Subwoofer Placement and Phase

Technically, the best place for the subwoofers (according to acoustician Norm Varney, who modeled my room on a computer and measured the room with a TEF analyzer during the acoustics installation) is on the wall behind the loudspeakers. I had them in this location for some time, but when I went to a large loudspeaker (the Wilson MAXX 2), that placement put the subwoofers physically in the way. I moved the subwoofers to the same location relative to the side walls, but positioned them on the rear wall. I heard no detrimental effect from this change. Note, however, that I use the subwoofers only for reproducing the LFE track on film soundtracks, not to augment the bass response of the main speakers for music. The latter is more critical, and might better resolve placement differences.

I set the subwoofer phase by reversing the the polarity of the main speakers by connecting the black speaker cable to the red speaker input, and the red speaker cable to the black speaker input. This inverts the polarity of the signal. I then play a test tone at the crossover frequency and while sitting in the listening position, have someone rotate the subwoofer's phase control. I listen for position on the phase adjustment that produces the maximum null (least amount of bass), and know that the phase control is set perfectly. I then return the speaker leads to their normal position.

This technique works because it's easier to hear the maximum null than it is to hear the maximum peak. When the phase control is set perfectly, the main speaker's woofers will move out when the subwoofer cone is moving in, cancelling each other. When the main speaker's correct polarity is restored, the main speakers and the subwoofer are maximally in-phase.

• reply

My favorite quick and dirty way to find the best subwoofer placement in a room sans test gear or even calibration discs is to put the subwoofer in your listening position (yup hoist it onto your chair or sofa) and then walk around the room and listen to the bass response.

Some spots will be thick and some thin and just like Goldilocks, our job is to find the spots that are just right...

Play tracks with as many different pitched bass notes as possible as your room will interact differently at various tones. Find the best compromise position where the room doesn't augment or reduce the bass too much.

• reply

One follow-on point I would add to the discussion is that, for many subwoofers, a certain amount of phase shift occurs across the sub's operating range, meaning that you can't always have the sub perfectly in phase with the main speakers at all frequencies. This point was drawn to my attention by Mike Creek (of Creek amplifier fame, and also designer of the terrific little Epos subwoofer).

So, we sometimes face a dilemma: Should we set-up best-case phase matching for the crossover region (which generally gives the best overall sense of integration) or for some lower frequency (which generally gives the best sense of low frequency depth, power, extension, etc.)? Ideally, it would be nice to have things both ways, but in practical terms that isn't always possible. As a matter of preference, I personally tend to go for better integration within the crossover region (reasoning, as does Alan Taffel, that proper integration must come first). That said, however, I've observed that some movie soundtracks sound best when the system is tuned for maximum low-end power and extension (albeit at the expense of some mid-bass clarity).

My point: It pays to keep your sonic priorities firmly in mind when doing subwoofer setup/tuning--particularly when making phase adjustments.

• reply