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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.

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    Written by Srajan

    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 Chai the Bengal cat in a tiny village overlooking the estuary of Ireland’s Shannon river at County Clare’s border with County Kerry. Srajan derives his income from the ad revenues of 6moons and his contributions to Darko.Audio.

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