I've never heard plasma speakers myself but a have friends who have heard them and claim that they are the best they've ever heard, with better highs than even ribbon tweeters and great midrange and bass as well, with amazing detail. According to my friend, they don't seem to have any tradeoffs or weaknesses at all except for the minor fact that they emit poisonous gas as they play and bursts out helium when you turn them on. Anyone ever heard one? I've found this on a Speaker design website:
"No resonances at all, and accurate pulse and frequency response up to 100kHz or more. Low distortion too ... like a really good amplifier. Actually, the "diaphragms" do have mass. But itís not much. Itís the same as the surrounding air, so the acoustic coupling is 1:1. The efficiency is a little difficult to state, though, since the output tubes of the Power amplifier are supplying a high voltage that directly modulates a conductive gas with very complex electrical properties.
I first heard the Hill Plasmatronics at the 1979 Winter CES, and I must say I've never heard a tweeter that even came close to that one. The exhibitors darkened the room for dramatic effect, and you could see this weird violet glow through the grill cloth that looked for all the world like a gassy triode ... but it was the tweeter! Not only did it glow, it pulsed along with the music!
The rest of the speaker, though, was a pretty mundane paper-cone setup in a huge cabinet ... oh well. Even so, the Plasmatronics was a wild thing, a taste of the future, like a SR-71 Blackbird in an airport full of commuter-shuttle 737ís. Not too surprisingly, the inventor was a plasma physicist at Los Alamos Labs.
(Talk about being ahead of your time! This was 10 years before the fall of the Berlin Wall, and Dr. Hill was already thinking of ways to peacefully convert atomic swords into sonic plowshares!)
There are a few little problems with this glimpse of Paradise. Previous generations of plasma speakers, such as the DuKane Ionovac tweeter of the Fifties, used RF heating to ionize air, making it conductive. Thereís a problem with this. If you ionize air, some of the oxygen molecules (O2) are stripped apart and then recombine as ozone (O3). You also get nitrous oxide (NO2), which is formed by combining nitrogen and oxygen at very high temperatures.
Well, a dose of laughing gas may or may not enhance the listening experience, but ozone really isnít too healthy, since it irritates and burns the mucous membranes and the eyes. The natural home for these highly reactive gases is far up in the ionosphere, not in your living room.
The Hill Plasmatronics avoided the air pollution hazard by having its own built-in supply of helium, which is a noble gas and thus unreactive even when ionized. Helium is also biologically inert, and being much lighter than air, promptly escapes to the upper atmosphere and outer space. Even in the best-insulated houses it will be gone in a matter of seconds, so it is completely safe.
I remember seeing the helium tank, pressure gauges and all, in a special compartment inside the subwoofer enclosure. Imagine cracking a valve and hearing a very faint hiss of helium gas every time you turn on your hi-fi. Oh, I nearly forgot, you had to swap the helium tank for a fresh one every month. Helium is not a renewable resource, and is only found in a few natural gas wells, so itís not as inexpensive as other industrial gases.
There are still some interesting plasma-speakers that havenít been tried yet. For example, one alternative to tanks of helium is a flame Speaker (flames are plasmas too), using flammables that release no toxic byproducts of combustion. This leads us to hydrogen and oxygen, preferably generated on-the-spot by splitting water by electrolysis. (Youíd water the loudspeaker like a plant!) The hydrogen and oxygen pipes go up to a copper Wire mesh (a hemisphere would be the right shape), and the flame is trapped on the surface of the copper mesh.
The system has a computer-controlled Power supply that splits the water, monitors the gas flow, and automatically sparks the flame when the correct hydrogen-oxygen ratio is reached. You then polarize the plasma with a high-voltage supply and modulate the flame with a high voltage audio transformer or direct-couple to 211, 845, or 212E plates from the built-in power amplifier. (The flame is modulated in the same way as a conventional electrostatic speaker.)
As far as I know, nobody has ever built a complete system like this before. I hereby throw the idea into the public domain, and wait to see if anybody is crazy enough to actually Build it.
Donít expect to get UL or VDE safety certification for a "loudspeaker" that mixes hydrogen and oxygen gas, high voltage, distilled water, AC power, an open flame, and a microprocessor, all in a domestic environment. Imagine the reaction of the reaction of the insurance company if they discovered how it works!
Aside from these trivial non-audiophile considerations, the plasma-flame speaker would have truly exemplary performance ... very low distortion, perfect impulse response, and a bandwidth of 100kHz or more. Another benefit of the confined-flame speaker is the "diaphragm" can be as big as you want, limited only by concerns like combustion noise, room heating, and fire hazard. As a compromise, a 6" diameter hemispherical flame front certainly wouldnít be too difficult to build. That would deliver response down to 200Hz or so. It would be lab-standard flat from 200Hz to 100kHz, and no cabinet resonances either.
Just imagine a cold winterís evening with twin pale-blue flames illuminating the copper-mesh hemispheres, the faint hiss of hydrogen & oxygen gas, the quiet murmuring of the water electrolyzer, and a pair of eighteen-inch-high Western Electric 212E direct-heated transmitter tubes to make it all sing. Add a Jacob's Ladder for visual stimulation and the picture is complete; Bride of Frankenstein has an electrifying night over at Nikola Teslaís bachelor pad. Careful with that Zippo lighter, Nick!"
Edit: The bass doesn't come from the plasma speaker but a separate subwoofer, according to friend.