If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

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 (fB)The magnitude of the difference between the two interfering frequencies. Scalar with SI units of Hz.

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, f1 (purple) and f2 (green). The waves are interfering constructively in the green highlighted regions, leading to a larger amplitude when the waves combine in fB. The white highlighted regions are where destructive interference occurs, and the amplitude of fB 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 (f1 and f2) interfere to produce beats with frequency fB.

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.

Want to join the conversation?

  • duskpin seedling style avatar for user BoiRo
    On the previous practice, I am confused about the pressure graphs. From what I am understanding from the practice, it is saying that when there is more pressure, there is more displacement and thus a larger amplitude/volume, but when I look it up, I find the opposite. Can someone please help me understand what I am missing here?
    (7 votes)
    Default Khan Academy avatar avatar for user
  • spunky sam blue style avatar for user aungsoewinfamily
    How much beat frequency can we not perceive the beat sound?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • aqualine ultimate style avatar for user JLD
      The human hearing range is between 20 -20,000 Hz.*

      Thus, you should use frequencies between those value if you want to perceive the beats.

      20 Hz > f1 and f2 < 20,000 Hz

      beat frequency = |f1 - f2|

      Moreover, I believe that the frequencies need to be relatively close to one another (eg, 440 Hz and 445 Hz).

      *although there is considerable variation between individuals, especially at high frequencies, and a gradual loss of sensitivity to higher frequencies with age is considered normal.
      (1 vote)