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
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
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.