That's how I learnt of Martin's latest sub.versive project following my feature on How many revisions until finalization might the Vox sub undergo? R&D can involve detours. Those quickly become time consuming and costly. About off-boarding filter/gain electronics, it neatly eliminates their exposure to the high-pressure environment of the woofer enclosure. With Martin's Vox 3Awf on permanent install in our upstairs system, I already knew his standards not just on sound but design and build quality. 2 x 15" woofers add up to the 353.5 inch² cone surface of a 21" woofer. Air motion would be robust. A RiPol¹ is considered a velocity not pressure converter. Hence this wouldn't be a monster boom box. A RiPol avoids energy storage and box resonance across its operative band. It prioritizes speed, accuracy and dryness, not bloom and room pressurization. Lateral out-of-phase cancellation produces very little output at the sides. There's far less interaction with a room's side walls and the room modes at 90° off the sub's axis. That makes RiPol bass directional so far less room sensitive than omni box bass.

¹ RiPol is Axel Ridthaler's trademark for a 'folded baffle' dipole variant based on a Siegfried Linkwitz idea. Its Ridthaler/Hoos patent DE19830947C2 sold in 2004 and expired in 2014. These graphs show how the same woofer loaded into a sealed box, standard open baffle and folded RiPol baffle radiates differently plus how the latter nearly halves the driver's resonant frequency versus the sealed box.

"Am conversing with usual collaborator Chris Ellis to see what he can contribute acoustically. Contacted Purifi about their Eigentakt amp. Still loads of options to sort through. The 15" woofers I'm starting with are already specified for dipole use but may want certain RiPol adjustments. Audio Technology are very flexible with their modular system. Should we isolate certain driver parameters that need optimizing, they're excellent to work with." The switch-back trek from concept to commercial realization had commenced.

Music bass versus home-theater bass. Except for the biggest church organs, acoustic instruments don't breach infrasonics. A piano's lowest note is A0 at 27.5Hz. A 5-string upright can hit B0 at 31Hz. The open low E on a bass guitar is 41Hz. A kick drum can make 50Hz. Synth bass can go lower but few producers of music for home playback record data which nobody hears. They'll assume a practical/theoretical 20Hz limit. Movie soundtracks mastered for commercial cinemas anticipate different hardware which can cover sub bass. That poses greater challenges when relocated to the home and the type speaker one finds there. It's why home-theater fans call subwoofers mandatory. It's not just about raw extension but coupling it to full power. With music where nothing explodes, drills into permafrost or launches rockets into outer space, that's no requirement. The primary benefits of a properly filtered sub/sat music system are expanded dynamic range in general; more bass extension particularly with smaller mains; audibility of the occasional synthesized very low bass; and audibility of spatial effects which may occur at very low SPL but also very low frequencies.

An active music sub has other advantages. It dedicates a muscle amp of extreme damping ie. braking power to just bass reproduction, then drives its woofer actively so amp output couples to woofer voice coil without interceding parts. That equals superior control. The same bass amplifier might behave overdamped, dry and harmonically lean if it had to drive the main speakers for their full bandwidth. As a dedicated bass amp, it can be groomed for pure beast mode. That needn't cost much. Enter OEM class D.

Particularly subs designed to be filtered well below 80Hz can turn their woofer/s into a very different type driver than a 2-way whose woofer doubles as midrange to meet a tweeter. Even a three-way's dedicated woofer may need to cover more bandwidth than can be optimized for true low bass bravado. And it'll be passively driven by an amplifier selected for full-bandwidth sonics.

Then there's a sub's ability to adjust bass volume separate from mains volume to adapt to room and taste; and being placed separately. Unlike dipole/Ripol subs, standard box woofers develop their highest output in a room corner. That reduces their excursion which reduces distortion. Corners are hardly ever where main speakers end up. In short, active low bass by way of an enclosure dedicated to the first or first/second octave becomes a specialized solution optimized for one very particular task.

By comparison, passive speakers with low-bass pretensions multi task. Having to be good at everything, they'll rarely be as good at pure bass as a superior active sub. That's plain good sense. Integrating such a sub has always been the real challenge. Hence Martin's interest in Pál Nagy's Gradient Box which over the $4'500 Wilson crossover adds full remote control over its settings and costs far less.

Ascendo's 50-inch subwoofer next to their 15" for proportion shows how below our specialized music sub, there exists another category of infrabass generator to hit single-digit Hertz. It mandates yet another type of hyper-specialized driver and still far more power to drive. That's beyond my interest and software but madness kudos are still due. If you want to do a max 125dB at 20Hz and remain loud at 5Hz, the 400kg man-tall Ascendo will attack your organs and the structural integrity of your home. Personally that's military weaponry I'd never want in our home.