Play a tone that climbs and climbs, forever, and never gets any higher. It sounds impossible. It takes seven sine waves and one volume curve.
Think of an Escher staircase: every step goes up, yet you end where you started. Your ears can be fooled the same way. The trick is called a Shepard tone, after the psychologist Roger Shepard, who published it in 1964. Later the composer Jean-Claude Risset made a smooth, continuous version, the Shepard–Risset glissando.
Take one note, say C, and play it at once in several octaves. Each octave doubles the frequency: 40 Hz, 80 Hz, 160 Hz and so on. Your brain files all of them as the same note name, just higher or lower.
Now do two things. First, slide every one of them upward together. Second, control the loudness with a fixed bell shape: the lowest and highest tones are almost silent, and the ones in the middle are loud. A tone that rises out of the loud middle gets quieter and fades away at the top. A new tone fades in from the quiet bottom. Nothing ever jumps, so you never hear a seam.
Watch the bars: every single one really does rise. But they are evenly spaced, so after exactly one octave of climbing, the whole stack looks identical to how it started. The loud middle has been handed from one bar to the next, like people passing a bucket along a line.
Your ear is not good at following one tone's loudness through a crowd. It hears the whole shape sliding up, and the shape keeps coming back to the same picture. That is why the climb never arrives anywhere. The counter above keeps growing, while the range of the sound never changes.
This works because pitch is not a single line. A note has a height (how high it is overall) and a chroma (its name: C, D, E and so on). Chroma repeats every octave, so it behaves like a circle, or a clock face. Height behaves like a ladder. An ordinary scale changes both at once. The Shepard tone keeps the height fixed by the volume curve while the chroma runs around the circle again and again. It is a musical barber's pole.
Sound designers love it because it builds tension without ever resolving. The score of Christopher Nolan's film Dunkirk (2017) is described as using Shepard tones, and the effect is also reported in the Batpod engine sound of the Dark Knight films, where it suggests constant acceleration without gear changes.
Try it with the direction flipped, then drag the picture yourself. Notice that dragging up one full octave puts you exactly back where you began.