Audio Illusion - The Shepard Tone
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.