Beat

two steady tones. one throb. from nowhere.

slide a tone through 440 — hear interference, tune it to silence

Two tones close in pitch do not sound like two tones — they sound like one tone pulsing, because their waves slide in and out of phase and that drifting phase relationship is the actual throb you are hearing, not either frequency alone. Slide the second tone to match the first and the drifting stops — the same trick a piano tuner uses to tune by ear rather than by a meter.

Concepts: sound, interference.

An interactive you play with one thumb, on Wundr.