The "Sonic Barber Pole"

Have you been known to say "I thought I heard…"? Or have been certain you heard something correctly only to realize your perception was completely incorrect? For those who are getting older or have been with a partner or spouse long enough, both of these are increasingly common occurrences. But rest easy, friends: the seemingly infinite Shepard scale illusion laid out in this post will be misheard by all.

The Shepard scale was developed by cognitive scientist Roger Shepard, and relies on a combination of pitch and volume to achieve its never-ending effect. Shepard's effect is quite simple and begins with two identical pitches an octave apart. While the tones ascend through the scale, the higher tone gradually diminishes in volume, while a third tone below the original lower tone begins to sound almost inaudibly and increases in volume as it ascends. Eventually the original high tone becomes inaudible and the scale returns to its original position, only to continue ascending. A crude realization sounds a little like this (complete with helpful spectrogram).

The image below is a little confusing, but nicely illustrates the Shepard principle. The graphic representations of pitch below are color coded by volume, with purple indicating the lowest amplitude and teal / yellow indicated the highest. We can see that, just as the higher tones fade, a new set below becomes increasingly strong and takes their place.

...Via Wikipedia

Jean-Claude Risset, a French electronic composer, subsequently altered the scale to blur the tones together, which in my opinion creates an infinitely neater effect. Here's a looped audio example of the Shepard-Risset glissando, in which a complete cycle occurs every 30 seconds or so.

The Shepard scale is sometimes referred to as a "sonic barber's pole" and shares many characteristics with this classic optical illusion. Both contain seemingly endless information loops, and both mask information that assists in fooling our actual perception. Both also share one trait of all sensory illusions: playing on our brains' ability to assimilate limited amounts of information. When we listen to a Shepard scale, we are able to compare the current tone with the previous tone that we've heard, but not all the previous tones in the scale. This sort of auditory "aperture" is identical to the visual aperture that makes the barber pole illusion possible: by limiting the candy stripes to a narrow vertical cylinder, it gives the illusion that they are endlessly swirling down. If the same visual material was presented in a horizontal aperture, we would be fooled into thinking it was endlessly swirling sideways.

So, the next time you botch an important project detail or forget an item on your honey-do list, try blaming the pitch and amplitude of the instructions - it just may get you off the hook.

(...Ear trumpet image via Leeds Museum | Barber pole via Vimeo)