Every key here plays the same construction: octave copies of one pitch class under a fixed, bell-shaped loudness window across the spectrum — loud in the middle register, fading to silence at the extremes. Play C in any octave and you'll hear the same tone; the keyboard's octave has no effect, because there genuinely is no single octave in the signal. That's the whole illusion (Shepard, 1964) — this page just makes it playable instead of a fixed demo. Compare it to the reference version, with the continuously-rising Risset glissando and the ambiguous tritone paradox, on the Audio Illusions page.
These shift which octave band is loudest, not the pitch — that's the point.
Narrow width: one octave band dominates and the illusion weakens — you'll start to hear a real pitch. Wide: many octaves sound at once and the ambiguity is total.
A continuous drone under whatever you play — octave-spaced tones glide together so new ones fade in at the bottom exactly as old ones fade out at the top. It never arrives anywhere; that's the trick played on tempo, too (see Risset rhythm on Audio Illusions).
play with your keyboard (a s d f …) or click the keys — try the same letter an octave apart