Best setup for two subwoofers.

Les creative edge, to the best of my knowledge none of the subwoofer studies published by Harmon researcher Todd Welti (presumably Toole's source) investigated asymmetrical placement, simply because that introduces too many variables. Another researcher, Earl Geddes, demonstrated improved smoothness from asymmetrical placement in a paper that is unfortunately no longer up on his website, possibly because it gave away too much to potential competitors. Todd's consumer-oriented paper is no longer up on the Harmon site either.

Based on conversations I've had with both Earl and Todd, the main thing is to have multiple bass sources spread fairly far apart, as that is where the greatest benefit accrues. The exact positioning of the subs is generally less critical than with a single sub.

It is interesting that Earl and Todd arrived at using distributed multiple subs as a solution to room modal behavior completely independent of one another. Neither was aware of the other's work until Todd published the first of his papers.

Duke

From what I understand the only concern one truly has to worry about with two or more subs is that they are not placed in a way where one is out of phase with the other. By setting up multiple subs in ways where they generally mirror each other or sit along side/front walls equal distance from corners will generally have each sub work in tandem reinforcing the bass signal and keeping boom and nulls to less effect. Now of course if you have subs with a variable phase control you can with effort tweak the subs to work more in phase and thus positioning errors will not be quite as much an issue in terms of phasing.

Now of course two subs if not sitting in a best compromise situation for what may be determined as the best sitting locations for listeners can still aggravate boom and tubbiness of bass along with nulls in bass.

Dr. Toole suggests the best choice if budget allows (and factoring extra electricity useage) is FOUR subs each in a corner. They will cancel out well over 85% of the room effect of boom, tubbiness and nulls. They will provide the smoothest bass for the most listeners in room. On top of that they will work more efficiently as the corners reinforce their output causing the sub amps to work less hard. He suggests you may be able to settle for even smaller subs say 4-10's rather, than 4-12's or 4-15's. In such a case where you use FOUR subs one in each corner it no longer matters what your room is shaped like. It could be square, long rectangle, short rectangle, L shaped, or whatever.

If you want the easiest placement for multiple listeners with only TWO subs again setting each up down a wall and out of a corner and where each is relative to each other similar in the room placement you will get a better compromise for more listeners in room.

If you only have ONE sub the best thing to do is to put it as close to a mid placement on a wall, front, side or back to give the best compromise for multiple listeners in room. If only one listener will likely ever listen then put the single sub in a corner but understand once another listener or more then sits he/she/they will get a compromise in sound that may not be too pleasant for them.

If this is indeed Dr. Toole's position, then I disagree with him:

"...the best choice if budget allows... is FOUR subs each in a corner.... They will provide the smoothest bass for the most listeners in room."

I question whether that's really what he said, because Todd Welti's investigation showed that four subs with one at the midpoint of each wall in a rectangular room was smoother than one in each corner.

In the paper mentioned in my second post above, Dr. Geddes compared four subs each in a corner against three subs in a specific asymmetrical configuration (one in a corner, one along an opposite wall, and one closer to the ceiling than to the floor). The three subs asymmetrical were not only smoother, they also had less frequency response variation from one listening location to another.

I have been manufacturing four-piece multisub systems since 2006, and my experience plus my customers' supports the argument for asymmetrical positioning even if it's confined to the horizontal plane. I am unaware how many of my customers tried a symmetrical configuration, but to the best of my knowledge all of them have chosen to use an asymmetrical one.

Regarding the relative phase of widely-spaced subs, it is far less critical than intuition would lead us to believe. The reason is, we cannot even detect the pitch of a bass note before several cycles have reached our ears. By then the room's effects have totally swamped the first-arrival sound. For all practical purposes, in the size rooms we listen in at home there is no such thing as "direct sound" in the bass region. By the time we hear it, it's all reverberant sound. This is precisely why a distributed multisub system offers such a significant improvement in perceived bass clarity and smoothness, instead of sounding like mud: The only place a multisub system offers improvement is in the reverberant field, but from a perceptual standpoint that's all there is in our rooms anyway.

Duke

Duke,

The best thing I can suggest to you is to log on to Scott's Home Theatre Geeks. Look up the Episode with Dr. Toole and enjoy listening to one of the most respected men audio reproduction of sound, how humans perceive sound and speaker R&D.

http://twit.tv/htg16

You will find his assertion that 4 subs placed in each corner of the room will give the most efficient and likely best sound of sub bass for multiple listeners in room. Four subs placed mid way on the 4 walls may give better general fidelity but are not as efficient in bass output to power input.

Dr. Toole is also fine with two subs placed generally midway on walls if costs are an issue.

Thank you very much for the link, Les creative edge.

In the first half of the interview, I didn't hear anything on the subject of speakers, rooms, subwoofers, subwoofer placement, or psychoacoustics that I didn't already know. Not trying to be a smartass, but those are topics I have studied. Now I did learn some of the "why" behind limitations at the microphone end of the chain, and behind the fallacy of pursuing "ideal" room dimensions.

Much of the surround-sound discussion in the second half was EXTREMELY educational to me, as historically I've been a two-channel guy. Toole obviously can teach me a great deal about multichannel, so a few minutes ago I ordered his book. I have you to thank, Les creative edge, for making me aware that Toole's book contains a lot of valuable information I am not familiar with.

Back to the first half of the interview. Fascinating that Todd Welti, whose work I'm familiar with, was actually commissioned by Toole to investigate multisubs. I hadn't appreciated that it was Toole's idea first.

As far as subwoofer configuration goals, Toole apparently places top priority on bass efficiency rather than smoothness, as he prefers four in the corners over four at the wall midpoints. My top priority is smoothness, which comes at the expense of efficiency because destructive interference is the mechanism by which maximum smoothness is achieved.

The comparison I would like to see, but which I don't think anyone has made, is four subs at the midwall locations vs four subs in a thought-out asymmetrical configuration. I bet they both would sound pretty darn good.

Again thank you very much for this discussion and the link, without which I would have continued in my mistaken belief that there probably wasn't much in the book that would be new to me. At some point I wish to become competitive in the home theater sound system marketplace, and Toole's book will be a primary resource.

Duke