Srajan
Fleetfoot Max: it’s my favourite band. Did I finally become dyslexic? Not yet. As I’ve chronicled on 6moons, for the last 4+ years I’ve attempted to transpose the behaviour of ribbon headphones to my speaker systems. You’ll appreciate the challenge if you understand how ribbons work and how Raal-Requisite and Raal 1995 implement them in headsets. Ribbons in headphones are ultra-low-mass transducers which are fully conductive to be driven directly; carry no dead weight; and handle the entire bandwidth without crossovers stitching together drivers of disparate size, mass and makeup. They also don’t operate in a reverberant environment with LF standing waves, reflections, boundary gain and time smear. Then there’s all of it with multi-way speakers in an untreated normal room. Kipling’s famous ‘the twain shall never meet’ might as well have meant that rather than East and West.
Nonetheless, I failed to get the memo to experiment with bridging this gap between head-fi and speakerfi. A major discovery there was cardioid bass. It describes directional bass systems which exploit deliberate counter-phase cancellation to not radiate low frequencies in a typical omnipolar fashion. The Kii Three and its descendants popularized the idea in a compact monitor. Makers like Modalakustik, sound|kaos and Voxativ applied an expired Axel Ridthaler patent on Ripol or ‘super-dipole’ so cardioid radiation to their subwoofers.
Whilst DSP can strategically level mountains to flatten out response peaks, when using just two channels it can’t do a thing about late-arriving reflections. Though the post-EQ amplitude response will be more linear, omnipolar radiation from wavelengths long enough to wrap around speaker enclosures continues to cause lingering bass. That doesn’t take the direct route to our ears but snookers around our room by reflecting off all its hard surfaces. Longer path lengths mean a lengthier time delay. That means late bass and what arrives late is slow. So slow bass is no myth. It’s not that bass frequencies propagate through the air at a lazier pace than treble frequencies. It’s about the room becoming ever more dominant as frequencies descend. By eliminating the sidewalls as reflective sound sources, the lateral out-of-phase cancellation of dipole bass is advantageous. To this cardioid bass adds weaker output at the front wall so even less structural gain. The upshot is that strategically shaped LF radiation reduces how much acoustic energy we put into our room in the first place to minimize its resonant signature. It’s a form of ‘free’ room treatment in the sense that aside from a directional bass system, we don’t need ugly stuff hanging from our walls, ceiling or crowding up corners.
Ideally, proper subwoofer integration means an active crossover for mirror-imaged high and low-pass outputs for the speakers and sub plus a separate volume control for the latter. My main room’s dimensions create resonance modes at 35Hz and 70Hz. Those my Ripol sub seriously downplays since a 4th-order filter means it’s already in 6dB at 100Hz. Were the speakers to handle this bandwidth, they’d ride the standing waves like a rodeo champ clings to a bucking bull. Hence the sub’s primary rationale is not to add lower bass. The Qualio IQ speakers on their own are good to below 30Hz. The sub’s primary task is to replace their omni bass with directional bass. That means far crisper stoppage and far less resonant lingering. What’s the downside? Complexity so cost. The 2×15” sound|kaos sub is passive to need amplification and filtering. Filling those spots is a pair of OEM Pascal class D amps in Gold Note case work; and a Gradient Box II active analog precision crossover from Lifesaver Audio. That’s four extra components and the cabling to connect them.
Recently I tried an alternate solution which halves the box count and eliminates all extra cabling except for two generic power cords. Enter PSI Audio’s AVAA C214 active bass trap from Switzerland. It’s a dense compact cylinder a good half meter tall and less than a foot across. Think hi-tech plant perch. It’s a black or white metal build with a perforated grille behind which work two specialized 5¼” drivers, a microphone, DSP and amplification. On the back facing down are an IEC and power switch. Above it facing up are a blue and red gain switch for plus and minus and a small temporary LED. The adjustment range is from -12dB to +6dB. A small permanent green LED behind the front grill confirms power status and a smartphone app puts the otherwise manual gain adjustment on its screen. The operational bandwidth of AVAA (or ‘Eva’) is 15Hz-160Hz. Her absorptive powers are said to be up to 45 x higher than a passive bass trap of equivalent size. It’s thus a domestically super-friendly alternative to the classic passive bass trap which to be effective below 160Hz must be voluminous. Rather than plant a whole forest of passive absorbers in our room, PSI suggests that we plant just two stubby shrubs. At £3K each, that’s still chunky change especially since in standard rectangular rooms, we’ll want two.
The recommended best placement is as deep in the two front corners as possible so with minimal clearance. If you run decorative floor mouldings, that’ll be perhaps 2 centimetres off the walls. It’s close to out of sight, out of mind. ‘Eva’ isn’t in the signal path. There’s no DSP. This is a purely acoustical solution which creates negative acoustic impedance as though we’d opened a 1½x1½m window on the floor in each of our front corners. Because ‘Eva’ absorbs LF pressure rather than just trims bass amplitude as DSP can do very surgically, we lose time-domain ringing so endemic to untreated rooms. Because our front corners tend to be the two areas where greatest acoustic pressure compounds, placing ‘Eva’ there has the greatest effect.
What’s this to do with Fleetfoot Max? Simply that this creates maximally fleetfooted bass whose damping or stoppage is far more consistent with the higher more room-invariant bands. Interference below the Schroeder frequency is seriously diminished because longitudinal and transverse standing waves are cut. Rather than look at a big subwoofer between my speakers–plus two mono amps to power its two drivers plus a preceding active crossover to route filtered signal to my 2.1 transducers — I’d be looking at two stumpy thin columns if I bought a pair of ‘Eva’. In fact, the one in the left corner would hide behind the big plant to be virtually invisible.
I wrote ‘seriously diminished’ because for the very cleanest response I reactivated my Ripol sub to run that plus my two ‘Eva’ loaners. The Ripol sub adds reflective sidewall elimination which PSI don’t address. The more important point is this. Without sub and active xover I could – for the first time – run my good-to-25Hz 3-way speakers wide open and get virtually the same results. That’s something to write home about for listeners whose staircase into the bass-ment doesn’t consist of perfectly even steps of equal hardness but includes irregularly sized steps aka response peaks; and others of very squishy consistency or texture we call ‘bloomy’ if we’re casual, ‘boomy’ if we’re serious. Again, DSP can even out irregularly sized steps so that when a piano plays a downward scale across a few octaves, no tone is louder than the next. What DSP with two speakers can’t do is eliminate the time smear from reflected bass which in a room with four walls, floor and ceiling dominates the direct sound by a very significant percentage.
To keep this in context, the two rooms I tried this in are 4 x 8m in a short-wall layout; and 6 x 8m also in a short-wall layout. How effective a pair of AVAA C214 might be in much larger spaces I don’t know. Neither do I understand exactly how this Swiss invention works. I’m merely confident that it does what it says on the tin. One target audience for these devices should be listeners whose exposure to premium head-fi has them now intolerant of or even allergic to fat bloomy bass that’s not recorded but generated by uncontrolled room resonances. Such listeners have a rather good notion of the recorded tonal consistency of their music. They instantly hear how all speaker systems in untreated rooms create response and time-domain aberrations which their headphone systems don’t. It’s this kind of listener in particular who will immediately appreciate the benefits of our compact active bass traps; and understand that DSP alone can’t get us there. For casual listeners used to fatty ringy bass and loving it because that’s all they know, making it leaner and drier could seem less important or counter-persuasive. And then there’ll be those who acknowledge the issue but find £6K out of the question to solve it.
But the old ruse of blaming our significant other who won’t let us install professional room treatments just got cancelled. Purely from a cosmetic and footprint perspective, what audiophile other could forbid their music maniac a pair of AVAA C214 especially when it also cancels bass leakage into the adjoining room? Perhaps it’s finally time to bite the silver bullet and pursue this novel active if purely acoustic room treatment that’s now perfectly domesticated and even available in white?
