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.
10 x 10 is small but I still have two channel setup that sounds good and where I sit gives nice separation which I would think would happen if dual subs where used. Or is a sub and/or dual subs dependent on the size of the room?

Otherwise, if a 10 x 10 room is too small for dual sub, is it also too small for 2 channel? I mean am I actually listening to a mono setup because of the size of the room?
Unless you want a real lot of bass.A lot of rock? Shaking the room with movies? Or if you want to catch the lower bass your
present speakers won't go down to,then go with one good 8" sub.With one good 8" sub,it may blend in well,and still give
you those lows you just want to hear.It all depends on what you want to do.
It's probably useful to think of "woofers" rather than subwoofers. Some subs aren't really subs at all, in the literal sense. (See the thinline models from NHT with response to 38hz). Remember that bass issues are pretty severe below 150hz in most rooms. Good monitors, together with a pair of (so called sub) woofers placed flush against the walls, will probably help mitigate these issues - versus floorstanders which place their woofers out away from the walls. Note that the bass response in such a monitor/"sub" system will be smoother, even if it isn't particularly deep.

Good Luck

10x10 room will tend to amplify 55Hz (1125ft/s / 20) - not ideal since many speakers have port tuned to about 30Hz and get already hump on 60Hz. Bipolar speakers cancel this effect by directing sound in both directions (canceling). Similar effect can be perhaps achieved by using two subs facing opposite directions. AFAIK sound at low frequencies is not directional. Two subs are often used to reduce room effects (peaks and valleys) and not for the amount of bass or imaging.
If you are looking to enhance lower frequencies without much care of accuracy, then by all means get a sub. Tune it low, and you will be fine. Reduce the volume if needed. You can pace a sub almost anywhere, for any reason and get good results...IF you do not demand accuracy and perfect synergy with the speakers, do it. I would only purchase 1 sub, 10 inches min diameter. Later add an additional one IF your space increases.
Look for a passive sub. RA Labs (Roy Allison) made some good ones and they show up in the used market at very low prices. Essential they pull out the lows and create a great mid range.
BTW, as to placement:

The whole idea of 2 subs is to have each sub excite the room differently. This usually (not always) means asymmetrical or "random" placement. One key is to keep the subs close to the nearest wall (or walls, corners sometimes work well)so that there is less cancellation of the original bass wave from the sub with the reflection of that wave off the walls. These cancellations - and accompanying reinforcements - cause significant irregularity in bass response below 150-200hz from most speakers. Unless your room is large enough - speakers >15ft from the nearest wall - to behave like an anechoic chamber, you are unlikely to get the smooth response you see in that speaker's anechoic FR test. This is no reflection on the speaker, just an illustration that deep bass should be generated near the walls in "normal" listening rooms.

With due rerspect to Vandermeulen, with the exception of digital room corrected speakers, I have never seen accurate (as measured) bass response from a freestanding speaker that can approach that from a correctly implemented subwoofer systems.

And note: I have measured the bass response of my systems for years.

My desire is for a smoother, richer sound as opposed to "more" bottom, if that makes sense. I have one sub already and my bottom end is fine but keep hearing that two subs would give a better audio presentation. I've been thinking that the room I have my system in may be two small for two subs but maybe two "small" subs may do the trick.
I don't think you could localize any frequency below 110hz, based on the length of your room. Also, are recordings stereo below a certain frequency? I don't think they are, below about 80hz,
Magfan CDs might be but I remember that LPs were recorded mono below about 100Hz.

As for smoothness and richness it is better to get one good sub like JL Audio Fathom 113 that can auto correct for room and whole system response with microphone placed in listening position - instead of two average ones. Studies made with multiple subs usually are oriented toward theater environment to flatten peaks and valleys in whole room.
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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

Ok, so, if in a small room, a single subwoofer will suffice, what kind of small sub would be best? Ported? Sealed? Down-firing? And is there any sub that anyone could suggest?
I was going to write a long reply, but see that Martykl has already written more or less what I would have said. So I'll tick an agreement with him, and summarize in different words:

Multiple subs placed using a moderate amount \of trial and error will nearly always provide a smoother, less erratic bass than would a single one. The biggest problem with a small room or a large room isn't its size but just that it is a box with incredibly irregular low frequency response from all the multiple standing waves. Multiple subs let you choose placements that compensate some of the unavoidable holes and peaks that any of the subs singly will give you.
Ok, so, if in a small room, a single subwoofer will suffice, what kind of small sub would be best? Ported? Sealed? Down-firing? And is there any sub that anyone could suggest?

In a small room, a sealed sub will likely be easier to integtate. SVS makes a nice 12" sealed sub that has lots of flexibility at a decent price.
If you want really deep bass from your subs, JL makes a great product, albeit at a premium price. SVS appears (from test results, not listening experience) to be a great value; a bigger, less attractive cabinet with very nearly the same performance as JL at a real discount.

From my own experience, Rythmik (in my system) might trade away a little bit of performance below 35hz, but works very, very well for music only systems, where full, clean output at these frequencies is rarely an issue.

If you're music only, I'd also think about the $300 NHT "lifestyle" woofer I mentioned. I have not heard them, but NHT usually makes a great product and I suspect that a pair of these small cabinet woofers might work well for music in a small room. Each unit combines 2 10" metal drivers and a 200w amp in a cabinet 6" deep. The primary quarter wave cancellation should be pretty benign when the front of the driver is that close to the wall. Even though you won't get true "sub- bass", this could be a really cost effective music solution in a room like yours.

Money back policy from Audio Advisor minimizes risk.

Good Luck

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The first thing you need before two subs IMO would be to adjust the square dimentions of your room. Place as much of your furnishings along opposite walls. This will make the ten foot dimention slightly smaller and open a door even if its to a closet along an adjacent walls to make that dimention slghtly longer. As one other has said before, twist your listening triangle so your head has no common distance to a room boundry. And may powdered sugar fall gently upon your ears.