They don't. Multiple woofers just move more air, thus sound more powerful with the illusion of deeper bass.
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Assuming that the woofers are identical, they will have the same frequency response. However, when using two subs in the same room, it is possible to generate somewhat smoother low frequencies due to the effect of the room acoustics. A single subwoofer may excite resonant modes in the room which interfere with the acoustic output of the sub, thereby producing phase cancellation, or nulls, at certain frequencies. A pair of subs, however, will often acoustically load the room differently than a solo sub, so the effects of phase cancellation are reduced, thereby increasing the output.
There is another thread below, "Subwoofers and standing waves", that explores this topic as well.
The truth is that these drivers all have output down to, say, 22Hz, but a single driver can't provide enough usable output to rate the speaker + or- 3dB to 22Hz -perhaps only 28 Hz.
Add more drivers, you have more usable output at low frequencies, and the crossover and cabinet design integrate them into the total frequency spectrum smoothly
As Marty and John have explained, the extra drivers mean you can play those frequencies at louder volumes. Cabinet size is also a factor, and the taller cabinets have more internal volume.
I'd also be very suspicious of any speaker that claimed usable bass at 28Hz from a single 6.5-inch woofer. Barnyard epithets come to mind.
There are several reasons to use several small drivers rather than one large one. First, it allows you to build narrower cabinets, which the women-folk appreciate. (Two 6.5-inch drivers can probably move about the same amount of air as one 9-incher.)
Also, multiple drivers means drivers at different heights, with different floor-bounce arrival times. Floor bounce can cause suckouts somewhere in the midbass range. Multiple drivers will have different suckouts--so you get more of them, but none as deep as if you had a single driver.
Not necessarily. Multiple drivers ameliorate one problem, but there are other, and better, ways to deal with even that one. (One is to put the woofer near the floor; that way the suckout frequency falls well above the woofer's range.)
And how much that matters depends on the room you're in. In fact, given the room problem, there is no one design that's better than the others. Which is why it's a good idea to audition any speaker in your room before you buy (or lose your return privileges).
There are also some other reasons why a designer might use multiple smaller woofers rather than a single large one. If the woofers are to be front-mounted, the physically narrower drivers allow for a narrower baffle, which in turn permits theoretically wider dispersion and less diffraction from mid and high frequency drivers mounted on that same baffle. Smaller cones of lower mass but greater stiffness than larger cones allow easier motor control over acceleration/deacceleration. The smaller radiating diameters of these 'mini-woofers' allow them to be carried up higher in frequency before the crossover rolls them off without paying as much of a penalty in reduced dispersion (frequency beaming) as would be the case with a larger-diameter woofer - thereby letting the midrange driver roll in later with the attendent power-handling benefits of its not having to cope with some lower-down frequencies, yet still maintaining smooth power (polar) response through the lower midrange. And using multiple woofers of a single size and simply varying the number of such drivers employed in different models up and down a manufacturer's line increases production efficiency over needing to have many different driver sizes.
In all cases, whether with multiple or single woofers, the possible LF performance limits will have a lot to do with the total driven cone area, the total motor power applied to that driven cone area, and the total enclosed cabinet volume (as well as the particular cabinet design and tuning, and the performance attributes of the drivers used). So for your question, the larger speaker models with more of the identical woofers exhibit increases in all three of these parameters, and therefore with all other things being equal should be able to go deeper, louder, and with less distortion than the smaller models with fewer of the woofers.
(The same sorts of things hold true for midranges and tweeters too, even though the choice there is not between multiple smaller drivers and single larger ones, and excepting that of course LF extension and cabinet volume are not part of the critical equation as with the woofers, but multiple mids or tweets can afford higher headroom and lower distortion just the same. But with their relatively small diaphragm excursions, it is not generally necessary to increase these capacities in home speakers beyond that achievable with single mid/HF drivers, and most models featuring multiple mid/HF drivers do so more for reasons of shaping radiation patterns, such as in a D'Appolito mid/tweeter/mid array or a THX tweeter line.)
But as to whether multiple smaller woofers are somehow "better" than single larger woofers, I don't think you can make such a generalization. For one thing, there is no getting around the necessity for greater cabinet volume if really want to go low with authority (barring the use of compensatory active EQ as in ultra-compact subs, which increases distortion and lowers headroom), and the most efficient way to package this internal volume is not to restrict baffle width if you don't want to dictate a speaker that's very tall and deep. By the same token, one 12" woofer on that wider baffle makes a more space-efficient package than do the four 6" woofers that would be needed to total the equivalent cone area. And for what it's worth - and for reasons that I'm not sure I fully understand (maybe others can elaborate) - all the speakers that I've ever heard (or are reputed) to display truly whomping deep bass unfailingly have physically large woofers (8"-10" min., more likely 11"-15", whether singly or multiple), not a larger number of smaller ones.
There are several factors at work here. Bigger cabinet size results in deeper bass. Something else to consider is that the electrical characteristics of the driver change as more power is applied. By sharing the load with multiple drivers, less heat is generated in the motor and the electrical characteristics ( and therefore the tuning ) remain more consistent over a wider range. On top of this, bass extension is rolled off as the driver has to make longer excursions. Using multiple drivers reduces the excursion level that any individual driver has to make, resulting in greater extension. This is especially noticeable when listening at volume. Besides frequency response, reduced excursion due to sharing the load also reduces distortion. The further that the driver "throws", the more distortion that you have. Therefore having two ( or more drivers ) throw only half as far as one would will typically be slightly "cleaner" than if you just had one doing the work.
With all of that in mind, multiple drivers give you greater extension, more consistent extension regardless of volume, better power handling, lower distortion AND a bigger piece of furniture to move around : ) Sean
PS.... Multiple drivers also have their drawbacks but that is another subject and thread : )
As a Soliloquy 6.5 owner, I can verify it works very well. I have a pipe-organ CD with 24Hz tones and this speaker can rattle the room with them. I tried this CD on a pair of Avalon Eidolons and they were pitiful. You could hear the 2nd harmonic but not the fundamental. In general, for HT I don't really need a subwoofer, but for the sub-20Hz effects the SVS sub I have does have a little more oomph.
The small drivers also allow them to be very quick on transients. Kick drums, etc. sound very realistic.
One thing is kind of unusual, the 3 bass drivers are not all just driven in parallel. They each have different crossover frequencies. Not sure why this is but the speakers have a very coherent presentation.
I would add to Sean's thoughts that a larger woofer, with its greater diaphragm area, also doesn't have to make as large an excursion as a smaller woofer would. The more cone area you get - whether by using multiple smaller woofers or one larger woofer - the less excursion is required to reach the same SPL. A larger woofer also can (up to a point) use a larger motor/coil assembly, which can handle higher power (dissipate more heat), just like multiple smaller ones. Point being, either way you go (bigger woofers, more woofers), the method and goal are the same: Increase total cone and motor size, along with cabinet size, and it will give increased bass capacity.