The Sonic Barber Pole: Shepard's Scale

The Shepard scale audial illusion is often compared to the neverending staircase where you seem to be continually moving up the staircase (or down depending which direction follow the steps) , yet end up where you started.





































































Invented by Stanford professor Roger N. Shepard, the Shepard scale is a famous auditory illusion. An auditory illusion is like a visual illusion but involves hearing instead of sight. The Shepard's scale sounds to the listener as if it's continually going down-- or continually up, depending if the notes are played the opposite way. However, the sound is really a repeated scale of tones, much like the repeated loop of steps of the staircase that takes you to where you start. The tones do not continually go down (or up), it only seems that way.


How the Shepard Scale Works

The Shepard scale involves a carefully and calculated manipulation of tones, just as the steps in the never-ending staircase were carefully crafted to trick the eye. Shepard's manipulation involves not only which tones are played but the volume of the individual notes in the tones. Each tone in the scale is comprised of several notes played simultaneously, and at each moment different notes in the tone are played at a different volumes. Some notes are played loudly, while others are near inaudible. The changes to volume has the listener focusing on certain notes and oblivious to others. As with the steps in the staircase, the key to the trickery is each tone seems to sounds lower than the preceding (higher if the notes are played the other way) even if it really isn't. The listener judges subtle gradations in tone by comparing it to the preceding note, not to tones from twenty or thirty seconds ago and certainly not to future notes. If each tone appears lower than the previous, the listener will think the sound continually going down.


So how the heck can you make a lower tone sound higher?

At first this may seem like an impossible task, but it's simple when you manipulate the volumes of the notes within the tone. Just look at the below tones.

The above a and b are the same tone, each with the same three notes. How would you lower the notes in tone b, while make it sound higher than a? The answer is by changing the volumes of the notes.

If you alter the volumes of the notes, you can make lowered b sound higher. The black notes are played loudly, while the whites are played very softly. As you can see, the loud notes of b are higher than the two loud notes of a.

The changing of the volume works as an audial mask, with masking or hiding of information being a common element of both audial visual illusions. If you put your fingers or pieces of white paper over the white notes in the bottom picture, tone b will also look like a higher tone. This shows you that the Shepard scale really is much like a visual illusion.

The repeated Shepard scale involves a much more complex and larger variety of tones, notes and volume manipulations.


Compare to the Barber Pole

Shepard's scale is often called the sonic barber pole, as it works in a similar way to the barber pole visual illusion. Both have repeated loops of information that appear to continually move up or down, and both involve masking of information that aid in the perception.

Barber poles have diagonal candy stripes that rotate horizontally. When viewed from a specific angle, the stripes appear to be continually moving up or continually moving down (direction dependant on the direction of the rotation).
In the above animated version of the barber pole, the stripes appear to be continually moving down. At least upon reflection, it should be obvious that the stripes can't be moving this way, as stripes never move below the grey box. Plus, there would have to be endless supply of stripes coming from above.

Looking at the above barber pole you simultaneous see that it's impossible for the stripes to be continually moving down and that they look as if they are moving down. Your conscious knowledge doesn't change the illusory perception.

A large portion of or sight (and sound) perceptions are performed in non-conscious parts of our mind. With the earlier never-ending staircase, if you viewed only two or three steps at a time as you walked down the staircase, it would seem as if you are continually moving down. With the Shepard's scale you are essentially doing just that, you are following the scale comparing one tone to the tone before. You never see the proverbial entire staircase of tones to get an idea of what is really happening. To the newbie listener who doesn't know what's going on, the tones seem to be moving continually down, and there's no reason to think twice about it. Even when informed that it's really repeated loop, it still sounds as if the sound is continually moving down. Conscious knowledge doesn't change the illusion to your ears.

The masking of candy stripes information is essential for the illusion, just as the masking of the notes is essential for the Shepard's tone. The illusion only works with a physical barber pole when it is viewed at a specific angle. When you change your viewpoint, and see some rotation movement at the edges, the illusion disappears. With the above animation, the grey box covering stripes is essential for the illusion. If the grey box were removed and you saw all the machinations behind, you would plainly see it as a simple repetition of lines. glass

(c) david rudd cycleback, all rights reserved