What is your position on sub driver size versus speed?
There was a post here by Uncle Eric that basically said it was a non-issue (that post is mysteriously deleted).
IOW do you feel that smaller are faster & larger are deeper thus 2 are needed?
If not why is there is an 18" and a 12" in the Signature 1812.
This is a hotly debated topic but of course, I do have an opinion. In short, I agree with Uncle Eric, but let me explain in my own (numerous) words.
As I see it, the implication that folks make when they say that a 15" or 18" driver is "slower" than a smaller 10" or 12" driver is either that the mass of the driver is such that the motor cannot move its weight with the same precision as that of the smaller driver and/or the cone is flexing enough so that the woofer sounds boomy or muddy. The latter effect will have more impact on the perceived boominess and muddiness of the driver (in that it will produce more distortion) so let's address that first. This of course relates to cone stiffness, and a larger cone made of the same material will bend more and, in the worst case, buckle under the load of long throw bass combined with cabinet pressure. This is overcome with stiffer materials and/or cone reinforcement, and while it is true that a larger cone is subject to less linearity along the cone surface, this effect is minimized with a stiff enough cone. Too often, large drivers are made out of the same materials as smaller drivers and so aren't stiff enough - this is where this perception likely started.
Regarding the driver being heavy to be moved correctly, there are three main forces opposing the motion of the cone structure: the weight of the cone, the pressure inside the box, and the resistance of the suspension (i.e. the surround and spider). It's true that the bigger the cone the more mass there is, but you'd be surprised how much the air pressure in the box and the suspension stiffness affect cone movement too.
All of these factors can be overcome by motor force, otherwise known in speaker land as BL. BL is the combination of magnet force combined with the length of voice coil wire in the gap. The higher the BL, the more motor force is applied to the cone. As a speaker designer, one has to ensure that there is enough BL to overcome the cone mass, suspension stiffness, and box design you are planning on. Not enough BL and your cone doesn't move correctly, and too much and you spent too much on the driver. Of course, one also needs sufficient amp power to drive the motor. We also use a high gain servo to make sure the cone movement is true to the input signal.
An often overlooked spec in comparing speakers is magnet weight. We use 310 ounce (19.3 pound) magnets for the DD-10 and DD-12 and 380 ounce (24 pound) magnets for the DD-15 ad DD-18. Why so big? Because we never want to run short of motor force and we need more BL for the 15 and 18.
Now let's talk efficiency. SPL= sound pressure level = amount of air moved by the speaker cone. Given similar travel distances (all DDs go up to 1.75" peak to peak), the formula is 2 pi r (i.e. cone area) * travel. Some simple math shows that a 15" cone moves a heck of a lot more air than a 10 or a 12. And an 18 more yet. Yet, the additional motor force to move the larger cones is disproportional to how much more air they move - so the larger the cone the more air is moved for less energy.
Lastly, let me comment on the phrases "slow" versus "fast". To us, these terms almost entirely describe the amount of perceived distortion the speaker is producing. That idea is somewhat radical, but we've based our designs on it for almost 20 years with a fair bit of success. We've strived for very low distortion in all our subs, and the DD-15s and DD-18s have no more than the DD-10s and DD-12s. So, we consider them as "fast" as anything going.
Re the 1812 - what we did there is let the 18 handle the lowest bass, since it's efficiency lets the long excursions needed for 40 Hz and below be reproduced with optimal use of BL and amp power. The 12's job is to handle the upper bass frequencies and not spend its headroom trying to recreate 20 Hz. The result is a sub that will play louder than either alone, or even in tandem, because we've optimized the efficiencies of both. This does not mean that a stand-alone 18 cannot play 100 Hz or that a stand alone 12 cannot play 20 Hz - but combined (and crossed over internally with special software) we can squeeze out more clean bass since the efficiency of each driver is optimized.
Sorry for the long post!