Auditory illusions
An illusion is something that manipulates ones perception. Many of us are familiar with optical illusions, seeing things moving that really sit still, colors that are not there, images that seem to make sense but on further inspection would be impossible. This results from our brain’s desire to find patterns in everything. This is true with our perception of sound just as much as with vision. This phenomenon is called auditory apophenia. Our brains are wired to find order, to see patterns, to draw meaning from every stimulus we encounter. We hear words and phrases where there is only random sound, we hear melodies in the chaos of what should be discord. Our ears can be tricked into hearing pitches in any manner of ways, infinitely ascending, moving up or down depending on the listener.
All of this is the result of our attempts to organize the world. One incredible phenomenon that shows just how strongly we are preconditioned to hear and decipher language is called the phantom voice effect. When we are presented with a repeating series of nonsense syllables our brains strive to find meaning in them, they will begin to piece together bits of what they hear and paste them with bits of what we know to form something discernable. A common sound used is the repeating syllables ba-ma- ba-ma- ba-ma- ba-ma- ba-ma- ba-ma-… Each person will hear something different, in their own dominant language. A comic book fan might hear “Bat Man Bat Man Bat Man,” an alcoholic might hear, “Bar Maid Bar Maid Bar Maid,” and so on. This is a common explanation provided for people who attest to hear the “voices” of ghosts in recordings or in their homes. We hear what we want to hear, or fear we will hear.
Another fascinating phenomenon dealing in musical pitches is called the Shepherd’s Tone. This is an illusion named after Roger Shepherd who first created the idea and rough theory; it was then perfected by Jean-Claude Risset, who refined it to the perfect illusion we have now. The effect of the illusion is a continuously climbing scale that never stops rising or falling. To picture how this works, consider a brass trio consisting of a trumpet, a horn, and a tuba. They all start to play a repeating C scale (C–D–E–F–G–A–B–C) in their respective ranges, i.e. they all start playing Cs, but their notes are all in different octaves. When they reach the G of the scale, the trumpet drops down an octave, but the horn and tuba continue climbing. They're all still playing the same pitch class, but at different octaves. When they reach the B, the horn similarly drops down an octave, but the trumpet and tuba continue to climb, and when they get to what would be the second D of the scale, the tuba drops down to repeat the last seven notes of the scale. So no instrument ever exceeds an octave range, and essentially keeps playing exactly the same seven notes over and over again. But because two of the instruments are always "covering" the one that drops down an octave, it seems that the scale never stops rising.
