[James Romeyn]

Are the subs same brand and model?  Both using same output?

It would seem not being symmetrical would introduce separate placement issues for each one.  Why would this be better?  Just curious as it's not what sub manufacturers generally recommend.  Keep in mind that when same sound frequencies meet they increase amplitude at that frequency, no cancellation as long as they are in phase.

There are well over 100 pages documenting the advantage of three or more assymetrical subs here and www.DIYaudio.com  as suggested by Dr. Earl Geddes and Duke who posted above.  My experience is that any system with fullrange speakers is permanently flawed (meaning the main speakers must not go below about 100 Hz and a minimum of three assymetrical subs define the current state of the audio art).  There's one exception but its impractical and costly.  Lastly, the subs can be of widely varying architecture with no downside whatsover. 
 
The philosophy is one of the two most significant things I've learned about audio in over 40 years of doing it for a hobby and/or for profit.  The worse the room's modes the better is the advantage.  Nothing else seems capable of providing the characteristics of large (commercial) room sized bass in a small (domestic) sized room.
 
You're in a for treat if/when you try it.         

[James Romeyn]

I'll have to check it out because it seems a waste to have extra subs then turn the volume below the other sub or rest of speakers.  My thought process would be to have all the subs at the same level to pump increased air pressure into the room which what I thought the goal was.

The affect of the room's nodes are the overiding factor with bass wavelengths. The lower the cutoff/greater the output the worse are the nodes.  So the first goal is to smooth the nodes, not more absolute output.  Assymetrical multiple subs smooth the nodal effects.  EQ is of not recommended because EQ is a static one-dimensional cure for a 3-d problem; EQ moves the node effect from one location in the room to another, often degrading performance at other locations and overall.
 
Read about the placement ideas, borrow sub(s) for at least three in your room and try it out. 
 
Cheers!     

[polarbare]

With respect to multiple subs and asymmetric placement, would you set the phase of one subwoofer to match the fronts, then set the phase on the second (and others) to match where it is on the one you chose to match the fronts, or would you adjust each sub seperately to match the phase of the fronts? I'm sifting through the monster thread on diyaudio http://www.diyaudio.com/forums/multi-way/134568-multiple-small-subs-geddes-approach.html, but figured I would ask here, as it might be a lot quicker 

[James Romeyn]

With respect to multiple subs and asymmetric placement, would you set the phase of one subwoofer to match the fronts, then set the phase on the second (and others) to match where it is on the one you chose to match the fronts, or would you adjust each sub seperately to match the phase of the fronts? I'm sifting through the monster thread on diyaudio http://www.diyaudio.com/forums/multi-way/134568-multiple-small-subs-geddes-approach.html, but figured I would ask here, as it might be a lot quicker 

My understanding, for 3 or 4 subs:
No constant rule defines the sub polarity vs. mains.  You will be shocked the degree to which the sub placement can not be identified.  They blend seamlessly with the mains as long as there is not too much overlap.  If necessary look into passive HP filters at the input to the main amp. 
 
Sub A (no constant rule which sub) is inverted relative to the remaining subs, which are in phase with each other but inverted relative to sub A. 

I will soon post a formula at my website defining what may be the best plan for siting the subs.   

You gotta try it!

[Duke]

Jim, that is very interesting!  I've played around with inverting the polarity of one sub, but never could decide whether there was a clear net improvement.  I'm confident your ears are considerably better than mine (or at least you're a lot more tuned-in to what they're telling you), and I think you're going to push the edge of the multisub envelope beyond where I've been able to go with it.

Duke

[polarbare]

Thanks for the info. After doing a bit more reading, I came across this page http://mehlau.net/audio/multisub_geddes/ describing the process for integrating subs. It appears there are no hard and fast rules regarding phase - just tune to where you get the best sound. I tried sifting thru that large thread at DIY audio - I could only get through the first 50 and last 20 (of ~130 pages), and there was only about 20 posts that actually dealt with placement and issues. The rest of what I saw in that thread is arguing over acoustics theory - not useful to me. I'd rather just try it out (know how to set up and measure) and see what it sounds like. If I like it, I'll keep it. If not, then I'll try something else.

Back on topic. I only have two subs - I had another, but gave it to my brother before stumbling across this technique. Maybe I should borrow it, try this out, and then give it back. A question about placement - in the above link, they describe one sub must be corner loaded close to the mains, a second on a side or back wall, and third pretty much wherever you can put it as far away from the others as possible. For the sidewall, how far away from a corner does it have to be before it's not considered corner loaded? For my current room, the sub would be about 4 ft from the corner if on the sidewall, and about 3 ft if located on the back wall (but only 2 to 3 ft from the listening postion).

[Duke]

For the sidewall, how far away from a corner does it have to be before it's not considered corner loaded? For my current room, the sub would be about 4 ft from the corner if on the sidewall, and about 3 ft if located on the back wall (but only 2 to 3 ft from the listening postion).

I think your three feet from the corner is far enough to no longer be considered "corner loaded".

If only one of your subs as a steep-slope (24 dB per octave) lowpass filter, that should be the one closest to your listening position as that's the one you're most likely to be able to hear as a separate sound source.  If all of your subs have 12 dB per octave lowpass filters, then use the crossover frequency and level controls to make sure you can't localize it in the mix.

[polarbare]

Right now I have a pair of rythmik F15Se's - both have a setting for 24 dB/octave.

Looking at my room (and the crap in it ), I think I just need a bigger place  Not to mention I just ordered 6 of the GIK tri traps. Speaking of which, any cons to putting a bass trap above the corner loaded sub (it will be on a stand as opposed to sitting on the sub itself).

I still have no idea where I am going to be able to place a third sub and not have it corner loaded. I might have to just make do with two until I move.

[Duke]

Polarbare, try the third sub anyway if it's not too much trouble to do so.  Even if it's not in an ideal location, it will still contribute some smoothing compared to just using the other two.  As long as the subs are spread out somewhat, the number of subs is more important than the location of any one of them. 

You might experiment with reversing the phase on one of the corner subs.  That might result in some zigging where the other one is zagging.

I don't think you have to worry about the bass trap being in the same corner as the sub.  If GIK tells you otherwise, well they're the experts on bass traps, not me.

Duke

[polarbare]

So what software/hardware do you recommend to take measurements with (I was about to order a tascam 144 and a calibrated mike, but figured I ought to ask)?

[Duke]

I'm really not up on what's available in reasonably-priced microphones and measurement systems, sorry.  The stuff I use is a fairly pricey professional package from Linear-X.