Brief History of Plasma Transducers

Here is a brief, incomplete history of different transducers classified today as plasma transducers.

One of the earliest talking film sound systems called the Tri-Ergon system developed by Josef Engl and Hans Vogt. It used a glow discharge microphone, named the 'Kathodophone' or 'Kathodofone'. Some of these early electronic recordings survive to the present day, on 78's of the label 'Tri-Ergon', many which can be heard on YouTube. Invented in 1919, this microphone predates the modern dynamic microphone. Surviving recordings demonstrates it functioned well, with wide frequency response.

In 1925 Thomas Philip, a Westinghouse engineer used a Glow Discharge microphone for a short period for live radio broadcasts, but many problems made this apparently unreliable and prone to severe RFI, this has been mostly forgotten to history.

In the early 1950's the Corona Discharge speaker was created and tested. In the same period the corona triode was investigated, lacking the heater of it's thermionic valve equivalent.

In the mid Fifties researchers studying the effects of sonic booms on wildlife built a plasma speaker to simulate the high pressure wide bandwidth sonic boom. SPL levels of 140db were achieved over a wide frequency range.

The Ionovac or Ionophone, was sold by a number of brands and developed by Siegfried Klien. It was quite popular with owners but apparently unreliable. Sold by Dukane, Magnet, Fane, B&W in their P2 (used as a monitor at the BBC,) and Sony in a very rare licensed version of the B&W P2. Still popular with owners, new replacement parts are once again available online.

In 1965 the STL-Ionophone was built as a high frequency transducer, and used as a scientific testing microphone, many papers of which can be found online. No sound clips or examples are available online AFAIK. Proves use of plasma microphone for animal studies, in addition to turbulence studies in a wind tunnel. Could also be used for wind speed measurements, registered as a DC signal.

Alan Hill sold the Plasmatronic speaker from 1972 to the early eighties. With a large helium tank the problem of ozone generation was eliminated, but a constant supply of compressed gas became a requirement. Woofer reportedly not up to par with the fast plasma speaker. Few were built (under 100, probably ~80,) as the price was very high.

Nelson Pass developed and demonstrated a Full-Range Glow-Discharge Plasma speaker called the Ion cloud panel, constructed of ~30 sections of tungsten wire from photocopy machines, in a clear acrylic case. The speaker drew so much juice the power at the 1982 CES convention went down with loud musical passages, though many report hearing the clearest, most realistic and fast performance from any speaker ever. It didn't even require a crossover, yet Nelson Pass was hospitalized and in recovery for more than a year from ozone exposure while developing and testing it, and the idea was shelved permanently. No other speaker in history is more interesting in my opinion, and there is no other I personally wish to hear more than this one. Nelson Pass states "It took several kilowatts to get any sound out of it, but it was the most physically and sonically transparent loudspeaker I've ever run across. That is to say, you could see right through it, and it sounded like it wasn't there. It was quite remarkable in that regard."

Also in the mid eighties the Plasmasonic headphones were sold for around less than a year. Many reported excellent sound quality, but bankruptcy, possibly spawned from the poor service life of the anode/cathode, abruptly ended this idea. Frequency response goes down to below 10hz IIRC. Some report odd physical ear fatigue possibly caused either by ozone or electromagnetic radiation.

There are reports of another Plasma headphone from around the same time that never materialized as a product, with a reported frequency response of DC-60Khz. Also ear irritation, caused by either ozone or IR radiation, severely limited it's longterm usability.

There are modern examples of Plasma tweeters, many from Germany, and a few DIY projects, all with reportedly excellent sound quality. They are generally frequency response limited to 1-2K minimum, and often extend to 100khz and beyond, depending on supporting hardware.



References:
Glow-discharge microphone - United States Patent 1638893
Stereophile Magazine - Vol. 6 No. 1
Bowers&Wilkins Blog -
airspeaker.co.uk - excellent resource with much more info