Loudspeaker designers constantly strive for lower and lower moving mass in their cones and panels, but what would a driver sound like if it had no moving mass—or no moving parts at all?
The massless driver is the holy grail of loudspeaker design. It’s the moving mass of speaker cones, domes, and panels that introduces a whole host of distortions—distortions that designers devote an inordinate amount of time, money, and effort combating. A diaphragm, no matter how light, has inertia that, when it is at rest, makes it want to stay at rest. It’s not just the diaphragm that moves in dynamic loudspeakers, but also the voice-coil former, voice coil, and suspension. And when all those parts are moving, they tend to keep moving after the drive signal has stopped. It’s not hard to imagine how inertia plays havoc with a loudspeaker’s ability to faithfully reproduce music’s dynamic structure: Transient leading edges are not as steep as they are in life, and notes don’t end as quickly and cleanly as those produced by live instruments. Moreover, loudspeaker drivers misbehave in a whole host of other ways—breakup modes, temperature-dependent dynamic compression, non-pistonic motion, non-linearity at high excursions, and magnetic eddy currents, to name just a few. To be sure, great advances have been made in these areas during the past twenty years, particularly as a result of exotic-materials technology. But at the end of the day, we’re still listening to cones, domes, electrostatic panels, or ribbons moving back and forth.
Enter the corona-plasma tweeter, a device that produces sound with no moving parts. As explained in more detail in the sidebar, an electric arc stretching between two electrodes causes the air around the arc to become ionized. By modulating the arc with an audio signal, the ionized plasma around the arc is made to expand and contract, creating sound—no diaphragm, and no diaphragm- induced distortions.
Plasma transducers have a long history dating back to 1900(!) but have never gained much traction in audio (see the sidebar). One company committed to the technology is the German firm Lansche Audio. The speaker company bought the rights to a corona-plasma tweeter in 1999 and has since offered many loudspeaker models built around this exotic driver. Lansche redesigned the corona-plasma tweeter in 2006, and currently every product in its lineup is based on this unique driver.
After hearing something quite special from the second-to-top- model Lansche No.7 loudspeaker at the 2012 CES, I asked for a review pair. The No.7 is a three-way, seven-driver system employing four 8.7" woofers, two 4" midranges, and the horn-loaded corona- plasma tweeter in the baffle’s center. The $108,000 No.7 is a tall, narrow design finished in exquisite woodwork. It is distinguished from conventional loudspeakers by the side-panel vents that are required to cool the corona-plasma tweeter and its integral electronics. The No.7 also departs from typical loudspeakers by incorporating an AC power jack (the corona-plasma tweeter needs a power source) and a rear-panel on/off switch. The rear panel also houses dual ports (the woofers are reflex-loaded), two sets of binding posts, and small jumpers that allow you to adjust the woofer and tweeter levels, respectively. The woofer adjustment is limited to flat or a 3dB cut, and the tweeter level can be set flat, increased by 1dB, or cut by 1dB or 2dB.
Lansche has fitted the narrow enclosure with outrigger “pods” that not only stabilize the loudspeaker, but allow for fine height adjustment. You simply turn the knobs to raise or lower each corner until the loudspeaker is level. The pods also incorporate vibration isolation, although I found that placing Stillpoints Ultra 5 Isolators beneath the pods rendered a significant improvement in sound quality.
The corona-plasma tweeter is crossed over at a lowish 2.5kHz, meaning that a significant portion of the audio spectrum is reproduced by this transducer. The dual 4" midranges handle the range from 200Hz–2.5kHz. The crossovers feature a wide range of slopes, all the way from third-order (woofer low-pass) to first-order (tweeter high-pass), and all are made from premium components such as Mundorf, Duelund, and EPCOS capacitors. The tweeter high-pass circuit comprises a single Dueland CAST capacitor. This Danish capacitor is quite exotic and expensive, and its makers claim that it is the world’s best for audio applications.
The No.7’s enclosure is fabricated from a composite that combines MDF with a ceramic material, coated internally with heavy foam. The large enclosure is reinforced with seven horizontal braces and two vertical braces, then veneered with gorgeous wood (the review samples were Indian Applewood). A knuckle-rap test on the side panels reveals an enclosure that is less inert than that of many six-figure loudspeakers. Sensitivity is a highish 92dB, but the No.7 is harder to drive than the sensitivity would indicate.