sound that we call beats.
Figure 1: Two sound waves with different frequencies (f, start subscript, 1, end subscript and f, start subscript, 2, end subscript) interfere to produce beats with frequency f, start subscript, B, end subscript.

Learn more

For deeper explanations of the beat frequency, see our video about beat frequency.
To check your understanding and work toward mastering these concepts, check out the exercise on analyzing the interference of sound waves.

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