Yet another big sub gut-check (long)

[DS-21]

Apologies in advance for the wordiness, but I'm considering building/commissioning a new sub for my home system. After jumping from the explosions in "Munich" a friend made an unsolicited offer on my current sub - the first commercial sub I've owned since my early teens, a Tannoy B475 - that's hard to refuse. The driver I will use, primarily because I bought it on a lark and it has sat in a closet since then, is one of the 15" SVC TC2+ that Oaudio cleared out last year. Because the woofer has a pretty good bandwidth, I think I might further lower the excursion demands on my mains (8" Tannoy dual concentrics, System 8 NFM II) by building the sub as a TV stand (30" LCD) and using it up to 150Hz with a 4th order LP thereafter. Whatever design I end up with, I will EQ it using a Velodyne SMS-1 and power it with an NHT A1 Class G monobloc. I know the SMS-1 is in relative disfavor here due to being overpriced and less powerful than the BFD, but after playing with a friend's SMS-1 I swapped my BFD for one because it looks so much better in my equipment rack and it makes tuning so darned fast. (Another plus of the SMS-1 is that my Harmony remote also controls it.) Also, the A1 may be modestly powered (200W/8ohms, 300W/4ohms) compared to pro amps, but it runs cool, looks nice, and turns on automatically when it senses a signal. IOW, I will buy nothing for this sub except for the wood/cabinet, a 6" Aeroport/PSP flare, and a Speakon terminal. (I prefer Speakons to binding posts.) Everything else is on hand.

I've built at least 25 home subwoofers for myself and others over the past dozen years or so, but one common element in every sub I've built is that it was not designed for use above a 60-80Hz fairly steep lowpass. So I have some questions about this design. (Also, oddly enough there isn't much in the way of discussion on the Oaudio 15, perhaps because it was only out for a short time compared to the 12, so might as well have a thread in the archives about it...)

Ordinarily, the box that looks like the best compromise between output, size, and extension to me would be something in the 180L range tuned to about 18Hz. As you can see from the graph below, that would yield an F3 in the low-to-mid-20's and with the 300W amp I'm using will provide a theoretical anechoic 108.5dB at 20Hz while staying well within xmax until about 16Hz. Even considering likely power compression losses, that's still more 20Hz output than everything tested in the Way Down Deep articles save the Contrabass and the Genelec! My one issue is that 3dB wiggle starting at 220Hz in the ported model. I suspect, with a 4th order LP at 150Hz, that it may still be audible. (I don't want a downfiring port, because I don't want to lift the enclosure above the ground.) I'm not sure what the flare will do vis a vis port resonance, if anything, so if somebody could point me in the direction of something to read on that topic I'd appreciate it.

I also modeled this woofer in a PR enclosure. I used 8 Peerless XLS12-PR's simply because TC Sounds does not provide adequate data on their 15" and 18" PR's, and I know of no other current source (with Janowitz and Lamdba out of the picture) for very large high-excursion PR's. Doesn't look very promising, to be honest, especially considering that when TC gets theirs back on track the PR will cost more than the woofer did!

A sealed enclosure was also modeled. The same 180L enclosure, stuffed, yields almost a textbook Bessel alignment, with a Qtc of 0.572. Compared to the same sized vented box, the sealed box is ~4dB less efficient at 20Hz. Also, below 25Hz run with 300W it exceeds linear xmax, so let's drop one more dB from its potential at 20Hz. IOW, two Peerless XLS12's + XLS12 passives in ~80L of airspace would pretty handlily outmuscle the sealed Oaudio 15 at 20Hz despite taking up only half the volume. But the sealed box does not have the port resonance issue, and I love the sound of low-Qtc sealed boxes. Still, especially since I won't be corner-loading it's hard to look at the models and not practically see a whole second woofer's worth of LF output for free in the ported box compared to the sealed. I'm very open to being convinced that low-Qtc sealed is the way to go, though. No LT's simply because I don't believe that the SMS-1 will allow me to do it without using all of its filters in that process and because at this point I'm not interested in buying a more powerful amp. Also, only once (with JBL's amazing W15GTi "car" woofer) have I found an LT to sound as good as a larger box with innately lower Qtc and just EQ to bring up the low end a bit.

I mentioned plots above. Here's a Unibox saved graph showing most of them.



(For full-sized version, click below)


Included for reference (and because otherwise Unibox would give me ugly dashes above the graph) are plots for the previous subwoofers in this room, twin JBL 2235H's loaded in 160L reflex enclosures and fed 200W each, and the sub I use in the nearfield in my home office, which is a (by DIY standards) fairly common design, an XLS12+PR based on the Peerless application note. I no longer have the cabinets made for the JBLs, though I'm saving the 2235H's for a future Tannoy Kingdom-inspired speaker that will use the 12" Duals currently in my home office system with the 2235H's for midbass grunt, and a separate supersub for bass. Also, for fun, a possible future driver to find a home in the cabinet I'm going to commission for the TC2+, B&C's forthcoming 21" driver is modeled in the same cabinet, albeit with a lot more power.

Lastly, as far as aesthetics, my living room is fairly midcentury modernist. My current TV stand is an Ikea Middus table in silver and white, with the expansion leaf left off. I was considering painting the new enclosure white (unless Corian turns out to be financially viable; then it will be naturally white, but presumably more matte in finish) and sizing it such that it the Muddus steel legs (they are a rectangular frame of welded square box-section tubing, ~18.875x29.125) would be kind of like cheekpanels, and give a nice lip to grab onto when I move the sub. I could even go a bit art deco-y and build the white middle section a few inches taller than the frame sections. Any reason you can think of not to use those pieces on the sides of this enclosure?

[cjd]

So, it looks like you're selling yourself short on box size. In that case I'd be more inclined to suggest you go sealed and dial in a bit of EQ to bring up the bottom end. Sound quality should be better that way, perhaps a fair bit.

Crossing as high as 150Hz will, of course, be localizable, which really suggests running a single sub that high isn't such a great idea unless it's centered between your mains in which case you might get away with it.

Otherwise, as long as the box itself is solidly constructed and well braced, I don't see anything further to add.

C

[Paul H]

29 views and no response - I imagine a few people fell asleep part way through the somewhat lengthy write-up :twisted:

You've opened up much for discussion, but I have a hard time getting past a 150Hz crossover for a single sub. I find a sub very easy to localize with an 80Hz crossover - never mind an octave higher which would clearly identify and differentiate the sub from the bass sections in the two speakers. I'd use a lower crossover.

You appear to have a strong preference for sealed, but noted the spl that sealed gives up to ported. You didn't mention your room size or typical listening levels. If the sealed sub provides enough output it's an easy decision...

Have fun,

Edit: Chris wrote in while I was typing - great minds and fools ...

[DS-21]

So, it looks like you're selling yourself short on box size.

A larger box just won't work in the space. After all, it's going to essentially be a stand for a relatively small and thin LCD panel. Which, admittedly, is a little bit of a pity, because this woofer in 10 cubic feet (283L) can give me a screaming 111.1dB at 20Hz (19Hz F3!) without exceeding xmax (300W drive) until ~16Hz, according to Unibox. It's so much more fun to use an EQ to cut excess bass than to need one to lift the low end up...

Crossing as high as 150Hz will, of course, be localizable, which really suggests running a single sub that high isn't such a great idea unless it's centered between your mains in which case you might get away with it.

Doh! All of those words and I forgot to mention placement relative to my speakers! For reference, the sub will be directly under my center channel, which is centered between the mains. Vertically, the center of the Dual Concentric on all three mains is a little over 5' above the ground*, so I imagine there will be a 3' or slightly more vertical separation between sub and LCR.

I'm not wedded to the higher crossover, mind, but I am going to try it out. If I find localizability to be an issue, or do not perceive any treble improvement from the lowered midwoofer excursion on the front dual concentrics. Regardless, a higher sub LP is just a band-aid, but if it works for now then all the better.

As for listening levels, the irony is that despite my general preference for pro gear over "home" stuff is that I rarely listen that loudly. I do occasionally like to listen to the more "heavy metal" Romantic orchestral stuff (Mahler, Bruckner, Shostakovich, Sibelius, etc.) at 7th-row-center-in-the-Musikverein volume and not have the people around me know how loud it is until they try to hold a conversation, though.

Room size is trickier. I'm not there at the moment, but I'm going to guess that it's about 21 x 15 x 8 or 9, opening up (double-width "doorway" sans door) into a kitchen of dimensions I couldn't ballpark off the top of my head. The center of my couch is just under 9' from the TV and thus the center channel and proposed sub. The room is carpeted except for a small landing in front of the fireplace, with well-stocked bookshelves placed behind the couch and roughly at the points of first reflection on the sidewalls but no dedicated acoustic treatments.

*That placement works well for me both when sitting and when standing, because of the on-axis null endemic to any coincident/concentric driver) there will be about. Because of the even power response, stage height is not an issue.

[DS-21]

ut I have a hard time getting past a 150Hz crossover for a single sub. I find a sub very easy to localize with an 80Hz crossover

Seriously? Except in one huge, huge room I've never had a need to go below 80Hz even with a sub that's right behind the listening position and mains 12' away. IOW, I've found that if levels and phase well-integrated, everything that could rattle is far away from the sub, and the mains have enough volume displacement to handle peaks in the half-octave below (60-80Hz) without much in the way of dynamic compression a sub crossover at 80Hz of at least 2nd order is basically impossible to localize.

You appear to have a strong preference for sealed, but noted the spl that sealed gives up to ported.

I have a strong theoretical appreciation of low-Q sealed boxes. But actually I haven't done a sealed sub for myself in this decade. My previous seven home subs were all vented designs. Usually more of an EBS (I despise that newfangled "LLT" designation when we already have a perfectly good TLA for the concept) than this enclosure, though neither of my last two bass subsystems (the JBL 2235H-based design described in a link above and the commercial Tannoy unit) fit that rubric. I did just help a friend design and build a compact sealed/LT'ed sub that sounded phenomenal, but I suspect the woofer he used (a JBL W15GTi "car" woofer, with their "Differential Drive" tandem vc motor) is a better performer at all excursion levels than a TC2+.