How could laser research qualify someone to design a loudspeaker? The connection is really much more direct than it seems. Twenty-odd years ago, Dr. Hill envisaged a loudspeaker that would use a field of ionized air as the transduction element, but didn't feel enough was known about plasmas (footnote 1) to perfect such a device. At about the same time, a firm called the Dukane Company started producing such a device anyway: The "Ionovac" tweeter. It was not a huge commercial success, partly because of its (for those days) outrageous price and partly because add-on tweeters have never been big sellers. (The Ionovac was subsequently made by ElectroVoice until phased out in 1963.) Nonetheless, the Ionovac is still considered by the knowing to be the best supertweeter ever made, and there are few audiophiles who would sniff at its 240kHz (±2dB) response.
While developing the high-efficiency laser, Dr. Hill found it was necessary to control the shape of the plasma of ionized gas that does the lasing. And it occurred to him that shaping might be the key to a high-efficiency, wide-range "Ionovac."
His first efforts, using a relatively low-temperature plasma (and an absolutely Mickey Mouse mockup), were disappointing. It produced sound, over a respectable part of the audio spectrum, but at ridiculously low levels of efficiency. Using higher ionizing voltage, and a mixture of air and helium as the plasma medium, he was able to sustain a much larger plasma field (thus significantly extending the low-end range) and to yield practical efficiency figures. Then it was necessary to do additional trimming of the system to produce the flattest possible frequency response across the board.
In the final production version, flat response is maintained (with 1dB) down to around 700Hz. The upper limit is claimed to produce "significant acoustical power" out to beyond 100kHz. It was deemed impractical to try and carry the low end because of cost and power-supply considerations. Even in the production version, the required driving amplifiers (built into the system and all tubed) are rated at 500Wpc.
The range below 700Hz is handled by conventional cone drivers: a 5" midrange and a 12" woofer, which must be driven by their own (choice optional) amplifier.
The speakers connect to the main system preamplifier via a 30' cable and an "electronic interface"a small box housing the system's electronic crossover circuitry, balancing controls, and a series of LEDs that display the system's output level at any given instant. The interface unit is located at the main preamp end of the interconnecting cable.
Gas
Beside the plasma driving amplifier and the transduction device, each speaker enclosure also houses a large bottle of compressed helium gas (footnote 2), which is fed on demand to the plasma field when the speaker is operating. (When the system is off, the helium flow is automatically turned off.) The bottles must be recharged after each 300 hours or so of operationrepresenting s little under 6 months of 2-hours-a-day listening sessions. Refills cost around $30 per bottle, which translates into an operating cost of 20¢ per hour for helium alone.
For people living within convenient delivery distance of a major city, there should be no trouble locating a helium supplier. (You'll find them in the Yellow Pages, under "GasIndustrial and MedicalCylinder and Bulk," or under "Welding Supplies and Materials.") For those people who live 'way out in the boonies, recharging may involve shipping the empty bottles to some distant supplier and waiting, perhaps for weeks, for their return. (Anyone who can afford a pair of the Plasmatronics should certainly also be able to afford a second set of gas bottles to be put into use when the other set is away being recharged.)
Practicalities
Each speaker weights about 300 lbs with its fully charged bottles. And when both amplifiers have been running for an hour or so, their combined heat dissipation dumps about 3500 BTUs (just over 1kW) into the roomdandy on those chilly winter evenings but a dubious blessing on a hot August afternoon.
With all the design complexity, the question of reliability must inevitably come up. As of now, the speakers haven't been around long enough to establish ay sort of reliability record, although their ability to withstand accidental overloads and foolhardy listening levels has already been demonstrated. They seem to be very rugged, but whether or not production samples will be inadvertently sabotaged by a parts vendor remains to be seen.
Those of us who have read alarming things about the toxic effects of ozone may wonder how much of a problem it is with this system. Well, the Plasmatronics do generate ozone, but in such small quantities that after three hours of continuous operation, it could barely be smelled at a distance of 12" from either speaker. This concentration of ozone is so far below the toxicity (or of potential damage to rubber and plastics) that to worry about it may be symptomatic of some degree of neurosis.
There is provision for biamplifying the two lower-range cones, but this is one of those rare instances where biamping is not recommended. The built-in crossover has phase-correction circuitry; electronic crossovers do not. As a result, biamping the Plasmatronics speakers introduces audible frequency-response irregularities (which are absent when their own crossovers are used), neatly shooting down the system's remarkable blending of drivers.
Listening
We auditioned two versions of the Type 1 speaker over a 3-month period. The first was early production, and while that part of the audio range covered by the plasma driver was impressive (more details subsequently), we were unhappy with the low end. The cones blended superbly with the upper range, but the bass was somewhat loose, floppy, and ill-defined. We were inclined to blame that on the driving amplifier, which was one we had never been enamored of: the Audio Research D-100.
Subsequently, Dr. Hill made changes in the cone portions of the system and also found what he felt to be a better drive amplifier for them (the Threshold 4000A), and that was the version of the system we auditioned for this report.
So, how does the current version sound? Quite simply, mind-boggling! One's first reaction is that there is just no transducer there at all. You seem to hear through the system to the program source. Stereo imaging and depth are as well reproduced as form any system we have heard, and the most immediate response to all this is that the system sounds incredibly alive.
Footnote 1: To a physicist, a plasma is a volume of ionized gas. (An ion is an atom having more than or fewer than its usual complement of electrons.) The gas within a plasma has an extremely low density, relative to the gas surrounding it, Thus, when cool gas is heated to the plasma state, it expands in volume and imparts a pressure wave to the surrounding, cooler gas. Using an audio signal to vary the volume of the plasma produces the alternating compressions and rarefactions of a soundwave.
Footnote 2: Helium is inert, odorless, and completely harmless. Deep-sea explorers have breathed a 50/50 mixture of oxygen and helium for days at a time without any effects other than a comical raising of the voice pitches that makes grown men sound like Donald Duck. (Excluding nitrogen from the "air" prevents a nasty diving disorder called "the bends," which results from the formation of nitrogen bubbles in the blood stream when a diver returning to the surface undergoes rapid decompression.) The raising of voice pitches is due to gaseous helium's very low density, which provides less acoustic loading the vocal cords than does normal air, causing them to vibrate more rapidly.