Shepard Tone

Each step in a Shepard sequence shifts the audible pitch class upward. At the same time, high octave components fade away and low octave components fade in. After twelve semitone steps, the pattern can return to the same pitch classes and a similar spectrum, even though each neighboring step seemed higher. Your ear follows local upward motion and misses the quiet handoff at the edges.

Several sine-wave components are stacked at octave distances, such as A2, A3, A4 and A5. Their loudness is shaped so the middle register is prominent and the extreme low and high components are faint. This makes it hard to assign the whole sound one clear octave height.

No. A speaker has a finite frequency range, and the sound reuses components through a crossfade. The 'endless rise' is a perceptual effect created by the relationship between consecutive tones. If you isolate and follow one component, it eventually fades out; another one takes its place below.

A rising Shepard sequence can build tension in a film, game or electronic track without a conventional melody reaching an obvious top note. A continuously sliding version is often called a Shepard-Risset glissando. The effect works best when the layer balance hides the octave handoff; ordinary chromatic notes on one piano register will simply climb and stop.