@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 ;-) .
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 ;-) .