They're only 89db sensitive, so they will move a lot of air and they'll play deep, but not loud. At all. They also reach their xmax really fast due to low (15) BL.

More importantly, they have 20.14 BL and 95dB sensitivity, and double the RMS power handling of the Lab 12. So they can take a lot more power AND they make more noise with that power.

You can tune a pair of Lab 12's to 29Hz in a 150 litre box and they'll be flat to about 33Hz, but they reach 13mm xmax at 675w input, or about 119dB SPL. 125dB max for two double 12 enclosures. So equal to a pretty entry level 1x18, but with a lot more air movement.

What's worse is you can hardly even EQ the 30Hz up, because of a broad excursion peak running from around 25 to 50Hz, centered around 38Hz.

You could load them to a larger box and/or tune them lower, but then the xmax runs out even faster., and that excursion centre point will drop even closer to 30Hz.

By contrast, 2x BMS 12s305 tuned to 39Hz in the same box is flat to 38Hz, with an SPL of 128dB for one double vented enclosure.

If 30Hz is a priority, you can still get 123dB at 30Hz by tuning the BMS box to 32Hz, but in this design there's an excursion trough centered on 32Hz, so with 3500w of amp power, careful limiting and careful application of EQ, you can achieve 127dB at 33.5Hz. About 10dB louder than the dual Lab 12.

Edit: Subjectively I'm sure they'll sound amazing, and they'll sound huge. That massive double stacked magnet, the low Fs and accordingly the low centered excursion peak will give an incomparable reproduction of those lower registers, and if you're playing indoors you're unlikely to ever find their output wanting, with enough boxes in small to medium events, so when I say 'not loud at all', I really mean relative to a 218 or other similar enclosures. Loudness is definitely not the strong point of the box. If you tried to use these for outdoor events you'd run into issues, guaranteed.

The option I didn't consider: tune it higher and make the box bigger (180 litres, 35.6Hz). Then it can take more power. 126dB peak at 40Hz so a pair could reach 132dB. It's a fair comparison with a 218 sub, so long as you cross it to kicks or midtops as low as possible. (less than 60Hz ideally)

The plot just looks ugly in WinISD (5dB peak at 40Hz), but that's a pretty good place for a peak in a sub. You still can't get much true 30Hz in a compact box with these drivers, but you can get a good amount of 33-38Hz if you EQ the box. And if you're lucky enough to excite a room mode in that region then you'll definitely experience strong 30Hz.

I stand by the recommendation of the BMS, but so long as you're giving the drivers enough space it's not as much louder as I suggested. But it is 2-5dB louder, and plays truly to 30Hz without EQ.

Where the BMS gets really interesting is when we look at it's performance in a 115 litre box (57.5 litres per driver) tuned to 35.5Hz (with grey line dual Lab 12 in the same box)

Again with the BMS, if we're willing/able to use high power loading and careful EQ/peak limiting, it can approach 126, or even 127dB at 34Hz in that tiny box.  (plots pictured are taken at approximately the excursion limits of each pair of drivers, no EQ applied)

Given the relatively cheap price of the drivers and their high power capacity, you could load 4 of them into a 230 litre reflex box (100 x 50 x 65cm external?), chuck a Behringer iNuke 6000DSP at it.. Pretty heavy but still a lot smaller and lighter than the 218 that seems to be the benchmark here. 133dB at 34Hz for the price of a single 21" driver, and it would move as much air as 2x 21" drivers with 9mm excursion as well.

Apparently acceptable (or even detectable) distortion in peaks at 35Hz for most people is like 25-30% THD+N anyway. Certainly the lower the frequencies, the harder we find it to detect distortion.

My comment about use outdoors was based on my first ideas about their output, but if you're not able to supply enough amp power it may still be an issue. It'll be interesting to hear how they do.

Ports are large and long enough to keep vent velocity down to 40m/s even at 6000w input. You can get it down to 30 but you've then got so much port you'd need to add another 25 litres of volume, and virtually the entire baffle would be drivers and ports, but it is possible (and would allow for theoretical extension of the bass with EQ up to 10000w input with 40m/s vent velocity)

So it's about the size and weight of a single horn loaded enclosure. Very power hungry (only 96dB/watt despite 4 drivers!) but an iNuke NU6000 for 369 Euros can give it all it needs to reach 134dB at 40Hz, 132dB at 34Hz (it goes 3.4mm over xmax at 48.5Hz, and 3.4 again at 31Hz with that input level).

Given the extents people go to get that sort of level of output in the low 30's with stacks of horns I'd say it's a fairly economical and straightforwards option. It's the same size as the X1 bandpass subwoofer with a similar frequency response, but output is greater and obviously a lot more hifi. The drivers are categorised as 'bass midrange' so it's a one box solution. You could run it to 250Hz even.

All in you'd be looking at something like 1200 Euros a built, loaded and powered box, or you could build it in very compact halves for ~600 each. (Assuming you already have an LMS - limiter not optional! A bit extra for DSP in the amp.) Which on the market gets you something like this https://www.andertons.co.uk/p/TS218/subwoofers/alto-ts218-active-18-sub -10dB at 35Hz 

At the higher end of PA 218 subs though you've got stuff like this for ~3500 Euros. http://www.fbt.it/en/products/mitus/mitus-218sa

I can't understand how on Earth they arrive at their 145dB half space claim though. Amp power is listed as 2000w RMS 4000w peak. Even if the true peak is 8000w and the system is 102dB/watt, that's 'only' 141dB calculated.

From the spec sheet: http://www.fbt.it/documents/31106/394149/Mitus+218SA/cfa2476b-670b-4ca0-863b-cb2c13c03885

The amp can actually only provide 2000w for 2ms bursts.. it lists a 'max peak' rating of 4000w, but that must be somewhere in the microseconds so it's totally worthless. Published response chart has a peak of 103dB sensitivity, but at 35Hz it's only 95dB/watt.

So actually 136dB peak SPL, 128dB at 35Hz, vs 135dB peak SPL, 132.5dB at 35Hz with the 4x12

So it's a very close match, for three times the price, with a 15% bigger box, 12% more weight, and considerably less low frequency extension.

Impedance is the other thing - it's a 6ohm nominal driver. Did you possibly wire it as if it was 8ohms?

If you can get it, the BMS 12s305 wins hands down in this enclosure, even as a simple drop-in replacement with no redesign of the box (as Art Welter confirms). It solves the issue of falling response, has a standard 8ohm rating, weighs 2kg less, extends further, more efficient, etc. Not many clear downsides. In the UK the Lab 12 is very expensive also. 218 pounds vs 149 for the BMS.

(actually to match the response of the original box as closely as possible the ports should be tuned a little higher to 37.3Hhz, then the lower bass plots overlap almost exactly for the same watt input, so efficiency is actually the same except when you get to the upper bass (then it's up 3dB at 110Hz).

Your amp almost certainly shouldn't run 2ohms, it's probably a case of 'technically can but probably should't, especially not on sub frequencies' - the engineering department and the marketing department are often in disagreement on ohm ratings! So if you wired them at 3ohms, but it was actually presenting something between 2-2.5 ohms in subs played right at the limits of the amp, that would do it.

So for example, in your passband, the BMS rated 8ohms has impedances of:

66.5 ohms (peak) at 58Hz

23.5 at 70Hz

8.1 (trough) at 36Hz

21.5 ohms at 28Hz.

vs the Lab 

29 ohms (peak) at 46.5Hz

6.1 ohms at 70Hz

5.6 ohms (trough) at 33.5Hz 

7 ohms at 28Hz

So by these points at least, if the BMS is 8ohms then the Lab is more like 4ohms, and in many areas (28Hz and 70Hz ie) the impedance for the Lab is incredibly low - considerably less than half of the BMS.

Edit: Those drivers will take a LOT of excursion. The most likely cause of death would be amp power over their rating. Peaks will be less of a problem than sustained RMS tones, so with high power you'd absolutely need an RMS limiter.

I think excursion is something like 25-30mm each way, 50-60mm peak to peak on the Labs, before damage. If you forget to high pass, that might do it but you'd still need to get very unlucky with some unusual music that decides to blast some 20-25Hz tones at maximum amplitude. Even with 1500w input to the box, the excursion of the Lab at 25Hz is 25mm.

Very unscientific but tells you something. I guess. It was pink noise sub only. The amp QSC pl4 and processor BSS318 

When I made these I quickly realized I didn't have an amp that would do them justice.

So I took the drivers out and stored them. I then came across 4 amcron 3600 going cheap. Then every time I walked past the Arts I wondered. That went on for a few years.

So, a few months ago I reinstated them. 

Very hard to describe the sound. They just rumble in a nice way Not Woofy growling at you. I love them. Quite unique I think.

When I was testing, my workshop is 50mtrs away from the kitchen. My wife's friend

Asked if that was an earthquake. Wife "no just Dom and his speakers." They rumble.  

Cracking job Dom. 

Shame we lost Hemisphere along the way. This thread shows just how generous he was with his time and knowledge. 

Jon