Answer: Shepard Tone

If you’ve ever been at the cinema and noticed that the musical score for the movie or a sound effect within the movie seems to be perpetually climbing ever higher in scale (even though such a thing would be impossible), you’re likely experiencing an artful application of what is known as a “Shepard tone”.

Named after Roger Shepard, a pioneering cognitive scientist, the Shepard scale is a type of auditory illusion created by combining several tones together. The key to the illusion is that all the tones follow an octave-separated sine wave pattern and that one is a bass tone. When the tones are played in a rising and descending sequence, the combination of the sine wave pattern and the anchoring bass tone creates the auditory illusion that the tones are rising (or falling) indefinitely. When the transition between tones is continuous, it is known as continuous Risset scale or Shepard-Risset glissando.

This effect is used by game designers and movie directors when they wish to create a high sense of tension. In the video game Super Mario 64, for example, the endless staircase in the final castle has a Shepard tone-driven soundtrack that creates a sense of urgency and tension as the player runs up the winding stairs. The Batpod, Batman’s motorcycle in the Dark Knight trilogy, has a Shepard-tone-based sound effect for acceleration (so the Batpod sounds like it is smoothly accelerating instead of shifting gears). The recently released film Dunkirk uses a Shepard tone extensively in the musical score to create a perpetually rising sense of tension in the viewer as the story progresses.

A bit of trivia about an auditory illusion would hardly be complete without examples for the reader (soon to be listener) to enjoy. Here’s an example of the Shepard tone—listening to it, you can hear how it would be the perfect way to create the illusion of something like the aforementioned Batpod or the warp drive of a starship continually accelerating.