Slow bass - whatever happened to the speed of sound's sacrosanct constancy?

A cursory poll of current subwoofer designs competing for the lowest, loudest, baddest bass? It shows them all almost exclusively devoted to first driving down transducer resonances for the lowest possible reach. Remember, unlike midrange and treble units -- always crossed out well before approaching critical cone resonance threshold -- bass drivers, by definition, run without electronic low-pass filters. Downwards, they operate wide open to attain their low frequency extension. Why would you stop 'em short? Because they'll go out of control once hitting self-resonance? This lousy check-in with reality sets off self-fulfilling prophecies. Large driver diameters. High mass. Long strokes. Steep associated power requirements. Accelerometer feedback loops. Error correction circuits. And surrounding it all, the gleefully adolescent fascination with size and SPLs. Quite a kettle of (rotten?) fish, this.

Whether front-, rear-, side- or floor-loaded, woofers visible to the naked eye -- hence room -- suffer effective compression from the acoustical impedance facing them. They "see" the combined air volume of the room you park 'em in. You've aired out your stuff at the nude beach in public? Nobody so much as shot the breeze about it? Why the big deal with woofers then? Because they're 12 or 15 inches, perhaps?

Well, no. Woofers don't hang in a true free-air environment like your family prunes. They're resistively back-loaded by their own cabinets. That, while facing the room's bored stares on the fancy sides of their cones, creates a tremendous work load. Popular MO? Audio pornography. Respond with extreme excursions of the 3-inch kind. This clearly necessitates ultra-stiff cones to avoid break-up. High mass means high weight. To overcome its inertia requires colossal power to propel these King Kong gorillas into action. Desire precise clean bass? How to, on a dime, stop run-amok Rambos dead cold in their mighty tracks after they've gotten their fat asses into belated gear? That becomes the next issue. Hire a different actor?

Therein does lie the real rub, true. This brute force approach of woofers-on-steroids [see left for an example] gave rise to the concomitant observation of slow bass in the first place. Did the speed of sound constant momentarily -- and very unscientifically -- forget to operate in customary fashion at lower frequencies? Not at all. It's simply that a high-excursion driver takes more time to complete a single cycle of rarefaction and compression than, say a horn-loaded driver whose output is acoustically amplified to require a far shorter and thus faster excursion cycle for the same output level. By transitioning to larger and larger woofers with more grotesque strokes, another common victim in the races for high output/low frequency records became the now unavoidable time delay between the fundamentals and their upper harmonics. Astute listeners noticed that the bass lagged behind the remainder of the spectrum. It also didn't blend, fundamentals and harmonics arriving out of sync, clearly not part of the main speaker system. Far smaller drivers would be the ticket to gain speed and coherence - if they could go low enough loud enough, be liberated from asymmetrical pressure buildup, retain low driver mass and cone travel requirements and be guaranteed immunity from room reflections. What a wish list.

As I reported briefly in my CES 2003 report, VBT does approach low bass radically differently. My stunned exposure to their patent-pending solution in that hotel room had already regressed me. I'd slipped from the "common knowledge" of that's impossible to the head-scratching ignorance of hot damn, it works exactly as claimed. How to regain one's primate intellectual self-confidence in the face of such impertinence? Secure at least a rough appreciation for the underlying concept so you can think to yourself: Of course, that's - so obvious. Can't imagine how anyone wouldn't see that. Yeah, right.

While awaiting the outcome of the snail's pace patent process, VBT remains understandably hesitant to dish and talk out of school. Still, I wanted to grasp the core idea. The "blank" spots in the following descriptions are due to this reluctance to divulge crafty details to the competition; to my lack of the proper hollow Allen key that would have pried open the boxes for a look-see; and to my brain which goes mushy whenever faced with brilliant engineering concepts.

Here are some key ingredients that did survive the usual brain freeze:

  • Take the room's compressive air mass off the driver.
  • Present it instead with a constant acoustic impedance approaching free air.
  • Construct an enclosure in which the driver is optimally loaded to require minimal excursion for the intended frequencies while lowering its effective resonant frequency.
  • Combine with but also isolate this circular line scheme from coincident port loading for equalized, symmetrical pressure on both sides of the cone.

VBT's designer likened the challenge of subwoofer traction in the face of the usual compressive air mass to starting on ice while outfitted with regular shoes. He loads his 8" short-excursion woofer into a component-width squared chassis (17.5" x 17.5") with merely 6.75" height. He then nearly seals its down-firing rim terminus [a narrow continuous slot along each side wall - images on next page] with a special viscous foam four times as dense as the usual damping materials employed in transmission lines (recalling, in principle, Anthony Gallo's backloading scheme for the 3.5" full-range driver in his micro spheres ). This arrangement creates what he calls "a constant acoustic impedance environment approaching free air. It gives the driver shoes-with-nails - the necessary traction to start and stop with precision on thin infra-sonic ice". The actual port on the backwall? A lightly flared, counter-intuitively narrow 2-inch diameter affair.

The driver itself stares directly at the cabinet's inside top, and thus a very small, perfectly controlled, non-variable air mass independent of where the passive Magellan VIII will be placed. While the precise details remain elusive, Plummer cites a 20-30dB increase in effective output which, coupled to the port, raises to 40dB, meaning the woofer proper merely requires 3-4mm of excursion to do its job. Hence, it's as fast as the best midrange drivers. The underlying idea isn't so dissimilar from a 1-inch tweeter loaded into the minimal air cavity of your ear canal to produce sub-30Hz headphone bass. The remaining question? Would the Magellan be capable of producing the undistorted output levels I was used to from my firing squad of quadruple 10-inchers? And, would reverting to mono bass -- the companion VBT200 power amplifier, while capable to drive two Magellans, inexplicably sums them to mono -- minimize realism?

But first, some techno geek gratification as transcribed from VBT's website. The low-profile Magellan VIII bass module is based on VBT's patent-pending VARTL/Reflex Technology, dubbed an optimized alignment of a bass reflex system for sub-bass frequencies that focuses all of the useful driver energy at the port while simultaneously shielding the driver from the backlash of external acoustical reflections. The Virtual Acoustic Radial Transmission Line creates constant-pressure isobaric load conditions. These improve dynamic power handling while allowing driver modulation of the circular vent over a wider-than-usual frequency range, even below the aperture's cutoff frequency. This eliminates the need for fast roll-off subsonic filtering which normally minimizes driver control loss below 30Hz. The following impedance graph from VBT's site illustrates the VARTL loading principle. The balanced air load of free-air operation is indicated by the right-side-up and upside-down triangle curves. The x-curve indicates the negative pressure assertion by VARTL loading while the diamond-curve indicates the positive pressure added by the reflex loading. The o-curve indicates a closed vent and the stability of the driver's impedance under such extreme conditions.

Negative pressure is defined as the effect of an air mass disposed in front of the driver. The resonant frequency of the driver is reduced by the air mass associated with the damped transmission line (x-curve). This action is opposite to that of the reflex enclosure that loads the backside of the driver (diamond curve) as the cabinet air mass attempts to force the driver's resonance higher. The t-line action is analogous to adding physical mass to the driver cone - except that it is eliminated by the compressed air mass in the cabinet. The resulting operating impedance is at or near free air impedance (zero cone air mass differential - triangle-curve), an ideal fast-transient operating platform for an enclosed driver resonating a port. Or so according to VBT.