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Two nearly equal notes together go loud, soft, loud, soft. How fast is the wobble?
Beat Frequency lab · Last updated: 16 October 2026
Two notes that are almost, but not quite, the same pitch are played together. Instead of a steady sound, you hear a slow wobble: loud, soft, loud, soft. The lab shows the two sound waves and how the wobble rate depends on them.
What you'll discover
- Two notes of slightly different pitch combine into a throbbing sound called beats.
- The number of beats each second equals the difference between the two frequencies.
- Musicians use beats to tune instruments: the wobble slows and stops when notes match.
The science in plain words
Every musical note is a sound wave with a frequency, the number of vibrations each second, measured in hertz. When two notes play together, their waves add up in the air.
If the two frequencies are exactly equal, the waves stay in step and you hear one steady, louder note. If they differ slightly, the waves drift in and out of step. When the peaks line up, they add together and the sound is loud. A moment later the peak of one meets the trough of the other, they cancel, and the sound goes quiet. Then they drift back into step again.
This regular rise and fall in loudness is called beating. The beat rate is simply the difference between the two frequencies. Notes of 440 and 442 hertz give 2 beats each second; 440 and 444 give 4 beats.
Piano tuners and guitarists listen for beats. When they play a string against a reference note and hear a slow wobble, they adjust the string until the wobble gets slower and slower, then disappears. At that point, the two notes match exactly. Beats are an example of wave interference that you can hear.
Where it fits
- India, NCERT: Beats go beyond the NCERT syllabus for classes 6 to 10; the lab builds on frequency and pitch in Class 9 Chapter 11: Sound.
- United States: Grades 9 to 12
Common questions
What are beats in sound?
Beats are the regular loud and soft throbbing you hear when two notes of slightly different pitch play together. The waves move in and out of step, adding and cancelling in turn.
How do you calculate beat frequency?
Subtract the smaller frequency from the larger one. Notes of 256 hertz and 260 hertz together give a beat frequency of 4 hertz, so you hear four wobbles each second.
How do musicians use beats to tune?
They play their note alongside a reference note and listen to the wobble. As they bring the pitches closer, the beats slow down; when the beats stop, the notes are in tune.
Related labs
- Wave Interference: Two dippers, one duck. Where do the waves add up, and where do they cancel?
- Sound Waves: A speaker squeezes the air. Watch the squeezes march across the stage.
- Standing Waves: Buzz a cord at just the right rate and it splits into still points and big loops.
See all physics labs or every Taz science lab.
Try it yourself
The Beat Frequency lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.