Two subwoofers in smallish HT room?

I don't know about Rives suggestion about canceling room modes by having one SW at the back, but, years ago when experimenting with matrix quadraphonic sound I found that bass from the front speaker could be greatly reenforced by the rear speaker playing the same signal, but with phase inverted. This would happen if the speakers were one half a wavelength apart.

I tend to think of subwoofers as just another driver of the speaker system, so that each speaker should have its own colocated SW. I have three for my front channels, and would have two for the rears if I thought that the rear LF response was a problem.

Sogood51...I noted the bass reenforcement from an out-of-phase rear speaker as a BAD byproduct of matrix multisound. For me it seemed boomy. This is what you would expect because the reenforcement is peaked at a wavelength that corresponds to a room mode. I can't imagine why anyone would do it.

Sogood51...My observation of bass boom when there was an out of phase speaker in the rear did not involve subwoofers or crossovers. Just plain vanilla speaker systems.

I bet that Rives mention of killing room modes with a rear subwoofer involved driving the two subwoofers in phase with each other.

Gregm...If the two out-of-phase SW are physically half a wavelength apart they will add at that frequency. I guess some people use this to boost the SW SPL. My point is that if this distance between SW is the length of the room that frequency is already boosted by the room, and cutting it by a rear located inphase SW makes more sense to me..

Sogood51...Fun and games for sure.

I don't think this has anything to do with SW X/O frequency. The sound wave doesn't care whether it is launched from a separate box or not. Very deep bass will boom nicely if your room is large enough to sustain the frequency, and the speaker/subwoofer is capable of the frequency. And, by the way, if your ears are sensitive to the frequency.

When almost 30 years ago I first observed the bass-boosting effect of out-of-phase rear speakers I didn't like it. However, for exploding car movies I can understand why people like it.

Sogood51...It was a surprise to me, but I measured about 4.5 watts rms into each of my three SW system, a total of less than 15 watts, to shake the windows in my listening room, using a warble test signal around 30 Hz. The assumption that LF takes a lot of power does not stand up to objective measurement. My CarverPro ZR1600 amps drive a woofer system with one side and a MG1.6 with the other, and if I crank up the volume I can make the MG1.6 side clip (LED indicator) while the SW side is still loafing. Hard to believe, but that's the fact.

You might want to check Rives website and study up on room resonance. (Maybe buy his gadget also).

Sogood51...I am now set up with a crossover where I can alter the SW X/O frequency while music is playing by twisting a knob, and my ears and my spectrum analyser tells me what happens.

With a white noise test signal (flat 20-20KHz) I am able to vary the X/O frequency from 50 to almost 200 Hz with no audible or visible spectral change to the sound. This tells me that I have got the SW well matched to the mains. With music, I sometimes vary the X/O frequency.
With "heavy" sound, like organ, choral, or some orchestral, the six big cone drivers in the subs work better than the Maggies, and I crank it up to about 150.

My room resonates at about 60 hz. Always has with any speakers. Rives uses 70 Hz as his example on the web site. If you are lucky enough to have a larger room it will be lower.

Sogood51...My SW systems are built into the wall behind the Maggies, and "play through" the screens. So they are not moved around. I pull the Maggies out 4-6 feet when in use.

I do not run the Maggies full range. I think that getting the really LF signal out of the woofer is the main reason to have a SW. My X/O is 24dB slope.

The Behringer DEQ2496 analyser that I bought recently has really helped me understand a lot of things about my system that I previously could only guess at. I recommend it, and a crossover that you can diddle from the listening position. After the RTA shows you what your problems are, turn it loose in AutoEq mode, and be amazed!