Visual illusions are a common occurrence but how many have you seen (or heard) an aural illusion.
Eric Lippert wrote a neat
post regarding an aural illusion discovered by Risset who was able to create a sound such that it appears to be descending continuously yet never hitting the bottom. The key is playing several (3 is the minimum) same notes, separated by an octave (12 semitones), simultaneous but each at a different volume. The top note would start quietly but become louder as it descends whereas the middle note would start loud and decrease in volume as it descends. As
Eric says:
That's the essence of the illusion. The human ear attunes itself to the loudest part of the tone, and therefore doesn't notice that the lows are dropping out of the bottom and being replaced by very faint notes at the top. As the top notes descend they get louder and louder, and eventually the ear switches from hearing the middle note as the primary to the bottom note as the primary, and then back to the middle later.
Eric even took it a step further and wrote code to simulate it. I've compiled the code and generated the 50 second audio file (wav - 365 kb) which can be downloaded
here. Pretty cool eh? If you listen hard enough, you can tell when your ear picks up the new primary note.
He also points out that the human ear listens in ratios (think of notes as a specific frequency) which is very true. Have you ever seen a piano tuner at work or a musician tuning a string instrument? A lot of the times they will tune using a pair of notes instead of just listening to the individual notes. It is much easier to tune that way and is certainly the way I do it for the violin.
Definitely a good read if you don't mind a little calculus. That
post was also part 5 in his series of posts and I'm sure the previous ones are just as interesting. Hopefully I'll get around to reading them when I have time.
Next step will be to attempt this illusion on the piano....definitely not an easy task.
Tags: illusion music sound piano