Subwoofer Placement and Phase

Tom Martin -- Thu, 06/26/2008 - 20:05

Somewhere in his description of his Wilson X-2 setup, RH mentioned that he has the JL subs behind his listening sofa. Is this for storage or is this a placement that deals with phase problems better than the typical "behind the main speakers" setup?

Robert Harley -- Thu, 06/26/2008 - 21:47

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.

Lear -- Fri, 07/04/2008 - 10:55

Very interesting and helpful set up idea. How do you pick the sub location in the first place though? I mean if you don't have a bunch of test gear (or is there cheap test gear)?

Steven Stone -- Sat, 07/05/2008 - 08:55

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.

Steven Stone
Contributor to The Absolute Sound,, Vintage Guitar Magazine, and other fine publications

Robert Harley -- Sat, 07/05/2008 - 12:39

The original placement was suggested by the computer model. Steven Stone's technque works very well.

Chris Martens -- Tue, 09/30/2008 - 15:24

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.

Chris Martens
Editor, Perfect Vision 

Audiofan222 (not verified) -- Thu, 04/08/2010 - 12:39

For  stereo subwoofers, do you adjust phase individually for each sub, and assume that when they play together they will be okay, or do you need two friends adjusting the phase at the same time?

Audiofan222 (not verified) -- Thu, 04/08/2010 - 12:45

I have a question about phase.  If you have two subwoofers how do you set the phase of each.  Do you do it for one and assume it will be the same for the other?  Do you do it one at a time?  I know that having two people dial in each of the two subwoofers at the same time as Robert is suggesting would be too hard to do.  I am interested what might be the best way to set phase if you have two or more subwoofers in your system?

simond88 -- Mon, 05/24/2010 - 06:01

do it one by one as its the best to assure that each unit works
electronics supplies -- Thu, 08/05/2010 - 11:43

subwoofer phase and placement related to main ( stereo) speakers are quite easy to understand. However, where one has main and surround how does he decide/choose subwoofer phase and placement?
If the main speakers are asked to reproduce low frequency, I supposed phase and placement should be with the surround speaker, not the main ( more than 2 surround speaker, how do you manage to get it right?)
How about a situation where none of the speakers have to reproduce low frequency and are only used for home theater?

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