The ultimate in loudspeaker discant element
design, also reffered to as "Massless" Exotics
They give no resonances at all, and they have accurate pulse and frequency response up to 100kHz or more(!).
The best known examples are Ionovac, Magnat, and Plasmatronics. The designer if Plasmatronics was plasma physicist Dr. Teller of Los Alamos Labs.
Actually, the "diaphragms" do have mass. It's the same as air, so the acoustic coupling is 1:1.
Efficiency are difficult to state, since the output tube plates of the power amplifier are providing a high voltage that directly modulates a conductive gas.
Problems occur if air is used as conductive gas:
If you ionize air (by using RF heating) you
strip apart oxygen-2 and get oxygen-3 (ozone gas). Not very healthy, and illegal in the USA.
The Plasmatronics ionized helium gas, which got around that problem, but it required a fresh tank of helium every month.