What size subwoofer to get?

How big is your room? Do you have a tv or anything between your speakers? What are your main speakers-brand?What spl's do you listen at? Wood or carpet on floors?

I believe you are approaching your dilemma from the wrong angle. The size and quantities of subwoofer do not matter one bit unless you are already buying the absolute best. Why would anybody want to purchase two mediocre subs when they could have instead afford to purchase one good to excellent sub? The world is full of mediocrity. Isn't this what the 'high-end' is all about?

The quality of sound of the sub (just like a set of speakers) and the ability to fine tune the sub to your main speakers means everything.

Bad subwoofers come in all sizes. Good subwoofers come in all sizes.

Like any other piece of equipment, hopefully you are shopping for the best subwoofer you can find in a given price range.

Assuming that, I will make a few other assumptions and/or comments:

o You are looking for a sub that will faithfully reproduce down to 18Hz or lower. There is too much musical information can be found in the 18Hz to 30Hz range to purchase a sub and still not be able to reproduce it. In my opinion, a subwoofer is not a subwoofer unless it can go down to 22Hz or even below that.

o Since your floorstanders go down to 35Hz, you are looking for a subwoofer that you can dialin the cutoff filter somewhere around that same frequency. A subwoofer with a cutoff at 50Hz will most likely give you some kind of mid-bass hump with an over-emphasis in the 30Hz to 50Hz region. You may not be able to compensate for this hump.

o You are looking for a subwoofer that is musical. That is, it reproduces the lowest octaves with accuracy, definition, and speed. Not just flapping in the wind like an American flag. Again, size does not necessarily determine this. There are very bad sounding 8inch subwoofers and very good sounding 18inch subwoofers and vice versa.

o In my experience and opinion, in the lower budgets say $1200 to $2000, you need to be looking at 12inch, 15inch, or 18inch subs, because they stand a greater chance at faithfully reproducing the 18Hz-35Hz range. Usually, one must spend more than that to get a good 8 or 10 inch sub to accurately perform in that same frequency range.

o One extremely musical subwoofer is the Bag End Infrasub 18inch subwoofer which sells new for $1600 that is reported to reproduce all the way down to 8Hz. Now that's a sub. I've had this unit in my home for about one month. I had no idea that an 18inch sub woofer could be so tight and so musical. Almost too tight. But it was difficult to dial-in properly.

o I stepped down a notch and purchased a Triad Platinum 18inch sub ($2000 new) that is still quite musical but allows me to tailor it to my room and floorstanding speakers just a bit more.

o These two 18inch subs I mention replaced an 8inch subwoofer ($1400 new) that would knock my house off it's foundation, but had absolutely no musical definition to it whatsoever.

-IMO

Stehno is absolutely right. The cone size is not important. Since your current main speakers go to 35Hz you'll need a sub that can crossover below this level (unless you filter the signal going to the main speakers).
My main speakers go to around 35Hz and I'm crossing at 27Hz to avoid boomy mid-bass !

My suggestion would be to look for a used REL strata. It crosses very low (down to 25Hz), it's adequately powerful for all but the largest room, and if you don't like it or ever want to upgrade they hold their value and are easy to resell.

IMHO Better no sub than a cheap sub for 2ch music.

Slightly different advice:
1. A floorstander's low frequency (in this case 35Hz) can be expected to be its -3dB point (unless you're quite close to a wall). The crossover on a sub is its -3dB point. Therefore, a crossover point an octave above the low frequency spec point of your speaker is a good place to start. In your case that is 70Hz.
2. Though there are many design factors in subs, the amount of air moved by a large sub is much larger than a small one, requiring less excursion (often meaning less distortion) to move the same amount of air.
area = 3.1416 * r squared
10 -> 79 sq in
12 -> 113
15 -> 177 (two 10s)
18 -> 254 (two 12s)
3. Finally, a phase control as well as a volume control is useful in getting floorstanders and sub to sinq.
Good luck.

Stehno hit it on the head.

Rives also brings up some important points. One sub IS harder to place than two. On the other hand, two well designed commercially built subs can be quite expensive.

Just keep in mind that a vent of any type ( ported, passive radiator, slot loading and yes, even transmission line ) will have slower transient response ( reduced impact, less definition, increased ringing, poorer decay, etc... ) than a well designed sealed box would offer. The sealed box will also TYPICALLY have less problems with interfering in the upper bass since there is no leakage through the port or passive radiator "talking". Placement should also be easier since you only have to worry about room reinforcement / nodes from one radiating surface ( unless you have multiple drivers ).

The major drawback to sealed woofers is that a good one will be quite large. Having said that, two good quality sealed subs that are offer large box volumes would be VERY hard to beat if properly set up.

As to the Bag End, it is equalized to produce 8 Hz, but only at very low volume levels. As driver excursion and spl is increased, the bass processor rolls off the extreme lows so as not to muddy the more prominent notes slightly above this range. Kind of like a smart "loudness" control that one can find on receivers, etc... The biggest problem with Bag Ends is the construction of the cabinet, which is not as rigid or damped as it should be. It also takes a pretty good amount of power to really make this thing sing, which is true of almost all subwoofers. Sean
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