Beat frequency demo: hear the 4 Hz wobble from 440 + 444 Hz

How this is calculated

The math is direct: when two sines of slightly different frequencies combine, their sum is a tone at the average frequency, amplitude-modulated at the difference frequency. So 440 + 444 sounds like a 442 Hz tone whose loudness pulses 4 times per second. This is why tuning by ear works - a guitar string slightly off from a reference produces a slow beat, and you tune until the beat slows to zero. You can also hear beats between any two close sources: two out-of-tune orchestral players, two synth oscillators, even the two tines of a tuning fork that's been slightly bent. Move the slider on the lower oscillator and you'll hear the beat speed up as the frequency difference grows.

Verdict

Beat frequencies are the most teachable acoustics phenomenon there is. Two sines near the same pitch produce a slow wobble at the difference frequency, and a tone generator turns the math into something you can hear and play with. Use the summed view to watch the beats on the waveform too.