Two subs in a small room....

I have read that two subs smooth out the bottom end and give a much better sound. But, what if you have a room that is just not big enough for two subs. Is it worth the effort to get two "small" subs or is there a special placement for just one sub besides the usually place, behind one tower. What about placing the sub right in the middle?

By the way, my system is in a 10 X 10 room and it is in a diagonal configuration if you can picture that.
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Matchstikman, I like your diagonal configuration. You might try shifting your speaker-listener-speaker triangle a little bit so that you're not lined up perfectly on the diagonal.

I'm an advocate of using multiple asymmetrically-placed subs to smooth out the in-room bass, a technique I learned from Earl Geddes. Small rooms are especially prone to lumpy bass because the room-induced peaks-and-dips tend to be larger and spaced farther apart in a small room, which makes them more audible. Multiple dissimilar peak-and-dip patterns at the listening position can help smooth things out, which allows you to turn up the subs a bit higher (giving a more natural tonal balance) because you don't have strong bass peaks that boom, forcing you to keep the subs turned down low. Whether you use a symmetrical or asymmetrical setup, two subs will be better than one if you choose wisely.

You see, another issue in many small rooms is excess boundary reinforcement because the room boundaries are all pretty close to the sub(s). So you don't want subs that are "flat" anechoic; rather, you want subs that would roll off gently under anechoic conditions - which the room gain will offset. I'm not sure what would be ideal in a 10 by 10 room, but the 6 dB per octave rolloff of a low-Q sealed box would probably be a good starting point.

Finally, if you're going to place any of the subs well away from the main speakers, you want to make sure those subs aren't contributing any audible lower midrange energy. If they are closer to your ears than the main speakers, you could hear them as separate sound sources if that lower midrange energy is loud enough. So a steep-slope (24 dB per octave) low-pass filter is desirable.

I appreciate all the advice. I feel the 10x10 is small but that is what I have for now, but hope to be moving into a new house with something bigger next year, but with the economy I'm not sure. For now, I want the biggest bang for the buck with what I have.
I have two Martin Logan Dynamo 10" subs in an 11x20 room. I have the subs placed on the short (11') wall. From my experience, if you can make one sub work, you can probably make two work, but one will probably serve your needs in that size room. I would recommend a Velodyne SMS-1 sub equalizer to aid in integration. Also, I would recommend setting the low pass filter just below the lowest rating of your main speakers, so that the sub basically just fills in the low end. Also, show restraint in the volume control.
To incorporate Duke's and Bob's comments into my previous post.

1) An optimallly placed single sub together with a bass restricted main speaker will usually produce smoother bass than a full range speaker that has been placed for best overall performance. (The best spot for bass generation is usually different than the best spot for generating mids/highs.)

2) Optimally placed multiple subs usually allow smoother response than an optimally placed single sub.

3) Room EQ can achieve smooth bass response with freestanding speakers or with subs. If you don't want to EQ your main speaker's signal (that would be audiophile heresy!), think about room EQ for one or more subs.

Personally, I use 2 asymmetrically placed subs and the Velodyne SMS that Bob mentioned for bass EQ. My main signal path is interrupted only by an NHT active x-over (that I can't identify in blind A/B tests) which feeds my main amps and the SMS, which is turn EQs and feeds the subs. I get outstanding results - both measured and perceived.

I'd also add that the SMS has a built-in room analyzer that makes optimal placement of the subwoofers a hell of a lot easier. The SMS includes an active x-over, can control up to 3 subs and costs app $450. IMHO, it represents great value. I'm not crazy about the unit's built-in, basic (digital) high pass filter, so I added the NHT x-2 active x-over @ app $300. So the total budget for my sub control set-up is $750 plus the cost of sub(s).

You can either buy cheaper subs and apply the savings to fund the SMS and/or X-2 or you can always start with sub(s) and add these control unit(s) later, as budget permits.

Either way, I believe that you'd be going down a good road.

Good Luck