Why not more popular?

Great post @phillb!

The significance of your point about Clayton using drivers designed and built specifically for OB use cannot be overstated. All professional loudspeaker designers/builders know the importance of that, and proceed accordingly. I’m sure there are some DIYers who have used inappropriate drivers (those designed for use in enclosures), but we’re not taking about them.

Danny Richie of course also uses drivers made to his specs, drivers made with electrical and acoustic properties tailored specifically for OB use. These include his woofers, tweeters, and NEO3 magnetic-planar tweeter. One difference between the designs of Danny and Clayton is Danny’s use of a minimum-width front baffle, partnered with the use of side "wings" to increase the front-to-back driver separation the large baffles used by Clayton inherently provide. In spite of their different approaches, Clayton’s design talent and resulting OB loudspeakers have received high praise from Danny.

Also good is your mention of subs not blending well with OB’s. Planar loudspeaker lovers have for decades dealt with that reality. No matter how good the box sub was, it sounded "separate" from the planar loudspeaker. In the 1960’s the 24" Hartley was used by many QUAD ESL users, Mark Levinson using it in his HQD (Hartley sub, double QUAD ESL’s, Decca Ribbon Tweeter) loudspeaker. Infinity used servo-feedback sealed-enclosure subs with their IRS and RS-1b speakers, and many other attempts could be cited.

There are numerous reasons for the difficulty of achieving the desired blending---which I won’t go into here, except in one regard:

In the 1980’s, Finnish company Gradient introduced an OB/dipole sub designed and built specifically for the QUAD 63. They followed it up with an OB/dipole for the ESL (aka 57). Though compromised by sub-par design, build, and maximum SPL, it showed the way. OB sub design flourished amongst DIY loudspeaker designers/builders, including a very young Danny Richie.

Meanwhile, Harry Pearson at TAS had come up with the idea of replacing the sub towers of the Infinity IRS with the bass panels of the Magneplanar Tympani loudspeakers, two 16" X 72" panels per side. Mated with the m/t panels of the IRS (which housed line-source arrays of EMIM and EMIT planar drivers), the sound produced was the finest many people have ever heard---to this day. Finally: full-range (except for the bottom half-octave: below 30Hz) reproduction, completely planar. Unfortunately, the resulting combo required a LOT of floor space, so was impractical for most.

Along comes Danny Richie. His GR Research company was started to sell loudspeaker kits to the DIY community, with an emphasis on OB design. GR Research was offering a number of OB loudspeaker kits, along with a sealed sub. Though very much liking OB bass, he resigned himself to the fact that it’s limitations presented a challenge he was unable to resolve to his satisfaction.

But then Danny heard about a company introducing a new (patented) servo-feedback woofer (Rythmik), which just happened to also be in the State of Texas (in Austin). Danny proposed to Rythmik designer/owner Brian Ding his idea of mating Brian’s servo-feedback woofer/amplifier with a woofer designed by Danny for OB use, a pair of the servo-feedback woofers installed in an OB/dipole frame.

As I’m sure you know (or have guessed), the product came to market. If you have heard the bass produced by the Magneplanar Tympani loudspeaker, you already know what to expect from the GR Research OB/Dipole Sub, but without the latter’s missing bottom half-octave (and massive power amp requirements!). For any OB or planar loudspeaker owner who wants not just more bass, but bass that sounds like it’s being produced by their loudspeaker, it is now available.

@mijostyn: I expected to see the response in your paragraph number one, but not from you ;-) . Yes of course, planars are technically open baffle (though there have been some ESL’s made in sealed boxes). My statement was made in response to the post in this thread claiming that "Open baffle is basically a version of what Martin Logan has been making for years."

If that’s true (and it’s not---the factory-built OB’s from Spatial Audio and Emerald Physics use dynamic---cone---drivers, as do the DIY kits from GR Research---though some with a magnetic-planar NEO3 tweeter---and Siegfried Linkwitz.), you could make the same claim about planars dating much further back than Martin Logan, all the way back to the QUAD ESL, introduced in 1957. Then the KLH 9 a little later, the Magneplanar Tympani I in the early 70’s, Sound Labs and Acoustat in the 80’s, and numerous others. Why single out the Martin Logans?! Martin Logans claim to fame is the curved ESL (designed by Roger Sanders) and the ESL/dynamic woofer hybrid (or at least popularizing it).

As for baffle size and dipole cancellation, of course that cancellation cannot be completely eliminated by increasing the front-to-back driver distance alone (that’s what Infinite Baffle is for ;-), but you can lower the frequency at which cancellation commences somewhat by doing that. What Siegfried Linkwitz and Danny Richie do is use an H-frame to create the front-to-back driver separation instead of a flat baffle, as the frame provides more resonant-free sound than does a "normal" flat baffle (an un-normal one is the 2-1/4" thick one made by Ric Schultz of EVS, mentioned above) . The depth of the H-frame is limited by the "cavity" resonance inherent in each of the frames enclosed spaces (the deeper the frame, the higher the frequency at which cavity resonance becomes audible). Both Siegfried and Danny limit that depth to around 14".

To further minimize the dipole cancellation inherent in dipoles, both Siegfried and Richie incorporate the 6dB/octave shelving circuit I mentioned above (commencing at around 100Hz, I believe). Siegfried does it in his digital filters, Danny had Brian Ding of Rythmik put it in the A370 plate amp that comes with the OB Sub kit---analog, of course.

The reason full-range planars are so big is for the reason you state---their bass output is restricted by the relatively-limited (compared to dynamic woofers) ability to move air: the thin Mylar film only moves back-and-forth a tiny distance. OB/Dipole dynamic woofers are also less able to move as much air as a sealed or vented sub, so more of them need to be used. The Linkwitz and basic GR Research use a pair of woofers (the former 10", the latter 12"), but GRR also offers a 3-woofer version. Plus, you can make multiple H-frames and stack them. What Danny does at Hi-Fi shows (and in his own listening room) is use OBs at the front of the room, and a pair of sealed subs at the rear. That set-up won GR Research "Best Bass At The Show" award several years running at RMAF (as reported in show reports in TAS and elsewhere).

Clayton Shaw at Spatial Audio and most DIY’ers go a different way: using a pair of 15" drivers on a big flat baffle. When you do that, the midrange driver (if any) and/or tweeter will also be on that baffle. For better or worse: Danny Richie is particularly adamant that it is for the worse (though he holds Clayton and his speakers in high regard).

As with everything in audio, there are trade-offs made in all loudspeaker designs. You have to pick your poison, find the loudspeaker at your price-point which gives you the most of what you are looking for. OB’s are one choice, planars another (ESL, magnetic-planar, or ribbon), horns, and of course normal dynamic (cone and dome). Infinite baffle are REALLY out-of-fashion, though they may come back in when kenjit introduces his version ;-) .

Oh, and by the way:

The Magnepan "Concept" loudspeaker---the 30.7 For Condos---is basically not that different from Danny’s NEO Line Source/OB Sub design.

The Magnepan Concept has a smaller version of the 30.7’s midrange/tweeter panel, with an OB/Dipole woofer assembly in place of the 30.7’s large woofer/bass panel. That OB woofer has multiple small-diameter woofers (6.5", I believe)---six per side, iirc. And dipole-cancellation compensation, not unlike the GR Research/Rythmik.

Reportedly to sell for less than the 30.7, if it is ever put into production. For now, a pair of the GRR/Rythmik OB Subs, with a planar of your choice (ESL, magnetic-planar, ribbon), or an OB loudspeaker (one of the GR Research kits if you’re adventurous and confident, a Spatial if you’re not) will get you very close.

If size matters, the ET LFT-8b is only 13" wide and 5’ tall, much smaller than the Magnepan 3.7i. And it’s LFT magnetic-planar drivers are push-pull designs, with magnets on both sides of the Mylar diaphram for ESL-level distortion, unlike the single-ended 3.7i midrange driver. But the 3.7i does have a nice ribbon tweeter. The MG1.7i, only $300 cheaper than the LFT-8b, suffers pretty badly in comparison to the LFT-8b. I did that comparison.

For those not bored enough already ;-) :

Danny Richie was offering a very high-performance full-range OB loudspeaker when Bohlender Graebener was still making their fantastic NEO3, 8, and 10 magnetic-planar drivers. He used the Model 3 and 8 in a line source configuration, with a separate H-frame for the OB/Dipole Sub. He also did consulting work for a couple of companies who offered factory-built versions of a very similar design. I don't recall their names, but they were reported on in various show reports.

Well-known modifier Ric Schultz of EVS (Electronic Visionary Systems) took Danny's design and came up with his own variant: the same drivers, but with both the 12" servo-feedback woofers and the NEO drivers on a flat baffle with a base, comprised of three layers of MDF with Green Glue between the layers. Danny and Ric exchanged some lively comments vis-a-vis their two designs on the GR Research AudioCircle Forum.  

@jaytor, there are a few factors which pretty much answer both the major and minor questions in your post. Because I find this subject so interesting, I shall endeavor to remember to get to them all while also getting "into the weeds", as they say. Geez, I hope that doesn't sound as pretentious to ya'll as it does to me ;-) .

But first: I can't believe this needs to be said, but it apparently does. Martin Logan has not for years been making what we call open baffle loudspeakers. Not for years, nor in fact at all. Martin Logan's electrostatic loudspeakers are not open baffle loudspeakers, they are dipole planars. So are all the other "full range" ESL's (as opposed to ESL tweeters, like the RTR's in my pair of ESS TranStatics) that are being made, or ever have been. So are the magnetic-planar loudspeakers of Magnepan and Eminent Technology, and all the full range ribbon loudspeakers, past and present.

An open baffle loudspeaker is specifically defined as a (usually) dynamic driver (or drivers) mounted on a baffle, pure and simple. No sealed or ported enclosure, the driver(s) open both front and back to the enclosed room in which they reside. To be sure, some OB designs have included a dynamic woofer and either a magnetic-planar tweeter (as in some GR Research models), or a ribbon one. The point is, the drivers are not loaded by an enclosure (with technical implications. See below.)

OB's have a long history (my first loudspeakers were OB, purchased in 1968), but have remained largely unknown to most audiophiles due to a very simple and important reason: they have been almost exclusively a DIY product, not a plug-and-play one. There is a very active OB loudspeaker "underground" community, one whose members include guys like Nelson Pass. OB enthusiasts are in one way just like the original 1950's hi-fi amplifier designers, who started making their amps on the kitchen table.

Because OB's were built, not bought, there were never any OB loudspeaker dealers, no magazine reviews, no advertising, no nuthin'. And because there was no market for them, they have for years remained largely a DIY underground phenomenon. How many guys do you know who would even consider building a loudspeaker? Kenjit would, but he can't decide which of his designs is most perfect ;-) .

The word about OB's took a giant leap forward when the design genius known as Siegfried Linkwitz (a neighbor and close friend of Nelson Pass) started publishing his papers on OB design, and introduced a number of OB loudspeaker kits. The ultimate realization of his OB design was the model mentioned by another poster above---the LX521, and is very highly regarded amongst OB enthusiasts.

Danny Richie was another long-time OB enthusiast and designer, and in the 1990's started a company (GR Research) catering to the DIY loudspeaker community. His loudspeaker products were all kits, but not all OB designs. He offered sealed designs as well, of both loudspeakers and subwoofers. While admiring Linkwitzes OB designs in most regards, it was not without reservation. Danny is that perfect combination of knowledgeable designer and critical listener (as was Roger Modjeski), and in his products address every failing he heard in the work of Linkwitz.

For instance: Danny is a fanatic about resonance, and he heard a problem in the OB sub section of the Linkwitz 521. He found the 3/4" side panels would flex when confronted with deep bass, producing a sound he describes as "buzzing". His solution? Double the sub side panels to 1.5" and line them with Norez, a resonance damper he designed and has manufactured for him. He also greatly disapproves of the digital x/o inherent in the 521, but that's a subject for another time.

Danny hears resonances in just about all commercial designs, and for his sealed sub designed a "double-box" enclosure design: an inner box and an outer one, with a layer of sand between them. Whatta nut! Danny will tell you what's "wrong" with most OB's (though he expresses his approval of the designs of Clayton at Spatial. They are friends.), and why he does things his way. For instance: he finds that an OB with a large baffle (needed to lower the frequency as which the front and rear waves meet and cancel, commonly known as dipole cancellation. That frequency is distance related: the greater the distance between the front and rear of a driver, the lower the frequency at which cancellation occurs) draws attention to the baffle, preventing the creation of a deep layered sound field. So, to create the distance required between the front and rear waves, in place of a large, flat baffle, Danny uses a baffle only just large enough for the driver, and creates the needed front-to-back distance with side "'wings". To see it in pictures, watch any number of his GR Research YouTube videos.

But OB design is much more that just the baffle. A driver used in an OB application needs to posses different technical specifications and performance characteristics than does a driver used in a sealed or ported design. Danny designs drivers for both applications, and has them manufactured for him in India. He offers his 12" woofer in both OB and sealed/ported versions, each optimized for their application. His 12" woofer is identical in most aspects to the 12" woofer Rythmik uses in their F12 sub, but with a paper rather than aluminum cone. Danny prefers the lower mass and timbral tone characteristics of a paper cone, Rythmik's Brian Ding prefers the aluminum's great stiffness.

It's been a coupla years since I spoke on behalf of the remarkable OB/Dipole Servo-Feedback Sub co-created by Danny and Brian, so I hope no one minds if I do it again now ;-) . I am not alone in considering the bass produced by the Magneplanar Tympani bass panels my standard in bass reproduction, even if their maximum output is limited (Harry Pearson agreed with me). The GR Research/Rythmik OB Sub is the only one since that could compete with the Tympani. There is one fanatic who has an Eminent technology TRW-17 Rotary Woofer for deep bass (40Hz and below, flat to 1Hz at 120dB), a pair of Tympani's for bass, and a pair of Martin Logan ESL mains. Yeah, baby!

Anyway, Danny was already offering an OB sub, and when he discovered Brian Ding's new Rythmik servo-feedback woofer had an epiphany; the mating of OB and servo-feedback, to create a new standard in bass reproduction. Both men are located in Texas, and ended up putting their heads together in the project. Brian Ding says he finds the OB Sub to be too "lean", missing the weight and heft of sealed and ported subs. Danny disagrees! All I know is that the OB/Dipole Sub reproduces the sinewy timbre of an upright bass, the fat punch of my 24" Gretsch bass drum, and the lower registers of a grand piano better than anything other that a Tympani.

GR Research provides plans for both an H-frame OB structure into which the dual or triple woofers are installed, and an W-frame design (two woofers only). Danny has a woodworker making the H-frame as a flat pack kit, very simple to assemble: just wood glue and a coupla clamps required. Fantastic frame, side panels and top and bottom 1.5 inches thick, baffle 1" thick. Makes the Linkwitz look like a 90lb. weakling!

The Rythmik plate amp included in the Sub kit includes a 6dB/octave dipole cancellation compensation shelving network, which counteracts the roll off endemic to dipoles. Not just great bass, but plenty of it. It is the only sub I consider adequate to mate with planar loudspeakers. I use them with both ET LFT-8b and LFT-4 magnetic-planars, and QUAD ESL's.

What I haven't heard are the OB loudspeakers GR Research offers. Jaytor, what beer shall I bring? ;-)