Multiple Subwoofer Placement

Gedlee

WBF Technical Expert
Jul 21, 2010
364
0
0
Novi, MI
What is the methodology to select the LP frequency for the different subs, assuming one has access to high resolution acoustic measurement equipment?
This gets all tied up into the whole process, and I am not sure that it can be condensed down all that well. Marcus Mehlau reference earlier is the best.

Basically you feed the subs with an identical signal derived as the sum of all the channels (LP'd or not, but if LP'd then do it above 150 Hz.). Most receivers send out such a signal as LFE, as an option. If not, then you must do this somehow. My receiver does so I have not had the need to do anything different.

Take this signal into all the subs if active, or preferably a DCX2496 which then drives N rack amps (or N/2 if stereo amps), where N is the number of subs. The DCX can do up to three subs, more than that requires a second DCX. I prefer rack amps as I have had endless problems with plate amps.

I start by measureing the mains, then add the nearest sub, which will tend to end up the one with the most output. Adjust the sub for best "mix" with the mains as a spatial average. Now add in the next closest sub. You will likely have to tweek the existing subs setting for the best sum of the two subs and the mains. Keep adding subs as desired/required. In general, each new sub should have less output than the previous one. If not, then I suspect that something is not right since each new sub should only be a smaller correction to the existing result.

If it sounds like this is not a precise formula, then that is correct - there is no precise formula that works every time, just as there are no two rooms that act the same at LFs. So each process and result is unique. Thats just the way it is.
 

cjfrbw

Active Member
Apr 20, 2010
2,173
7
38
Pleasanton, CA
I have had the three subwoofer array for a couple of weeks now and have to say I really enjoy it. The third above the ear subwoofer is not a minor contributor. Going from one subwoofer to two was a big leap in bass quality, but the the third was probably another significant leap. Note that the emphasis is on "quality", not quantity.

I guess my array is an "unpolished" Geddes array ( I surmise Dr. Geddes has some proprietary implementations of his own), but works very well nontheless.

I have chosen to keep the YPAO Yamaha digital equalization in the equation because it tightens things up. Bass glissandos are better defined with the equalization in place. The bass can be turned up very loud and still enhances the main sound image. Those clouds of bass that can sully the sound staging seem to be much lessened.

I imagine that BassPig's crushing wall of sound bass array or Gary Koh's Genesis towers are the ultimate bass solution if you can fit them in your room. The Geddes array seems to be a very nice alternative for rooms that can fit three standard subwoofers.
 
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dougsmith

New Member
Sep 5, 2010
50
0
0
Gloucester, MA
I currently have two subs... I've been considering a third for a while but haven't yet convinced myself that the improvement will be worth it. My current response is flat down to 20 Hz (actually a little better than flat with a house curve) and quite uniform as well (+/- 3 db or so across the main listening area). Having one more sub that goes lower would probably enable somewhat higher volume with lower distortion at the bottom end and decent output down into the low teens or so. This wouldn't have much impact on music but might be fun for watching those occasional movies where you really want to get rattled. I posted my layout in another thread somewhere, but may as well post it here as well. I'm not sure exactly what it is about my room that makes it so well behaved - perhaps combination of good size and the cathedral ceiling.

.
 

cjfrbw

Active Member
Apr 20, 2010
2,173
7
38
Pleasanton, CA
I would suggest you "borrow" a small powered subwoofer that is easy to place that you could play with and place it in series with the one to back the left (??) of your apparent seating position. Place the new one above the coffee table by the piano above ear level. You may not decide to keep it, but you might find the result interesting.

I also did not think the third subwoofer could make a difference, but it did. Again, it is in terms of quality of bass and not just sheer quantity.
 

dougsmith

New Member
Sep 5, 2010
50
0
0
Gloucester, MA
I agree completely. The two subs I have are 12"-ers in round bandpass enclosures with the ports facing up. My plan was to replace the one up front with an IB going through the floor and move the one that is there now to the location you suggested. I think it would probably sound better, but I am giving the idea some time to incubate before I start cutting holes in the floor.
 

DWR

New Member
Jul 26, 2010
262
0
0
Western burbs of Detroit
Good thinking on that incubation period there Doug, cutting holes in the floor is serious, hahahaha. On a little more serious note if you go to this site http://www.hometheatershack.com/forums/diy-subwoofers/ you will find some useful info on IB subs and some good conversation on how some of their projects worked out for them. I learned quite a lot on how to design and build subs from these people and am very pleased with my results.

Dan
 

FrantzM

Member Sponsor & WBF Founding Member
Apr 20, 2010
6,469
0
0
Hi

If you can go IB by all means do go IB.. If you want to know more about IB go to this web site The Cult of the Infinitely Baffled Lot of info .. What will come of this is how easy it is to implement an IB. They are invisible and extremely high performance .. Low distortion .. Low cost $1000 for drivers and amps will provide performance that most commercial (if not all) can't even begin to think of ...
I am not much of a fan to place the sub behind the listening position but I guess you can make it work with the DCX 2496. As it's been suggested in this thread borrow some subwoofers and experiment with placement then go ahead .. cut the holes.. You will be glad you did .. :D
 

dougsmith

New Member
Sep 5, 2010
50
0
0
Gloucester, MA
Yes, I have spent quite a bit of time over at "The Cult" and already got some feedback in a thread I started there a while back. I am actually all set with amps since the rig I built was designed to accommodate a third sub from the outset. All I really need are the drivers and one or two nice wrought iron grills. Four IB15s would do the trick at around $500. Subs near or behind the listening position have been proven to work fine as long as they are appropriately level matched and don't play at too high a frequency. The ones I have start tailing off at around 100 Hz or so, so that shouldn't be a problem in my case.
 
Dec 27, 2010
68
2
8
My plan was to replace the one up front with an IB going through the floor and move the one that is there now to the location you suggested. I think it would probably sound better, but I am giving the idea some time to incubate before I start cutting holes in the floor.
What a great idea and I'm in a position to create some underfloor subs as I am currently having an extension built, which is where my system will be set up. The wooden floor is not yet laid so right now is the time to do this. Can anyone point me to some info/guidance on making underfloor subs? Edit - forget that question as having looked at the Cult website I realise that its all about such installations.
 
Last edited:
Dec 27, 2010
68
2
8
No one has the answer from the "subjective - can you hear it" approach since to my knowledge no one has done any valiud studies in this reagrd. So theory is all we have. Theoretically it doesn't make the slighteset difference what the distances are since in the end all that matters is how everything sums together. Our hearing does not have the ability to distinguish 'time of arrival" differences at these short distances and LFs.
Earl
I am constructing 2 IBs in my ceiling, 1 in the corner just behind where one of my Abbeys will be and the second will be off to one side of my listening position. For my 3rd sub I have a choice; I can use a conventional sub somewhere in front of my listening position or I can install a 3rd IB in the floor about 8 feet behind my listening position. Given what you say in the quote, do I take it that you do not see an issue with having a sub behind the listening position? Any other thoughts on my 2 options for the 3rd sub?
TIA
Ian
 

Gedlee

WBF Technical Expert
Jul 21, 2010
364
0
0
Novi, MI
Earl
I am constructing 2 IBs in my ceiling, 1 in the corner just behind where one of my Abbeys will be and the second will be off to one side of my listening position. For my 3rd sub I have a choice; I can use a conventional sub somewhere in front of my listening position or I can install a 3rd IB in the floor about 8 feet behind my listening position. Given what you say in the quote, do I take it that you do not see an issue with having a sub behind the listening position? Any other thoughts on my 2 options for the 3rd sub?
TIA
Ian
I have a sub behind the listening position, its not a problem. This sub should be setup last since the last sub added is usually ste at the lowest gain.
 
Dec 27, 2010
68
2
8
Earl
OK my thinking on my IBs has moved on and it has prompted some further questions. My original plan was to follow the received wisdom on IBs and moiunt 2 x 15" drivers facing one another in a manifold in the ceiling. (Mounting them in this way is said to reduce mechanical vibration.) I was going to have 2 such manifolds, each manifold being fed by a separate mono signal; in other words the 2 manifolds would make up 2 of my required 3 subs. However, due to space restrictions I have had to re-think how I mount the drivers and I am now looking at mounting them horizontally i.e. firing straight down. (I have done the sag calculations and the drivers are well within the 5% test for horizontal mounting.) So this means I no longer have to wire them in pairs (unless I choose to) and can now put each driver anywhere I like. So I could have 4 separate subs firing down from my ceiling (as noted elsewhere my DCX 2496 can output up to 6 mono channels). From what I've read of the multiple sub theory, the more the better and you only recommend 3 as a pragmatic solution and because you believe there are only small incremental benefits by going to more than 3 subs. Am I right in saying that and if so would you agree that having the 4 separate drivers dispersed across the ceiling should be better than mounting them and wiring them in pairs? In case its important, all these drivers are backing into the same void/roof space.
 

Gedlee

WBF Technical Expert
Jul 21, 2010
364
0
0
Novi, MI
Earl
OK my thinking on my IBs has moved on and it has prompted some further questions. My original plan was to follow the received wisdom on IBs and moiunt 2 x 15" drivers facing one another in a manifold in the ceiling. (Mounting them in this way is said to reduce mechanical vibration.) I was going to have 2 such manifolds, each manifold being fed by a separate mono signal; in other words the 2 manifolds would make up 2 of my required 3 subs. However, due to space restrictions I have had to re-think how I mount the drivers and I am now looking at mounting them horizontally i.e. firing straight down. (I have done the sag calculations and the drivers are well within the 5% test for horizontal mounting.) So this means I no longer have to wire them in pairs (unless I choose to) and can now put each driver anywhere I like. So I could have 4 separate subs firing down from my ceiling (as noted elsewhere my DCX 2496 can output up to 6 mono channels). From what I've read of the multiple sub theory, the more the better and you only recommend 3 as a pragmatic solution and because you believe there are only small incremental benefits by going to more than 3 subs. Am I right in saying that and if so would you agree that having the 4 separate drivers dispersed across the ceiling should be better than mounting them and wiring them in pairs? In case its important, all these drivers are backing into the same void/roof space.
The vibration cancellation could be a good thing, but beyond that I don't see any advantage to using pairs. I've never done IB so I don;t have a lot of experience to draw on.
 

keithyates

Industry Expert
Apr 25, 2010
11
0
0
Earl, [snip] I can now put each [subwoofer] driver anywhere I like. So I could have 4 separate subs firing down from my ceiling ... From what I've read of the multiple sub theory, the more the better and you only recommend 3 as a pragmatic solution and because you believe there are only small incremental benefits by going to more than 3 subs. Am I right in saying that and if so would you agree that having the 4 separate drivers dispersed across the ceiling should be better than mounting them and wiring them in pairs? In case its important, all these drivers are backing into the same void/roof space.
Just saw this interesting thread. At CES Earl, Pat Turnmire and I talked briefly about the approach my team developed to provide physics-based answers to exactly this kind of question. Strikes me that at least a few readers here might be interested as well. To keep it brief, I'll stick to the most basic questions, "How many subs, and where, exactly?", and ignore other relevant ones like "How should the delay settings, filters, gains, polarity etc. be set on individual subs?"

Goal is to model the actual physics of what's happening so the optimum subwfr config (quantity & locations) and LF acoustic treatments can be found for reducing the severity of seat-to-seat response variations. We can do simple optimization, where we assume all subs will play the same signal at the same time (as if they were all paralleled off the same virtual amp), or we can include in the optimization process any number of electroacoustic features, like individual sub settings for delay, shading (different output level settings), EQ, polarity, etc.

In crude outline, our process runs like this:
1: Create 3D CAD model of the room, including all the "geometric inconveniences" that simple mode calculators can't deal with, e.g. L-shape room plans, openings into hallways etc., alcoves, niches, doors, windows, seating platforms, soffits, sloping/cathedral ceilings, pony walls, big fluffy sofas, and so on.
2: Tag all locations in room where a pressure source (subwfr) can be physically accommodated and aesthetically tolerated, and set a max practical limit on the number of subs the client would consider buying (say, 3-6). (Side note: The commercial availability of high-quality enclosed, in-wall subs -- Wisdom S90i, Genelec 5041A, JL Audio Fathom IWS, and JBL Synthesis S4S spring to mind -- makes it a LOT easier to avoid lo-fi bass afflicting most rooms because they make it possible to drive the room's pressure map cleanly from multiple locations.)
3: Bring the model into an FEA (finite-element) package, feature/de-feature it as needed, mesh it and assign physical properties (Young's modulus, Poisson's ratio, acoustic impedance, etc.) to the walls, floors, ceilings, doors, etc.
4: Transfer the model into a CFD (computational fluid dynamics) package and create the resulting pressure plot throughout the listening zone at all frequencies below the Schroeder freq in whatever resolution (e.g. 15-120Hz in 0.1Hz increments).
5: Bring the results into mathematical analysis package running optimization routines to find the highest-performing sub configuration -- for example, 5 subs total, with 1 each in locations 7, 12, 23, 29 and 34.
6: Since we know WHERE in space all the antinodes are of all modes (axial, tangential & oblique), we can model the efficacy of specific LF treatments to further reduce standing wave ratios and increase spatio-pressure uniformity.

All of this is before we show up with the DSP/equalizer. In our experience, there's no substitute for getting the basic physics right!

--Keith
 

dougsmith

New Member
Sep 5, 2010
50
0
0
Gloucester, MA
Just saw this interesting thread. At CES Earl, Pat Turnmire and I talked briefly about the approach my team developed to provide physics-based answers to exactly this kind of question. Strikes me that at least a few readers here might be interested as well. To keep it brief, I'll stick to the most basic questions, "How many subs, and where, exactly?", and ignore other relevant ones like "How should the delay settings, filters, gains, polarity etc. be set on individual subs?" ...
All of this is before we show up with the DSP/equalizer. In our experience, there's no substitute for getting the basic physics right!
--Keith
How long does it take and how well does it work?
 
May 30, 2010
14,269
130
63
Portugal
Just saw this interesting thread. At CES Earl, Pat Turnmire and I talked briefly about the approach my team developed to provide physics-based answers to exactly this kind of question. Strikes me that at least a few readers here might be interested as well. To keep it brief, I'll stick to the most basic questions, "How many subs, and where, exactly?", and ignore other relevant ones like "How should the delay settings, filters, gains, polarity etc. be set on individual subs?" (...)
Keith,

It was a pleasure for me to read you at this forum - I still keep two of my oldest Stereophile issues because of your well written articles "A Matter of Diffusion" and the masterpiece "Are Audiophiles Music Lovers?".

Your answer also shows something I also belief - that in order to apply science to real audio rooms you must have enormous resources, a deep scientific knowledge and adequate tools, as real life acoustics needs accurate models and most amateurish simplifications are not compatible with science.

My income and audio ambitions will not allow me to use your services, but it is always good to know that real experts are available to those who want the state of the art. Happily for the remaining of us, the empirical expertise used by knowledge people can also lead to very acceptable results!
 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
319
2
18
42
Chicago, IL
www.seatonsound.net
Keith,

Your answer also shows something I also belief - that in order to apply science to real audio rooms you must have enormous resources, a deep scientific knowledge and adequate tools, as real life acoustics needs accurate models and most amateurish simplifications are not compatible with science.

My income and audio ambitions will not allow me to use your services, but it is always good to know that real experts are available to those who want the state of the art. Happily for the remaining of us, the empirical expertise used by knowledge people can also lead to very acceptable results!
Hi micro',

I'd say that's a bit extreme in dismissing all quantitative measures if you can't get down to the most minute of accuracy.

A bit of dissecting of the process Keith outlined above and at some point further examination of the results could yield some very useful targets and checkpoints we might be able to test for. In other words, there are often ways to trade lots of testing, moving, re-testing, for the math and computational power. No, such efforts won't allow us to check as many possible solutions, nor have quite the confidence that all efforts were exhausted, but they can certainly result in much better results than just random guessing and twiddling of knobs.
 
May 30, 2010
14,269
130
63
Portugal
Hi micro',

I'd say that's a bit extreme in dismissing all quantitative measures if you can't get down to the most minute of accuracy.

A bit of dissecting of the process Keith outlined above and at some point further examination of the results could yield some very useful targets and checkpoints we might be able to test for. In other words, there are often ways to trade lots of testing, moving, re-testing, for the math and computational power. No, such efforts won't allow us to check as many possible solutions, nor have quite the confidence that all efforts were exhausted, but they can certainly result in much better results than just random guessing and twiddling of knobs.
Mark,

Please read carefully what I said - I am not saying that measurements are not useful, but that simple measurements can not be used to fully describe the system. You have to use empirical knowledge to associate value to this measurements and I congratulate people who manage to do it. Please remember that to be considered scientific a problem must include a mathematical formulation - it is what Keith does in is approach.

Room acoustics is a boundary problem - solving it is not an easy job, even for bass alone. Toole includes some very interesting thoughts about it in his reference book "Sound Reproduction".
 

Nyal Mellor

Industry Expert
Jul 14, 2010
591
2
0
SF Bay Area, CA, USA
Just saw this interesting thread. At CES Earl, Pat Turnmire and I talked briefly about the approach my team developed to provide physics-based answers to exactly this kind of question. Strikes me that at least a few readers here might be interested as well. To keep it brief, I'll stick to the most basic questions, "How many subs, and where, exactly?", and ignore other relevant ones like "How should the delay settings, filters, gains, polarity etc. be set on individual subs?"

Goal is to model the actual physics of what's happening so the optimum subwfr config (quantity & locations) and LF acoustic treatments can be found for reducing the severity of seat-to-seat response variations. We can do simple optimization, where we assume all subs will play the same signal at the same time (as if they were all paralleled off the same virtual amp), or we can include in the optimization process any number of electroacoustic features, like individual sub settings for delay, shading (different output level settings), EQ, polarity, etc.

In crude outline, our process runs like this:
1: Create 3D CAD model of the room, including all the "geometric inconveniences" that simple mode calculators can't deal with, e.g. L-shape room plans, openings into hallways etc., alcoves, niches, doors, windows, seating platforms, soffits, sloping/cathedral ceilings, pony walls, big fluffy sofas, and so on.
2: Tag all locations in room where a pressure source (subwfr) can be physically accommodated and aesthetically tolerated, and set a max practical limit on the number of subs the client would consider buying (say, 3-6). (Side note: The commercial availability of high-quality enclosed, in-wall subs -- Wisdom S90i, Genelec 5041A, JL Audio Fathom IWS, and JBL Synthesis S4S spring to mind -- makes it a LOT easier to avoid lo-fi bass afflicting most rooms because they make it possible to drive the room's pressure map cleanly from multiple locations.)
3: Bring the model into an FEA (finite-element) package, feature/de-feature it as needed, mesh it and assign physical properties (Young's modulus, Poisson's ratio, acoustic impedance, etc.) to the walls, floors, ceilings, doors, etc.
4: Transfer the model into a CFD (computational fluid dynamics) package and create the resulting pressure plot throughout the listening zone at all frequencies below the Schroeder freq in whatever resolution (e.g. 15-120Hz in 0.1Hz increments).
5: Bring the results into mathematical analysis package running optimization routines to find the highest-performing sub configuration -- for example, 5 subs total, with 1 each in locations 7, 12, 23, 29 and 34.
6: Since we know WHERE in space all the antinodes are of all modes (axial, tangential & oblique), we can model the efficacy of specific LF treatments to further reduce standing wave ratios and increase spatio-pressure uniformity.

All of this is before we show up with the DSP/equalizer. In our experience, there's no substitute for getting the basic physics right!

--Keith
Hi Keith. Glad to have you on the forum!

Wow that is a lot of mathematical study but one that seems very sensible. How long does that process take in man hours?

On a philosophical note, this and Mark Seaton's response kind of reminds me of cooking. There was an article in the latest Wired where the ex CTO of Microsoft Nathan Myhrvold set up an experimental kitchen to reduce cooking down to its base physics and produced a 6 volume cooking book ('Modernist Cuisine') selling for $650. It definitely seems like it advanced the science and understanding of why something would taste good or not. However we all know that we can get amazing and just as good results from a more traditional empirical style of cooking using The Green Kitchen cookbook for $25. Interestingly the article concludes by saying some of the main proponents of the scientific style of cooking like Ferran Adria are getting out of it and going for something more empirical.

When is the AES paper coming Keith?
 

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