This is another Plasma Tweeter very similar to that made by Ulrich Haumann.
http://www.geocities.com/ResearchTriangle/Thinktank/1866/plasma.htm
The most bizzare type of speakers with phenominal sound quality. Extremely clear and open sound; Perfect transient response; no Doppler distorion; extension to 60khz!....
http://www.ece.villanova.edu/~cdanjo/plasma.html
Theory behind this project is that by picking up ambient sound with a microphone and reproducing it out of phase, we can actively cancel or "null" out background noise.
http://headwize.com/projects/noise_prj.htm
It is really simple. It is a modulated RF power amp with a controlled ionic discharge.
http://www.plasmatweeter.de/eng_plasma.htm
http://www.smarthome.com/howto04.html
http://www.enjoythemusic.com/tubecros.htm
http://sound.westhost.com/project28.htm
http://www.tkk.fi/Misc/Electronics/circuits/speaker_to_line.html