Perceptual Illusion

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.

Examples in the world

Just as the video I uploaded above, here is an detailed-demonstrated video for Shepard Tone, starting from 2'06''.

And here is another example video that also contains visual presentation along with audio example. In this video, the visual part, combined with the audio illusion, creates an even more powerful sense of falling.

Examples in Media/Art

The Shepard Tone, although discovered academically in 1980s, was used one of J.S. Bach's piece as early as 1744. Bach's "Canon per tonos" from The Musical Offering travels through all twelve minor scales in turn. The king's theme is in the top voice. The lower two voices are in canon, entering a 5th higher (or a 4th lower) at each repetition. C minor, G minor, D minor, A minor, E minor, B minor, F# minor, C# minor, G# minor,... repeating as long as one withes. By the time we get back to C minor, the whole piece is an octave higher than it started.

Another example is in video game: Super Mario 64. There is a door that leads to an "endless stairway". While the player is running up the endless stairs, the BGM(aka background music) is using the Shepard Tone to create the illusion of an everlasting ascending scale to improve the experience of the endless mode.

Interestingly, the Batman's BatPod in "The Dark Knight" uses a Shepard Tone effect to make the motorcycle to seem to have an infinitely rising tone.

Reflection

From all the examples I listed above, I feel like our spacial sense does not limit to eyes, or visual images, but auditory sense also contributes to it. That means, in order to create a real "stereo" experience of ascending or descending(climbing or falling), we have to manipulate how the perceives' ears perceive from our product. Now, Shepard Tone provide me a brilliant way into these kind of experience-creating. 

Not limiting to climbing and falling, sometimes people need to feel the presence of adrenaline to find something exciting. But how to make our perceivers hot blooded? Shepard Tone could be out way in- the everlasting ascending effect can be used to create the sense of a war or an epic fight is at the edge of breaking or something really exciting is going to break out and shock the perceivers' heart greatly. Shepard Tone could provide a great pre-war environment that help the perceivers to better prepared for getting their blood boiling.