# Four Subwoofers



## Ayreonaut (Apr 26, 2006)

Have you read this interview with Floyd Toole and Todd Welti from Harman?

(I've seen this repeated elsewhere that four subwoofers provide the best solution.)



> *Todd Welti: *...four subwoofers seems to do the trick -- and we came up with three configurations out of about 100 that we looked at that are the cream of the crop. Actually, there’s a fourth solution -- one that Floyd came up with -- that worked even better than the others, but it involves bringing the subwoofers out from the walls, which I generally didn’t consider.
> *Floyd Toole: *It’s not very practical, unless you could put them in the ceiling. It uses four subwoofers located at the 25% points from all four walls.
> *Todd Welti: *In other words, you shrink the whole room by 25% and put the subwoofers at the corners of that virtual room. You get incredible performance, but that’s just not practical in most rooms...


link to the white paper

link to interview

Well, I'm planning to implement a four driver IB sub in the fall. And I'm thinking that this is a perfectly practical solution. Instead of co-locating all of the drivers in one enclosure, I can mount each one in the ceiling at the quarter points as described. What do you think?


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## Sonnie (Apr 11, 2006)

Sounds interesting indeed. You are teasing us though... :nono: you shouldn't wait til fall... do it now so we can get a users opinion on how well it works. I agree about the only way it could work is via the ceiling. I can see four SVS PB12's sitting at 25% of my four corners... :yikes:


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## JCD (Apr 20, 2006)

I've had this thought before.. but wasn't sure if the coupling between the various drivers would be as effective. I think I've determined that it's ok in the end.

I posted a similar q on the Cult and was told to "save myself the headache and mount them all centered on the front wall" -- for reference, I was thinking I might mount them in the corners in the front wall and create a "wall of sound".

Anyway, that's what I got -- not much though huh?

JCD


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## Chrisbee (Apr 20, 2006)

If you put them all together in an IB you get a coupling effect. Just like you do if you co-locate box subs.

Four box subs pulled out from the walls is not the same as an IB with widely seperated drivers. 

I would imagine they would all be competing out of phase at certain frequencies depending on their spacing.


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## Ayreonaut (Apr 26, 2006)

Sonnie said:


> ...you shouldn't wait til fall... do it now so we can get a users opinion on how well it works...


Sure Sonny, you guys can chip in the cash so I can do it now. :laugh:


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## Ayreonaut (Apr 26, 2006)

JCD said:


> ...I posted a similar q on the Cult and was told to "save myself the headache and mount them all centered on the front wall" -- for reference, I was thinking I might mount them in the corners in the front wall and create a "wall of sound"...


This guy has a similar idea here.



Ingvar Öhman said:


> One example of an excellent placement of four
> subwoofers is an even distribution horizontally
> behind the main speakers.
> 
> ...


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## Ayreonaut (Apr 26, 2006)

The distances from the walls and corners will reduce the efficiency, but I looked into mutual coupling some more. 


Genelec Website said:


> When two subwoofers are positioned close to one another mutual coupling is the fortunate by-product. This is due to the long wavelengths, associated with low frequencies, causing constructive superimposition. For mutual coupling the subwoofers must be place within ½ a wavelength of one another (85Hz upper crossover frequency ½ wavelength is approx. 2m). For example, two subwoofers give a 6dB increase in acoustical output at the listening position - see table below.


 My room is 12'-10" x 18'-7". So located at the quarter points the four drivers would be 6'-5" and 9'-4" apart. By calcualting the half wavelengths, I found that all four drivers will be mutually coupled together below 60 Hz. This would seem to indicate no loss of power at the really low, power hungry frequencies.


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## Wayne A. Pflughaupt (Apr 13, 2006)

But you’ll still have the phase issues Chris spoke of, unless all of them are equidistant to the seating position. This will show up as ragged, saw-tooth response, and since it is a time-alignment issue, you’ll get different results at every listening position. You might note that Mr. Welti stated in the quote field from your initial post that four subs “aren’t practical in most rooms.”

It also should be noted that experience in sub placement like the ones evaluated in that white paper are relevant only in the room they’re performed in. Unless your room is identical, or at least pretty close, your results will be different.

Regards,
Wayne


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## Ayreonaut (Apr 26, 2006)

I wrote an e-mail to Todd Welti to see what he thought of our ideas.



Daniel in an email to Todd Welti said:


> Dear Mr. Welti,
> 
> I would like to seek your input on this. I'm an audio enthusiast, and I recently discovered your paper "Subwoofers: Optimum Number and Locations." and Floyd Toole's "Loudspeakers and Rooms for Multichannel Audio Reproduction." I also read the interview that you did with Wes Philips in 2004 on the subject.
> 
> ...


Mr. Welti was very kind to take time and respond to all of these. (Even though he's on vacation!)



Todd Welti in an e-mail to Daniel said:


> Hi Daniel,
> 
> first off, the most recent and complete reference regarding the subwoofer work we did can be found in the May issue of the AES Journal. There is some new stuff in there that might interest you.
> 
> ...


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## Sonnie (Apr 11, 2006)

Very interesting... now toss in this whitepaper on multiple sub locations.

ThomasW has actually suggested the possibility of multiple sub locations for my IB project. Possibly 2 subs in 4 locations or 4 subs in two locations.


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## Ayreonaut (Apr 26, 2006)

Sonnie,

I've been doing a lot of research on sub positioning lately, as you know. I discovered this article: link

Its a bit complicated, but after carefully going over it I was able to do the math. The gist of it is that for _one _sub in a _rectangular_ room the best sub placement to avoid room resonances is in a quarter point. 1/4 of the room width off the side wall and 1/4 of the room length off of the end wall.

That may not be practical is some living rooms, but sonically its the best spot. Fourtuantely I _can_ put mine very near one of the quarter points. I moved my sub there last night, and measured. I lost a few dB comapred to corner placement, but I got the most even FR (pre EQed) of any position I've tried. It really works.

Why does this matter? Can't I just EQ for any position? Well, I recently tried corner loading and then EQing out the ensuing resonances. Even though I gained a few dB abnd the EQed output measured flat, the sub still sounded boomy. I don't really know why. My guess is that the decay times come in to play since the room continues to ring after the sub stops. If you position in a quarter point, those modes get much less excited.

Theoretically, the only thing better for dealing with room modes is to use multiple subs in every quarter point. Instead of mitigating the LF axial room modes, it elimintes them. (I should add that two subs in the two front quarter points will eliminate the width modes and mitigate the length modes.)

I suggest poisitioning your two subs on the two front quarter points and measuring at all of your listening positions. Obviously this isn't a practical set up to keep, but it should give you the same response as subs mounted in the same poisitions in the ceiling above. 

Please let us know!


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## brucek (Apr 11, 2006)

> The gist of it is that for one sub in a rectangular room the best sub placement to avoid room resonances is in a quarter point. 1/4 of the room width off the side wall and 1/4 of the room length off of the end wall.


Interesting stuff. Wish I knew more about it. I have certainly read opposing views to the theory and article you site though.

Floyd Toole seems to support the theory that if we have proper room dimensions where we can spread the axial modes (and lesser so the tangential and oblique modes) evenly, then the optimum position would be corner loaded so as to most efficiently acoustically couple to the room modes. 

This provides us with added gain and headroom. Hopefully the resulting first order harmonics are evenly spread and not much equalization is required. That's the rub for sure - many are dealing with fixed room dimensions designed for ascetics and not subwoofer placement.

Your method essentially is in direct opposition to that theory in that you're trying to locate the dead zones so as to basically place the sub in a null and not generate any resonances. Fine, but unfortunately you're trying to power your way out of the null - you'll need a large amp. I suspect for optimum placement for a true null of the ceiling fundamental with your method, you'd have to mount the sub in the air about a quarter way between the floor and ceiling. Not possible with an IB or really very practical for any sub. Although the ceiling harmonic would be considerably higher than the other dimensions.

Who knows..... :dontknow: 

brucek


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## Sonnie (Apr 11, 2006)

I can see it now... four hanging subs... who will be the first to try it? heeheehe

I'm gonna be doing some experimenting this evening with moving one of my SVS subs around the room some. The 1/4 point definitely couldn't be the permanent spot, but it would be interesting to see the different response compared with each other. 

Last night when I was measuring the different spots in the ceiling I noticed the response was not a lot different from where my subs are now. There was a tad better extension down to 15hz and the dip I have moved from 40hz to 35hz.

What I'm thinkin' is moving the left sub down the left wall in various locations while leaving the other sub in check. I'll test the subs jointly as I have previously and also adjust phase on the one I'm moving and retest. I can stand to move that one sub to a couple of different spots along that wall... so I might pick up that weak spot at 40hz... or move the weak spot with that sub to another frequency area. 

Then eq'ing them might be more of a challenge too... being they will no longer be symmetrically located.


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## Ayreonaut (Apr 26, 2006)

brucek said:


> Interesting stuff. Wish I knew more about it. I have certainly read opposing views to the theory and article you site though.


 So have I. I've tried it both ways now.


brucek said:


> Floyd Toole seems to support the theory that if we have proper room dimensions where we can spread the axial modes (and lesser so the tangential and oblique modes) evenly, then the optimum position would be corner loaded so as to most efficiently acoustically couple to the room modes.
> 
> This provides us with added gain and headroom. Hopefully the resulting first order harmonics are evenly spread and not much equalization is required. That's the rub for sure - many are dealing with fixed room dimensions designed for ascetics and not subwoofer placement.


 Floyd Toole has a list of rules for good sound in rooms, and what we are coming up against is a conflict between two of them. Maximize the output from the subwoofer(s), but achieve a uniform performance over the listening area. For maximizing the output, corner loading is a good idea.


brucek said:


> Your method essentially is in direct opposition to that theory in that you're trying to locate the dead zones so as to basically place the sub in a null and not generate any resonances.


 Exactly. When we are trying to "achieve a uniform performance over the listening area" we can use what Toole calls Selective Mode Cancellation.


Floyd Toole said:


> So, why would we want to do this? Aha! Would it not be a good idea for everybody in each row of a home theater to hear the same bass sounds? Would it not be a good idea for a recording engineer to be able to move from one end of the console to the other without experiencing huge changes in bass? Well, this is how it can be accomplished. We are not saying, yet, that it is good sound, merely that it is the same sound. Once things are equalized in the sense of getting everybody hearing more or less the same sound, we then may need to equalize in the sense of changing the frequency response of the system.


I’ve tried it both ways. I equalized both responses and set the volumes the same. What I found was that in my room the position employing Selective Mode Cancellation does yield a more uniform response over several listening positions and a lower total level. But I also found that it sounds much cleaner. Tighter, or damped. That’s what I prefer. Many might prefer the higher output of corner loading and excited modes. It’s probably a misnomer, but some might call the difference HT sound vs. Musical sound.


brucek said:


> Fine, but unfortunately you're trying to power your way out of the null - you'll need a large amp.


 You’re right. To achieve the same output level you might need more sub and amp. Do you think Sonnie’s eight woofer IB and two amps will do the trick? :laugh:


brucek said:


> I suspect for optimum placement for a true null of the ceiling fundamental with your method, you'd have to mount the sub in the air about a quarter way between the floor and ceiling. Not possible with an IB or really very practical for any sub. Although the ceiling harmonic would be considerably higher than the other dimensions.


 Yes, a sub on the floor or in the ceiling will excite all the vertical axial modes. Fortunately only the first resonance will be in the subwoofer range for a typical room height. In my room with an eight foot ceiling it falls at 70 Hz. But since all of the listeners in a room will be seated at about the same height, one band of EQ will correct it for all positions, if it proves to be a problem.


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## Ayreonaut (Apr 26, 2006)

Sonnie said:


> I can see it now... four hanging subs... who will be the first to try it? heeheehe


 I think that to cancel the mechanical vibrations, he'll have to build four boxes and use eight woofers. Wait a minute ... weren't you going to buy eight woofers? :R



Sonnie said:


> I'm gonna be doing some experimenting this evening with moving one of my SVS subs around the room some. The 1/4 point definitely couldn't be the permanent spot, but it would be interesting to see the different response compared with each other.


 That spot worked for me with one sub, but be sure to try two in the quarter points some time.



Sonnie said:


> Last night when I was measuring the different spots in the ceiling I noticed the response was not a lot different from where my subs are now. There was a tad better extension down to 15hz and the dip I have moved from 40hz to 35hz.


 A spot on the floor and the one above it on the ceiling should measure very much the same.


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## Josuah (Apr 26, 2006)

Hm. The article posted on Audioholics just now about room acoustics states that placing a speaker equidistant from two boundaries creates a problem. So I assume placing a subwoofer at 1/4 in from a corner would create that same problem. ???


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## Jack Gilvey (May 8, 2006)

> That may not be practical is some living rooms, but sonically its the best spot. Fourtuantely I can put mine very near one of the quarter points. I moved my sub there last night, and measured. I lost a few dB comapred to corner placement, but I got the most even FR (pre EQed) of any position I've tried. It really works.


Great stuff. Definitely going to give it a try. I can certainly use a front corner for this placement on a permanent basis for a single sub. Even elevating the sub to 1/4 height might be do-able, at least with one of my little sealed 12" workhorses.



> Why does this matter? Can't I just EQ for any position? Well, I recently tried corner loading and then EQing out the ensuing resonances. Even though I gained a few dB abnd the EQed output measured flat, the sub still sounded boomy. I don't really know why. My guess is that the decay times come in to play since the room continues to ring after the sub stops.


 Yeah, I think you hit it. Eq-ing out some peaks isn't the same as not exciting the modal ringing in the first place.

What do you think this you linked to above...make sense?


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## Ayreonaut (Apr 26, 2006)

Josuah said:


> Hm. The article posted on Audioholics just now about room acoustics states that placing a speaker equidistant from two boundaries creates a problem. So I assume placing a subwoofer at 1/4 in from a corner would create that same problem. ???


Yes it will, if the room is square. If the length and width differ significantly, than the two quarter distances will be sufficiently different to avoid this.


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## Ayreonaut (Apr 26, 2006)

Jack Gilvey said:


> What do you think this you linked to above...make sense?


It would eliminate the width modes if perfectly symmetrical, but would excite all the height and length modes. You'd have to move them 1/4 of the length of the room toward you if you want to avoid exciting the first three length modes too much.


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## Sonnie (Apr 11, 2006)

Well I just did about 12 measurements.... every which way but loose. Very interesting indeed. I'll probably start another thread for it so as I don't invade on yours and it's a little off your topic anyway. I'll say this much... I now have a different sub location for one of my subs and I can definitely live with the location but it ain't 1/4 out into the room.


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## Josuah (Apr 26, 2006)

Ayreonaut said:


> Yes it will, if the room is square. If the length and width differ significantly, than the two quarter distances will be sufficiently different to avoid this.


Hm. Maybe I misunderstood the article. Or maybe I just translated it into here incorrectly. I thought it meant if you place a sub in the corner positioned 3' away from both walls, that is bad.


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## reed.hannebaum (Apr 21, 2006)

I think one thing to keep in mind is that there are different and often competing metrics used for determining the "optimal" location of the subwoofer(s) and speakers in general. Maximum SPL and flat frequency response are two we are all familiar with, and in the link http://world.std.com/~griesngr/asa05.pdf Dr. Griesinger introduces a third measure; sound pressure gradient.

In large venues (theaters, concert halls, cathedrals) reverberant spatial cues arrive at the listener in a chaotic fashion creating a sense of spaciousness. The attack of the sound comes directly to the front of the listener, but the delayed (50-100ms) reverberant sound seems to come from all directions. This seems to have a high esthetic appeal to most listeners. In our relatively small listening rooms we are able to recreate this effect at higher frequencies with multi-channel sound systems.

Often the low frequency sound in small rooms is lacking in sound pressure gradient, creating the perception that the sound is not external, but coming from inside our head. However, the conventional wisdom is that we can't effectively determine the azimuth of low frequency sound in small rooms, but Griesinger contends that this is not true. With a recording with at least 2 channels of uncorrelated reverberant sound, and by properly placing multiple subwoofers on multiple channels, the listener can experience simultaneously both high SPL and high sound pressure gradient; effectively recreating a spacious sound for low frequencies. However, what is sacrificed in his setup is uniform frequency response.

What Griesinger is trying to do may not be desirable to all people and for all types of music, but it does demonstrate that there are many dimensions to this subject. In general it can be said, if we try too hard to optimize only one aspect of sound reproduction, we will usually screw up two or three others pretty bad.


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## Jack Gilvey (May 8, 2006)

Ayreonaut said:


> The distances from the walls and corners will reduce the efficiency, but I looked into mutual coupling some more.
> My room is 12'-10" x 18'-7". So located at the quarter points the four drivers would be 6'-5" and 9'-4" apart. By calcualting the half wavelengths, I found that all four drivers will be mutually coupled together below 60 Hz. This would seem to indicate no loss of power at the really low, power hungry frequencies.


 Found another thread on mutual coupling:

http://www.audioheritage.org/vbulletin/showthread.php?t=5896

EV's formula has coupling occurring at 1/4 wavelength, which halves the frequency at which gain occurs.

Either way, I'm thinking (very generally) that subs so spaced might show relatively more gain/be flatter in a FR sweep compared to closer pairs as it's not occurring over the whole bandwidth, just low, as if adding an acoustic shelving filter the corner of which is determined by the spacing of the driver's centers..

I ran some sweeps yesterday comparing a pair of 12" sealed Adire DPL12's and was surprised to find that not only did Mr. Welti's wall-midpoint placement (side walls in my case) show the most uniform FR across my couch (still lumpy, but flat wasn't his objective) but seemed to show more room lift and a lower F3 than the pair together in a corner. He also mentions better "LF factor" for a pair at the midpoints, not sure if it refers to this phenomenon.

Maybe it's the spacing/coupling? Dunno. Anyway, just something I observed from casual measurements that I found interesting. This position also puts the subs closer to the couch, almost near-field, which would be a benefit with the necessarily smaller subs required for such permanent placement.


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## Ayreonaut (Apr 26, 2006)

Josuah said:


> Hm. Maybe I misunderstood the article. Or maybe I just translated it into here incorrectly. I thought it meant if you place a sub in the corner positioned 3' away from both walls, that is bad.


 3' away from both walls would be "bad" resulting in cancellations. 

Here's an example of a rectangular room (mine).
Length=18.6'
Width=12.8'
Height=8.0'

Multiply each of them by 1/4 to get the suggested sub location. Then the sub ends up 

4.5' off the end wall, 
3.2' off the side wall and 
2.0' off the floor. 

I get three different dimensions which should not create significant cancellations from the corner reflections.

(Mine is on the floor with the center of the woofer at just 0.7', but there's not a problem with the first vertical resonance.)


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## Ayreonaut (Apr 26, 2006)

reed.hannebaum said:


> ...Dr. Griesinger introduces a third measure; sound pressure gradient... What Griesinger is trying to do may not be desirable to all people and for all types of music, but it does demonstrate that there are many dimensions to this subject.





Dr. Griesinger said:


> Successful results depend on: 1. having an input recording that includes at least
> two channels where the reverberation is independently recorded, and thus uncorrelated
> with the other channels...


 This does illustrate yet another goal in what we could aim for. But there are few recordings that meet the requirements set forth by Dr. Griesinger. A lot of stereo music has mono bass and moviel LFE is mono. Dr. Griesinger uses the _difference_ signal between two channels to drive one of the subwoofers in his proposed setup.


reed.hannebaum said:


> In general it can be said, if we try too hard to optimize only one aspect of sound reproduction, we will usually screw up two or three others pretty bad.


 You're right, there are many parameters that define performance, and we each have to figure out our own priorities. I do think that anyone with one sub should try it in a corner, and in the middle of the front wall, and at a quarter point of the room and decide for themself. Likewise, anyone with two or four subs should try some of the arrangements suggested in the whitepapers referenced above and see what they think. A short list of logicaly calculated alternative locations is probably better than the old method of giving the sub your chair and crawling around on your hands and knees!


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## Ayreonaut (Apr 26, 2006)

Jack Gilvey said:


> ...not only did Mr. Welti's wall-midpoint placement (side walls in my case) show the most uniform FR across my couch (still lumpy, but flat wasn't his objective) but seemed to show more room lift and a lower F3 than the pair together in a corner. He also mentions better "LF factor" for a pair at the midpoints, not sure if it refers to this phenomenon.


 Nice work! I wish I had a four identical subs on hand to play with. (My wife is getting sick of me messing around with it, though she loves the results.) 


F Toole said:


> The LF factor metric is simply the sum of the energy over the bandwidth of interest (20-80 Hz here) produced by a given configuration, assuming a correction factor to normalize for the number of subwoofers: 20log10n.


 I think that LF factor is basically total output per sub and is related to the room lift that you mentioned.


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## superfly19 (Jan 19, 2009)

So what ever became of this? Did anyone ever try 4 IB's located a 1/4 from each wall. I'm gearing up to purchase four 18" Fi IB3s and I'm trying to decide if I should place them:

1) all four in one baffle centered between front speakers (about five feet off front wall)
2) two per baffle along the front wall, 1/4 off the front and side wall
3) two per baffle midway along each side wall 1/4 off the side walls
3) four baffles at 1/4 locations at each corner

My room is 27'L X 15'W X 10'H.

The room doesn't currently have a ceiling, it's open to the joists, and it seems any measurements would be skewed by the lack of a ceiling.


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## bambino (Feb 21, 2010)

superfly19 said:


> So what ever became of this? Did anyone ever try 4 IB's located a 1/4 from each wall. I'm gearing up to purchase four 18" Fi IB3s and I'm trying to decide if I should place them:
> 
> 1) all four in one baffle centered between front speakers (about five feet off front wall)
> 2) two per baffle along the front wall, 1/4 off the front and side wall
> ...


I havent read anything about any one doing that method but have read on other forums about the benefits of four subs in different locations around the room on other forums. :dontknow:


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## schmutziger (Jan 5, 2010)

I find this article quite good:
Which hopefully will be seen in my next 5:th post


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## schmutziger (Jan 5, 2010)

i have my subs equidistantly placed along the front wall...
maybe a link in the next post....
www.sonicdesign.se/subplace.html


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## acoustat6 (Mar 7, 2008)

Hello, SOOOOOOO if I have 32 woofers it should be very smooth, is it?

Hint, NOT without alot of bass traps and EQ:bigsmile:

Are these acting differently than if I had 32 drivers scattered around the room? Possibly. But my guess is that the four sub scattered in a room is a white herring (if that is the correct term). IN other words I am not sure there is much truth to it in the real world. 

Bob


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