Audio Illusion - The Shepard Tone

Background

This audio file tests whether your ears are properly connected... to your brain. Not really a test tone, but a sonic brainteaser ;-)

The sound file

The file starts with a steady tone which then increases in pitch and ends in a steady state again. Although the pitch has been continuously increasing, the beginning and the end of the file do sound exactly the same. How can it be?

The Shepard Tone explained

This sonic illusion was invented by psychologist Roger Shepard at Bell Labs. The trick is done by simultaneously sweeping different pure tones (i.e. sine waves) tuned exactly one octave apart, fading in at one end of the cycle and fading out at the other. The human brain has a hard time figuring out if a pure tone is a fundamental or the harmonic of another and easily "slips" between those two point of views.

External Links

• The Shepard Tone @ Wikipedia