Why two almost-in-tune notes throb, and how tuners use the wobble

Play two notes that are almost in tune and you hear one note that swells and fades: wah-wah-wah. Neither string is wobbling. The throb, called a beat, appears only where the two sound waves meet, drifting in and out of step.

The rule is simple: the number of beats per second equals the difference between the two frequencies. That is why piano tuners don't judge pitch; they listen for the wobble and turn the peg until it stops.