Phase cancellation is a phenomenon known to most people in the context of expensive headphones. The Phase of an oscillation refers to the exact position of a wave in relation to a specific point of time. Phase cancellation occurs when two waves, coming from different sources are perfectly matched in terms of size (amplitude), physical placement, and are perfectly out of phase (oscillation). When these variables are met with precision, a perfect sinusoid will sound silent. Meaning you will hear nothing even if both speakers are playing the same tone, but precisely 180 degrees out of phase with each other.
In specific headphones, this concept is utilized to quiet down outside noise. This works with a microphone listening for noise around you, and then playing it back away from you perfectly out of phase, thus “canceling” the noise.
In January of 2008, I designed an installation to exploit this phenomenon.
Two pairs of identical speakers, at head level, faced each other with enough room for a participant to walk between. Each pair were outputting identical sounds Achieving perfect phase cancellation is nearly impossible at high amplitudes, and without perfectly tuned speakers and setup. However, the result is very interesting. Participants described feeling vibrations within their skull, and other bizarre sensations in or around their head.
The installation at the Mondavi Center for the Arts, in Davis, CA, was developed for a John Cage tribute concert. Standing between one pair of speakers the listener heard randomly generated sinusoid clouds. The other pair was playing a recording of a John Cage speech from the 1970s. I will post stereo MP3 recordings of the sounds, as well as the maxMSP patches soon.