Setting up Multiple Subwoofers

[Wayne Parham]

Originally Posted by kevinh

You would have 2 'flanking' subs close to the main speakers and then the other (2 or more) subs positioned in the room. Is this correct?

What method do you use for placing the other subs?

That's correct. I run flanking subs near the mains when I have matched-directivity two-ways as my main speakers. But I do not use flanking subs with my constant directivity cornerhorns, because they are sort of "built-in" on that design. Constant directivity cornerhorns don't need flanking subs because they are within 1/4λ of the corner apex and so do not suffer from self-interference from the adjacent walls. Also, the mid and bass are blended between 100Hz and 250Hz, which smoothes vertical modes.

• Speaker placement and wavefront launch

As for the more distant subs, as I said earlier, placement doesn't matter. You can use a Welti method or the Geddes method. This new approach described by Romeyn would be just fine as well. The good news is, it doesn't really matter where you put the other two subs, as long as they aren't grouped together. That can help with WAF - Put them where they fit with the decor. When four subs are used, modal smoothing is assured.

Back in 2005, I did a lot of simulations as well as in-place measurements in a couple homes. I wouldn't consider two homes to be conclusive, except that I also did a LOT of simulations in different shaped rooms using different source locations. This started out as an excercise to compare the original Welti placement suggestions to Geddes pseudo-random placements.

What I found was that some placements were marginally better than others, depending on the room layout. But as long as four woofers were used, the differences were really small. If you used only three subs, the placements were a little more important, and if just two, placement became fairly important. Having just two really subs requires mains that have good extension, which then means you really have four bass sound sources, but I digress.

The thing that struck out at me most significantly was the differences above ~80Hz. Uniformity above 80Hz was usually poor unless (what I now call) flanking subs were employed. There are some Welti placements that have a couple subs flanking the mains. Those sound great. But this was missing on most other configurations. The conclusion I drew is simple - Include flanking subs or something equivalent, no matter what method you use to choose the more distant subwoofer positions.

This is really key here. It is the most important thing, in my opinion. When flanking subs are used, the response is good through the midbass and midrange. Without flanking subs, it most likely won't be, whether or not multisubs are used to smooth the lower bass range. So to me, that's the first priority. Run the first pair as flanking subs. Run them in stereo, one sub per main. Place it slightly below, behind and beside the main it is flanking, and run it up to somewhere in the 80-120Hz range, with a gentle low-pass, like second-order. That way there will be sufficient energy in the low 100s to provide modal smoothing, blended with the mains up through the lower midrange, up to approximately the Schroeder frequency. Being physically close to the mains, there are no localization problems and being between the mains and the nearest boundaries, self-interference from the reflections are mitigated.