Another perceptual illusion we use is the Shepard tone (also called the Shepard scale). The Shepard tone trick listeners to believe that the tone is continually ascending in pitch, although in fact the tone does not get any higher. This illusion is achieved by combining a higher tone and a lower tone into one. When the pitch goes up, the lower tone becomes louder and louder and eventually superimpose the higher one. Therefore the overall pitch of the tone seems to be going upward forever.
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