Unlike passive noise cancellation, which consists of sound-deadening materials like foam, or fingers in the ears, active noise cancellation uses the magic of microphones, computers, and speakers to actively cancel noise. Basically, a microphone picks up noise before you hear it, the computer figures out how to make noise that would cancel the acoustic wave travelling towards your ear, and the speaker makes this cancelling noise.
It's somewhat like the annoying pulsing you get when you play two notes on a guitar that is just slightly off. The low volume part of the pulsing is when the noise from onw string cancels the other.
This is being touted as perhaps the muffler system of the future. Imagine the sound deadening of internal combustion engine exhaust, without physically inducing backpressure. Theoretically, you could tune the engine so that it works more efficiently.
But combining some of this technology along with sound focusing technology, and some sonar-like stuff, imagine being able to:
- Detect on coming Harley-Davidson noisemaker/motorcycle coming
- pinpoint location using multiple distributed microphones
- project future location
- using multiple distributed speakers, generate acoustic waves that should all reinforce each other at projected future location of noisemaker.
- watch noisemaker explode into shrapnel.
You could also design burglar alarm systems in much the same way. Or blow up submarines, for that matter. I think that is the original idea that I first read describing such technology. | 
