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KIH #45 – Removing Repetitive Rooms

Repeat one. For this installment, it’s not about repeating a music track. It revisits an issue covered in an earlier KIH. Why revisit it? Because it can be a game changer. It minimizes problems with the front wall. Here I’m referring to the wall in front of you, i.e. the one behind the speaker. Some people call that the back wall. And from the speakers’ perspective, it is. Either way, now we’re clear on which wall we’re on about.

As wave lengths get longer, they wrap around speaker cabinets and propagate in all directions. That’s omni radiation into full space. For bass frequencies, we thus hear the sound twice: once direct as it comes off the woofer toward us; then as a second wave ricocheting off the front and side walls. We needn’t know math to appreciate that these rebounds are delayed sounds. As such, they add themselves to the direct sounds and muddy the picture. Call it echo to keep it casual. The more echo padding there is, the more swimmy things get. In smaller doses, we perceive it as warmth – the equivalent of a bit of reverb around a singer’s voice. It adds some fuzz or imprecision like cotton wadding.

The greatest build-up of muck happens in our room’s four front corners where three surfaces each meet. These become drone zones; high-pressure areas. Whilst our listening seat is elsewhere, uneven pressurization in the room still telegraphs. If we can minimize that contribution, our sound will be better in the hot seat and outside of it.

How to do that? In my recent Kii Three review on 6moons, I documented Bruno Putzeys’ solution. In brief, his compact 6-driver monitor has side- and rear-firing woofers staggered in phase and time via complex DSP such that strategic interference between these drivers causes radiation cancellation to the sides and behind the speaker. The technical term for the resultant dispersion pattern is cardioid or heart-shaped. It’s what directional microphones use to minimize the pickup of sounds from all directions. In a location-recorded interview for example, they’re meant to focus on the speaker, not the background din around him.

Put pragmatically, the Kii Three puts out directional bass. Its DSP/phase trickery is effective up to ~1kHz. It circumvents the fact that long wavelengths propagate in all directions. It means that the unwanted late second waves are much weakened. And that means that the front and side walls – more correctly, their effects – have been removed or better, minimized. Unless heard in action, we lack any in-room reference whereby to gauge how much time-delayed overlaid reverberance our setups suffer. To us they might sound reasonably or even very clean and transparent. Still, there’s a very high chance that by comparison, a Kii Three would show up plenty of remaining issues. Those we no longer notice in full. In that room they’ve been our status quo since day one. We’re used to the sound. Our perception compensates. We in fact can’t step back in time to when we first set up our hifi to still recognize the room’s full impact on the sound.

With the arrival of the Audio Physic Codex 4-way in our Irish residence, this same subject arose again. Why? Because, in a very clever way, designer Manfred ‘Manny’ Diestertich removed typical rear-ported bass issues without opting for the default sealed-bass alternative or going dipole. He uses a very compact woofer box inside the speaker’s main cab. Its woofer fires against the inside of the main cabinet sidewall within an inch or so. The rear wave does the same against the inside wall of the woofer box. The front wave exits a narrow slot behind the baffle bottom to hit the floor as its second boundary. The rest dissipates inside the enclosure. The rear wave inside the woofer box funnels through two tiles of porous transmissive ceramic foam. Those tiles face down. Again escaping bass hits the floor within an inch. Without port tube, there’s no deliberate resonance. Then, the very tight confines of this hidden 10” long-throw woofer exert very high damping and mechanical braking.

The end result is very low powerful taut high-torque subwoofer-style bass in full stereo which appears out of seemingly nowhere. Unlike rear ports tuned to an equivalent 28Hz, it doesn’t interact with the room in the same way. It causes far less overcast on the vocal range. Besides better bass, it means more transparency and linearity in the mid registers. Not as radical as the Kii Three, it’s still a very successful implementation to clean up common room issues. And this uses no DSP or room correction. It’s a completely passive solution.

Another passive approach is the Dutch & Dutch 8c pro monitor where 8 means midrange diameter and c stands for cardioid, again. This speaker is optimized for close-wall placement. Two rear-firing woofers in the cab’s sealed back half use the wall for gain and loading. Meanwhile slots in the sidewalls create dipole loading for the frontal midrange. This means deliberate lateral out-of-phase cancellation. It aims the speaker’s majority bandwidth at the listener not sides. Meanwhile rear-firing bass exploits the front wall.

It’s interesting how two of these three examples arrive from the pro world. It makes sense though. Recording engineers rely on hearing just their tracks, not whatever their studio space might add to it. There’s no good reason why you’d not use a Kii Three or Dutch & Dutch 8c at home.  Au contraire. Should you have something more traditionally audiophile in mind, do check out the Audio Physic Avanti and Codex models with their hidden woofers that replace the firm’s earlier signature scheme of force-cancelling lateral woofers. Something very clever and effective is afoot there…

Written by Srajan Ebaen

Srajan Ebaen

Srajan is the owner and publisher of 6moons. He used to play clarinet at the conservatory. Later he worked in audio retail, then marketing for three different hifi manufacturers. Writing about hifi and music came next, then launching his own mag. Today he lives with his wife Ivette and Blondie the cat in a very small village on Ireland’s west coast, between the holy mountain Croagh Patrick and the Atlantic ocean of Clew Bay in County Mayo’s Westport area. Srajan derives his income from the ad revenues of 6moons but contributes to DAR pro bono.

5 Comments

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  1. Don’t those who prefer the Bipole sound want the front wall reflection? For me, a Bipole looses a bit of focus but the gain in perception if soundstage depth and width is much preferred. So in my case would I want to cancel the front wall reflection?

    Thanks

  2. Another way to cure the room boom may lie in Elac’s Adantes line of speakers. What appears to be a woofer on the front baffle is a passive radiator, with the real culprit some distance behind, in it’s own sealed chamber. With no ports to chuff and bounce echo off that front wall, maybe I can rid myself of that room boom in the lower mids and bass.

    It was nice, at first, to hear it from a monitor in my small room (10′ x 15′) but it only comes in handy with movies. I need something with better balance and am looking forward to hearing it at the forthcoming LA Audio Show.

  3. Ultralight:
    Bipoles radiate in phase, dipoles out of phase. What the Kii and Dutch & Dutch do is exploit dipole out-of-phase cancellation to make radiation more directional, thus reduce room interaction.

    In your case, the design brief differs. Your speakers want more not less room interaction. Different goals -:)

    Tim:
    I’ll be checking out the Elacs in Munich if they have them on active display.

  4. Thank you Srajan for a great article. Set of speakers on a stand producing realistic bass forms lots of challenges no matter what technology is utilized. Certainly applaud Bruno Putzey for elegant and space saving option. As an alternative a set (a stereo pair) of good quality sealed subs with DSP amp can do wonders in even most difficult spaces as subs can be placed almost anywhere to equally pressurize the room. If equally spaced from main speakers they ultimately widen the soundstage to a room disappearing degree. Cheers!

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