https://www.vox.com/videos/2017/7/26/16033868/dunkirk-soundtrack-shepard-tone
Named after cognitive scientist Roger Shepard, the sound consists of several tones separated by an octave layered on top of each other. As the lowest bass tone starts to fade in, the higher treble tone fades out. When the bass completely fades in and the treble completely fades out, the sequence loops back again. Because you can always hear at least two tones rising in pitch at the same time, your brain gets tricked into thinking that the sound is constantly ascending in pitch.
