Are REL the most Musical Subs?

The Rythmik servo-feedback system also compensates for the increase in voice coil temperature, which all woofers suffer from, regardless of their excursion capabilities.

Rythmik offers sealed subs with a range of woofer diameters: 8", 12", 15", and 18", and dual opposed/double 15". Rythmik designer/owner Brian Ding states all his subs are equally "fast". Fast is more related to woofer "settling time" (returning to "zero" when the signal stops) than anything else. Of course a poorly-braced enclosure (Tekton anyone?) can make a sub (or speaker) sound "slow". But the major cause of bass overhang is room modes.

@mijostyn: Oh yeah, the Eminent Technology TRW-17 is unique all right. It doesn’t propagate sound waves with a woofer at all!

The dipole cancellation compensation circuit incorporated into the plate amp of the Rythmik Audio/GR Research OB/Dipole Sub does not "compensate for serious flaws". Dipole cancellation is not a flaw, but rather an inherent characteristic of dipole subs and loudspeakers. Arnie Nudell incorporated large "wings" on either side of the midrange/tweeter drivers in the Infinity IRS to prevent dipole cancellation. Siegfried Linkwitz and Danny Richie use either "M" or "H" frames in their OB/Dipole subs, for the same reason. The Rythmik dipole circuit progressively boosts the signal as frequency drops, resulting in flat response to 20Hz.

I’ll say it again: The only people who don’t know how good the Rythmik Audio/GR Research OB/Dipole Subwoofer sounds are those who have not heard it. I’ve owned servo-feedback subs before (in the Infinity RS-1b), but the OB/Dipole Sub is in a completely different class. It sounds very different from my Rythmik F15HP pair, and actually sounds more like the bass panels of my Magneplanar Tympani T-IVa speakers than anything else.

@bdp24 wrote:

The Rythmik servo-feedback system also compensates for the increase in voice coil temperature, which all woofers suffer from, regardless of their excursion capabilities.

How would it do so - other than acting as a limiter?

This debate primarily has your usual, lower efficiency and sealed sub design in mind that is built around a small, restricted size factor. As such heat dissipation from voice coils will be an issue eventually, but thermal limitations are not inherent to a wider design range of low frequency augmentation options that have higher efficiency as a core trait (of course with added overall size as a necessary implication) - certainly not in the context of domestic use.

The Rythmik dipole circuit progressively boosts the signal as frequency drops, resulting in flat response to 20Hz.

But this comes at the cost of eating away of the headroom that might (or might not) have been available initially, only exacerbating the issues that come from lower efficiency and what follows here both thermally and mechanically. If you multiply the number of such subs used to appropriately accommodate the clean SPL envelope that’s required, you would be able to at least partially alleviate this issue while also opting for a DBA approach, albeit at a higher cost.

On the other hand, when you have a pair of corner mounted, pro B&C 15"-loaded 20cf. per cab tapped horn subs with 97dB sensitivity (+ boundary gain) that deliver air-shaking +105dB levels at the listening position down to 20-25Hz, all the while exciting the woofer cones to only vibrate a few mm’s, you know you have actual and usable headroom in abundance and a wholly effortless reproduction at any desirable SPL. There’s no way to cheat around that other than blunt, core physics and letting size have its say, and don’t tell me audiophiles don’t need that kind of effortless bass delivery; if they heard it they’d most likely crave it, if it wasn’t for the size of such things.

I’ll say it again: The only people who don’t know how good the Rythmik Audio/GR Research OB/Dipole Subwoofer sounds are those who have not heard it.

The same could be said of those who haven’t heard properly designed and constructed high efficiency DIY sub designs (i.e.: not restricted to one or a few brands), sans all of your EQ-boosting, servo feedback, dynamic limiting, ultra high power demand and other electronic-digital trickery that will not be needed here (other than a quality DSP that will act as an elaborate crossover device). If anything it’s the purist subwoofer approach, and it could be even more so with the use of outboard quality amps instead of cheap plate dittos, etc.

I thought the OP was interested in a good option that’s available to buy, not a DIY approach thats out of reach to most everyone? It's interesting to read about the possibilities with DIY though, but to imply any other way is lacking enough not to be worth it is wrong IME.

For those who want the OB/Dipole Sub from Rythmik Audio/GR Research but don’t have the tools, experience, and/or interest in building the required OB frame into which the woofers are installed, GR Research has a couple of woodworkers who will make the frame for you. One of them also offers the frame in kit form, pre-cut and ready to assemble, only wood glue and clamps required. And either paint or wood veneer if you want the MDF finished.

@phusis: On the Rythmik Audio website designer Brian Ding provides exhaustive technical information on his subs. I believe details on the servo-feedback compensation for voice coil temperature variation and other functions may be found in the site's Technology section. Mr. Ding is not your average hi-fi designer, having a Ph.D. in electrical engineering.