What is the Shepard’s Tone?

The Shepard’s Tone is an audio illusion in which we seem to hear a constantly rising tone. Much like how the lines on a barbershop pole seem to constantly rise or fall, the tones in the Shepard’s tone seem to rise forever. In reality, there are new tones introduced in the lower regions as the tones in the higher regions extend out of audible range. The average pitch stays the same, but we perceive the scale to be constantly rising into infinity.

Example of a shepard’s tone:

Here is the spectrogram of that audio recording (both are from University of Wisconsin Eau Claire)