There is a sound that seems to rise forever but never gets higher.
The Shepard tone stacks notes an octave apart, fading the top ones out as new bottom ones fade in. Your brain hears endless climbing. It's an auditory barber's pole, and filmmakers love it: Christopher Nolan used it to wind the tension of Dunkirk ever tighter.
The illusion is named after Roger Shepard, who described it. Each Shepard tone is a blend of pure tones spaced exactly an octave apart. Imagine a quiet middle C sounding together with a loud C an octave above. Step up to C sharp and the lower note gets a little louder while the upper one gets quieter. By the time you reach B, the balance has flipped, and a faint new note has crept in below. One more step and you are back where you started, yet every step sounded like a rise.
The trick works because the loudest part of the sound always sits in the middle of the range, while the notes at the extremes fade to inaudibility. You never hear any scale begin or end. The effect is strongest when the notes are short and separated. The composer Jean-Claude Risset later made a version that glides continuously, the Shepard–Risset glissando, and even a rhythm that seems to speed up without end.
Shepard tones have also revealed something odd about how we hear. Play two of them half an octave apart and it is ambiguous whether the pair goes up or down. In 1986 the psychologist Diana Deutsch found that different people hear this 'tritone paradox' consistently but differently, and that native speakers of Vietnamese, a tonal language, heard it differently from English-speaking Californians.
Musicians have used the effect for decades. It can be heard at the end of Pink Floyd's 'Echoes', on Queen's A Day at the Races and in the endless staircase of Super Mario 64. Nolan explored it in The Prestige, used it for the Batpod's engine in The Dark Knight, and made it the backbone of the Dunkirk soundtrack, where it creates a feeling of ever-rising intensity.
Source: Wikipedia — Shepard tone · Text summarised from Wikipedia (CC BY-SA 4.0)
