Ear 5UX
Illusions you can hear.
An endless rise
Eight sine waves an octave apart slide upwards together. Each fades in from silence at the bottom and out at the top, and when the highest reaches the octave it started from, the whole set drops back by one octave — inaudibly. The result climbs for as long as you leave it.
An endless fall
The same trick downwards. Nothing falls for ever, but this one does its best for as long as you let it.
The missing fundamental
Four tones, all harmonics of 200 Hz, with no 200 Hz tone among them. Most people hear a low note anyway, because the ear infers the note that would produce that pattern. Add the fundamental and the illusion collapses into an ordinary chord.
A beat that is not in the signal
200 Hz in one ear, and in the other a tone a few hertz away from it, both steady. Most people hear a flutter somewhere in the middle of the head — a beat that is in neither tone and in no part of the signal, only in the hearing of it. Headphones are required; through speakers the two tones mix in the room and you get an ordinary beat instead. Move the rate and the flutter keeps up, at any time, while the tones are sounding: past about fifteen hertz it stops fluttering and becomes a roughness in the tone.
An octave that changes ears
Two steady tones an octave apart, one in each ear, swapping sides four times a second: 400 Hz on the left with 800 on the right, then 800 on the left with 400 on the right, over and over. Almost nobody hears that. The usual report is a single tone stepping up and down — not two tones at once — with each pitch pinned to one side of the head. Which side gets the high tone is a property of the listener rather than of the sound, and turning the headphones around is the test. Headphones are required here too: through speakers both tones reach both ears and there is nothing to mishear.
Silent until you press play.