The kind of illusion I chose is called Shepard Tone. A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves. When played with the base pitch of the tone moving upward or downward, it is referred to as the Shepard scale. This creates the auditory illusion of a tone that continually ascends or descends in pitch, yet which ultimately seems to get no higher or lower.

The trick involves not only which tones are played, but also the volume of the individual notes in the tones. Changes in volume cause the listener to focus on certain notes while ignoring the others. The illusion works because each tone seems to sounds lower than the preceding. The listener judges subtle changes in tone by comparing it to the preceding note, not to tones from twenty or thirty seconds ago.