Beats
Two close frequencies produce a throbbing sound.
About this topic
When two sounds of slightly different frequency are played together, they alternately interfere constructively and destructively, giving a periodic rise and fall in loudness called beats. The beat frequency equals the difference: f_beat = the difference between f₁ and f₂.
Musicians tune instruments by listening for beats: when two strings are almost in tune, you hear a slow wah-wah, and as the pitches match, the beats slow down and vanish. Piano tuners use this technique, and organ makers deliberately detune pairs of pipes to create a shimmering effect.
Worked example: a 440 Hz tuning fork and a guitar string at 444 Hz give 4 beats per second. If you tighten the string and the beat rate rises to 6 per second, you are moving the wrong way, since the difference is growing. Loosening it until beats disappear brings the string to 440 Hz.
Beats are an example of superposition in time, and similar effects arise with light and radio signals. The perceived pitch is the average of the two frequencies, while the loudness fluctuates at the difference frequency. A misconception is that the beat frequency is the sum of the two; it is the difference. Beats are only heard when the frequencies are close, within roughly 10 Hz of each other.
Key ideas
- Beat frequency = absolute value of f₁ − f₂
- Used for tuning
- Alternating constructive and destructive interference
Quick check
Use cases: where Beats helps
- Homework: review the key idea behind Beats before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Beats without notes.
- Visual learning: play with the interactive lab to see how Beats changes when you adjust its inputs.
- Teaching a friend: explain Beats in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Beats?
Two close frequencies produce a throbbing sound.
What will I learn in Beats?
Beat frequency = absolute value of f₁ − f₂. Used for tuning. Alternating constructive and destructive interference.
Is Beats part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Beats lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Beats?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.