Beat

two notes, nearly the same. listen to them argue.

tune by ear on an old oscilloscope: turn the knob until the wobble between two notes stops, then make it wobble in time

Two notes a few hertz apart slide in and out of step: in step they add up, out of step they cancel, so you hear one note swelling and fading, a wobble. The wobbles a second are exactly how far apart the two notes are in hertz, which is why a musician tuning a string listens for the wobble to slow down and stop. Turning the wrong way makes it faster; nothing but the wobble tells you which way is closer. On notes 5 and 7 you leave a set gap on purpose, so the wobble keeps time with a metronome, as a piano tuner counts beats. The trace is the sum of the two notes, drawn from the gap the knob sets; the gap you left is shown, never rounded to zero.

Concepts: sound, interference.

An interactive you play with one thumb, on Wundr.