While this may not be a surprise to those with electrical engineering backgrounds, this is a first for me — never seen this sort of thing before. While the effect is very cool, it’s potentially very dangerous and you shouldn’t go jumping the fence at the neighborhood 50kW shortwave station to try it out.
Wikipedia on plasma speakers:
Plasma speakers are a form of loudspeaker which varies air pressure via a high-energy electrical plasma instead of a solid diaphragm. Connected to the output of an audio amplifier, plasma speakers vary the size of a plasma glow discharge, corona discharge or electric arc which then acts as a massless radiating element, creating the compression waves in air that listeners perceive as sound. The technique is an evolution of William Duddell‘s “singing arc” of 1900, and an innovation related to ion thruster spacecraft propulsion.
The effect takes advantage of two unique principles. Firstly, ionization of gases causes their electrical resistance to drop significantly, making them extremely efficient conductors, which allows them to vibrate sympathetically with magnetic fields. Secondly, the involved plasma, itself a field of ions, has a relatively negligible mass. Thus as current frequency varies, more-resistant air remains mechanically coupled with and is driven by vibration of the more conductive and essentially massless plasma, radiating a potentially ideal reproduction of the sound source.
After doing some looking around, I realize that I’ve seen a similar phenomenon in the past while visiting the Boston Museum of Science — the singing Tesla coil! Here is a fun example: