Beats (Waves)
Definition
The slow wah-wah pulsing heard when two almost-identical notes play together — piano tuners listen for beats to disappear.
Technical definition
Two waves of frequencies f₁ and f₂ superpose to produce amplitude modulation at the beat frequency |f₁ − f₂|.
Example
A 440 Hz tuning fork sounded with a 442 Hz string gives 2 beats per second — the tuner tightens the string until the beating stops.
Common confusion
Beats are not a third tone being played — no speaker emits the beat frequency. Your ear hears the combined amplitude swelling and fading.