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KIH #40 – Openly baffled?

Open baffles. They’re the opposite of closed baffles. Yet nobody calls them that when the most common way of closing up an open baffle is to fold it into a box. So a box speaker is the typical opposite to an OB even when a spherical Anthony Gallo ball speaker is a closed baffle too. It’s just geometrically continuous rather than an assemblage of discrete flat panels. The key distinction isn’t geometry. That runs the gamut from actual monkey coffins to swooping KEF Blades to faceted Avalon to curvaceous Vivid alternatives. The key distinction over an open baffle is isolating the driver rear waves from their fronts, either by sealing them off completely in a closed box like Magico; or by manipulating the rear wave across a specific bandwidth via ports (most everyone else).

Generic 2-ways are sealed for the tweeters, ported for their mid/woofer. Most 3-ways seal off their midrange too and just port the woofer. Since dispersion transitions from half to full space with descending frequencies, all box woofers become omnidirectional eventually. “Leaking” sound through a port only adds strategic output at the port frequency. It doesn’t fundamentally upset the radiation pattern. Why are designers of box speakers so adamant about eliminating the rear wave? Unless you had an ‘infinite’ baffle—drivers mounted flush in a room’s wall for example—once wavelengths exceed baffle width, they begin to curve around it. Now front wave meets back wave, Harry actually hates Sally and cancellation happens. If you keep the rear in the box not out, it doesn’t. The majority of designers says that’s good.

Of course trapping half a speaker’s output inside a box not only is wasteful, it creates internal pressurization which fights the box. “The loudest part of the room is inside the speaker” isn’t a joke. Half the output in a fraction of your room’s cubic volume gets bloody noisy. Now the box must fight that noise acoustically with so-called damping or absorption (foam liners, wool/synthetic fill, traps); and mechanically with braces and general overkill aka heroic construction. Hi-tech designers exploit Comsol or similar software to simulate a given cabinet geometry’s air flow, pressure build up, mechanical flex and related behaviour to optimize it for ultimate inertness and resistance to stress.

This has given rise to very complex shapes which are expensive to manufacture. Typically enclosure walls thicken, their composition hardens, their mass and weight increase. That’s because the lower the speaker extends in the bass and the louder it is meant to play, the more acoustic and mechanical stress it generates. Now we see aluminium, resin composites, artificial stone, tempered glass, Birch Ply, concrete, Panzerholz, fibre glass and sundry compounds show up, either pure or strategically bonded in so-called constrained layers. Being a very visible object in the room, speakers are also expected to look fab, dahlin’… so finishing off their cabinets to contemporary luxury standards becomes another very serious job and expense. Be it veneer, automotive lacquer, leather, chrome, solid wood or various combinations thereof… finishing the box is a big deal. The more expensive the speaker, the bigger that deal. In fact, most high-end speakers spend more money (material and labour) on their boxes than anything else. Would that be a raw deal by any chance?

A little advertised by-product of boxing in the rear wave are through-cone reflections which, by their very definition, must be delayed in time. Thin speaker membranes aren’t airtight seals. After the rear energy reflects off the internal box walls, some of it escapes through the drivers unless, by some extreme measure, those internal walls and what’s between them are 100% effective at absorbing all of it. Enter the open baffle. Box, be ye gone. Sayonara, associated expenses. Auf nicht wiedersehen, internal reflections. Blimey-bye to choking off half the output which our system so dutifully generated. Even sensitivity can’t help but increase by 3dB though additional room gain could add to it.

But wait… what about those gnarly out-of-phase cancellations there at the edges of the baffle? Quite. Because of them, open-baffle woofers get big. For ambitious bandwidth, one commonly sees at least dual 12 inchers if not twin 15ers or even an 18”. That the same bandwidth or more could be generated by a properly ported 10-inch or smaller box woofer is part of this math. Bigger woofers mean wider baffles which, in contemporary aesthetics, are frowned upon just like contemporary beliefs think them contrary to good soundstaging. “Only narrow baffles get out of the way.”

Really? After my recent encounter with Martin Gateley’s unusual soundkaos Libération from Switzerland, I politely disagree. In fact, I had to find fault with a few of my own uninformed OB assumptions. Their trademark ‘figure eight’ dispersion pattern (sound fore and aft, very little to the sides) is actually very effective at minimizing sidewall, potentially even ceiling and floor reflections. Fewer reflections mean fewer room issues. Particularly weaker sidewall reflections make your room… well, wider. You no longer ‘hear’ the side walls by their typical effects on the sound.

That’s not true for the front wall however. Unlike a box speaker’s transitional dispersion pattern (narrow in the treble, consecutively wider with descending frequencies until those go omni), an OB throws full-bandwidth sound straight at your front wall. Because most dynamic tweeters are factory-sealed, a full OB might mount a second one firing back. The Libération’s was an open-backed Serbian Raal ribbon. Is bidirectional sound good or bad? If an OB gets moved too far from its primary reflective boundary, the time delay between direct/front sound and reflected/rear sound exceeds the window in which our ear/brain sums them as one. Now these sounds register as two: original and its echo. That creates more hall sound – your own, not what’s recorded. Overdone, things get overly reverberant and swimmy. Moving the speakers back closer to the wall also moves them back inside the window of oneness.

Just so, having actual ‘active’ sound back there behind the speakers, not imaginary sound set up by the stereo illusion, registers differently on the ear/brain. At least it did for me. Images felt bigger, not like laterally inflated balloons or extension-ladder fretboards but in the depth domain. Thinking about it, I concluded that, really, dipolar radiation is closer to live instruments and singers. Those sound more or less the same from all directions. They are not directional. Whilst their propagation goes beyond dipole radiation, such a pattern at least with Martin’s open baffles avoided the more diffuse imaging of real omnis. They had focus and staged positively huge on breadth and depth.

As though these box-less drivers were freer to move undisturbed—I saw similarities to how a gong radiates similarly free in the front and back—they sounded more gushing, liberated and elastic. Due to a more energetic ambient field of the open rear radiation, tone was richer and denser just like in real life. What at least Martin’s omni did differently too came in the area of punch. Conventional box speakers use their very close boundary resistance of inner cabinet surfaces and associated air compression to assist their drivers’ return stroke. An OB driver lacks that same resistance. All it sees is the same big open air front and back. That damping assist is weaker. It’s why drivers optimized for OB use must often stiffen their suspensions to make up for it.

In any event, the pop, crack, shove and snarl one hears from electric bass in an amplified gig come across more realistically with boxed than OB woofers I thought. Whilst going to about 30Hz, the Libération’s colossal 18” woofer in the listening seat didn’t have the same ‘violence’ or snap attack which traditional woofers or mid/woofer generate from their box loading. I viewed that as neither better nor worse but simply, different. Dynamics were superior however because this speaker put out literally twice the sound and did so with about quadruple the usual cone surface. Little voltage swings scaled up far bigger than they do with, say an ambitious 8” two-way of equivalent bandwidth.

Still, what struck me most about my tête-à-tête with the Swiss SOB aka salonfähige open baffle over the New Year was how beneficial chucking the box really was. Saying “you’re fired” to the biggest speaker expense had mostly clear marks on the plus ledger. This seemed slightly obscene considering how omnipresent box speaker are and how underrepresented open baffles have become in the commercial not DIY sector. True, they grow wider to approach other panel speakers in appearance like electrostats and Magneplanars (though their superior Xmax doesn’t require that they grow to very tall height to pile on planar membrane surface). Also true, many OB sport somewhat exposed derrières because magnets, baskets and hookup wiring aren’t concealed. The flip side is, most people don’t look at their speakers from that end; and these wider baffles not only don’t hinder soundstaging, their Fig. 8 dispersion reduces common room issues.

Then why aren’t OB more common? It could be that drivers which lend themselves perfectly to such loading have become rare. It’s likely that cosmetics play a role. It’s even more likely that endlessly tarted-up takes on the “sheet o’ Ply with some drivers” theme just don’t lend themselves to profitable commerce beyond a certain point. If you mean to sell expensive speakers, what can you really do to a flat baffle to make it very expensive? Use solid Marble? Plenty of dining room tables have their own flat stone baffle without costing silly money. Finish it in gold leaf? Perhaps for Dubai. Beyond spending top dollars on the very best drivers extant, this genre would seem to suffer limited commercial scale-up potential. Is that why open baffles are such anomalies in the retail sector? Even partial exceptions exist of course like the Nola speakers which mate box bass to open-backed mids and tweeters.

Whatever the case may be, for me to actually listening to a good OB that didn’t require multi-amping or DSP correction was quite the ear-wax remover. It made me appreciate why Emerald Physics and Spatial in particular are popular. You can get a lot of big-bore dense sound from a short quite affordable speaker and not spend hard-earned scratch on a box with complex finishing. Especially with smaller rooms where speakers default to squatting close to the walls, the OB concept’s dispersion pattern is a useful secret weapon. It’s definitely a worthwhile category to keep watch of even if finding a commercial open baffle in one’s neck of the woods could require some doing.

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.

21 Comments

  1. As someone who has spent many months with Spatial’s M4, I can confirm that some models to reset expectations. The M4 gave me tremendously tall soundstaging and played well despite the sometimes right-hand proximate side boundary of a balcony roller door. That you don’t need to drop large cash on muscle amps to get the M4 out of bed, they wake up earlier (lower volumes) and offer good dynamics without needing to crank it. Furthermore, the absence of a box is possibly why Clayton Shaw can offer such a terrific sound for just a few bucks more than the ever popular passive KEF LS50.

    More here: http://www.digitalaudioreview.net/2016/04/spatial-audio-factory-tour-hologram-m4-review/

    • Not only in materials and finishing but also construction. I recall Clayton Shaw being pleased that he could pass the savings from a streamlined build process onto customers; likely another reason why his M4 better many similarly priced rivals boxes in terms of staging and – to borrow from Zu’s promotional vernacular – pop and shove.

  2. Thanks for another good contribution. Is it possible to get the names of the speakers in the photos? Some are branded, but others are not. Its always interesting to follow up with visits to product sites. Thanks.

  3. I don’t understand why the creator of the highly regarded OB Audio Artistry speakers and his new OB Linkwitz speakers (that have been available for years) are not reviewed or even mentioned in the audio media.

    • I would venture to guess that because Linkwitz generally doesn’t advertise, his speakers likely won’t be reviewed on sites or periodicals who earn their living on advertising revenues.

      • Or maybe – just maybe – it’s because of Linkwitz’s hobbyist/DIY approach to system building that calls for active crossovers via DSP, multiple amps and quite a bit of end user config that has many writers (and buyers?!) steering well clear.

      • I suspected as much … I enjoy your site very much, thanks (it’s closer to the Gordon Holt’s Stereophile than most).

  4. You ignore Sigfried Linkwitz’s Orion’s and LX521’s, the former which he has been building for nearly 20 years with top SEAS drivers. He is surely one of the leaders/geniuses in speaker technology, whose goal has always been for the speakers to replicate the live event. Granted, multiple amps are needed to drive them, but so what?

  5. Glad to see this topic is getting aired out along with the speakers. When done right, it’s reassuring, like comfort food for the ears. I’ve mentioned this before when I walked into Spatial’s room at the last Newport Audio show. It just sounded right without the need for critical listening. You’re almost all the way there save for a tweak here and there.

    It’s only been about a month since I sold my Tonian Labs TL-D1s and I still regret it. They are a semi-open baffle design that just gets it right. It has a top, bottom sides and back but that back is about 1/5 exposed and one uses slats to dial it in to one’s tastes. That opening is directly across from the Fostex driver as well. Not much in the way of bracing or stuffing, like the body of an instrument. Granted, it’s not a true OB but it’s not a normal box either.

    As much as I liked the Tonians, those M3s have developed a big itch, the likes of which I haven’t felt in a long time.

    Tim P.

  6. I recognize Spatial X1’s, an Emerald Physics design and Jamo 909’s in the pictures. The Liberation’s look very attractive too, but which are the other models shown?

    I’ve always been under the impression that OB designs require larger rooms to work as intended, is that correct?

    On a tangent from the question of room size, the emergence of tall narrow box speakers with multiple smaller bass drivers claiming improved lower register response as a result has not been paralleled in OB designs. Is this related to a technical limitation of OB design?

  7. This wasn’t intended to be a complete listing of OB speakers by any stretch so Linkwitz fans needn’t be upset (of course his Audio Artistry has long since been defunct). I just mentioned and showed a few commercial iterations as examples to accompany the very general text on the subject. From top to bottom, Spatial Audio, Emerald Physics + Kyron Audio, Jamo, Zugspitz (a Bastanis-related brand), Pure Audio Project, soundkaos.

    Another OB brand is Reflector Audio which, clover-leaf like, group quads of woofers together with a horn-loaded compression tweeter in the middle.

    As to narrower OB, this would obviously increase the bandwidth of what wraps around and cancels at the edges; and because proper bass extension for OB relies on large cone surface, this would then need to stack perhaps 6 x 8-inch woofers or more in a line array which could get taller than intended. I suspect that’s why most of those I’ve seen end up with either one monster driver (18″ like soundkaos and the big new Spatial) or dual 15″/12″ woofers (Jamo, Emerald) but not a quartet or more of smaller units which would make the baffle narrower…

  8. Great stuff, Srajan, even if the size and design of most OB speakers would make life with any of the following difficult:
    – kids
    – pets
    – a significant other who doesnt share your passion to the exclusion of all else

    If you look closely at this vid, its clear that the listening room is a loft/attic of some sort – brighter than the usual basement man-cave but a man-cave nonetheless.

    https://www.youtube.com/watch?v=2ozT41TKqRM

    Personally, I like them (recording and YT sound quality notwithstanding), but Darko’s message of late appears to be that we need to be a little less weird and a little more inclusive of those who dont spend hours drooling over vacuum tubes and the like. Just sayin.

  9. Artie:

    Actually… have you seen how stumpy (short) the small Spatial speakers are? They’re cosmetically less intrusive than one might think. And yes, by the time we’re talking 18″ woofers, there’s no getting around the fact that such things, even if less than a meter tall, will never be petite. As always, a case for or against can be made with just a few easy arguments. This article simply shows another less common option to skin the speaker issue. Whether it’s right for a given household I couldn’t say – but that has never yet held anyone back to *talk* about it -:)

  10. Srajan, I take your point re the Spatials, even if that form factor seems to fly in the face of what I’m reading on the DiY forums re open baffle design. As with anything – particularly speakers – someone always comes along and turns ‘conventional wisdom’ on its head.

  11. After one of my maggies went up in smoke (literally), I replaced them with Emerald Physics KC !! + BOMB to my twin SVS subs. Also I had EP upgrade to WireWorlds internal wiring. Much easier to drive than the maggies with similar wide open effortless presentation

  12. My OBs have 2 tweeters(one front; one rear,) a midrange, and a 10-inch woofer. The real bass comes from an Infinite Baffle subwoofer. Mininum visual impact–maximum audio impact.

  13. I would add the Steinway lyngdorf model d as the clear exception to the rule. Very ambitious full scale dipole in very exclusive finish and branding, this is über high end!

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