How very facinating!
Talking Flames, was in 1990, the newest technical achievement from the Logos Laboratory for experimental music research. It uses a plasma in ionised air, an electric arc that is modulated with a PWM-audiosignal. Herewith a sound-radiator is created that does’nt make use of any moving parts such as cones or coils. It sets air into vibration in a most direct way. Moreover, it is also capable of converting digital audio-information into sound, using the ionised air arc as an digital to analog converter by simply integrating the PWM- signal. This digital loudspeaker operates on a very high voltage and pretty high frequency: de carrier wave should be at least two times the highest audiofrequency. Ignition voltage is ca. 20kV. The electrodes are made of tungsten enriched with 5% radioactive thorium. The points have to be sharpened every so often.
I’m going to keep an eye out for more information on this technique.