My multi location subwoofer thread inspired by Welti,Devantier,Geddes !!! - Page 6

Reading this and curious about new amp?

+1 on did you get new amp and what was your results, as I'm doing same in next 2 weeks with my own multi-sub attempt following the geddes approach.
(my drivers are coming in today, 18" SI for 2nd sub and 12" Dayton for 3rd sub, driving them with a iNuke3000. I'll be in build mode for a while - love that sawdust! )

Sorry to hear KG. I hope things work out the best that they can.

That sucks man. Sorry to hear that.

Chris

One thing I find interesting from a historical perspective is the thread on AudioRoundTable.com where Geddes begins to talk about his "random" subwoofer placement. This is virtually the same conversation in a discussionboard thread that Earl Geddes, Duke LeJeune and I had at the Great Plains Audiofest in 2005. In fact, it's the same three of us and a few other ART regulars having this discussion online. So you can really see where Earl was coming from, and how his approach evolved.

Since then, Earl has refined his approach from just saying "place them randomly" to saying "one in the corner, one at a wall midpoint and one in a random spot" to having people do a measurement sweep before installing each sub. But in the end, his approach is still basically the same, but with measurements to help dial it in. That's always a good practice.

Back when we first started talking, I expressed interest in the Welti multisub method. I was starting to form my "flanking sub" approach as an adjunct to the Welti method, because I felt that the upper midbass and lower midrange needed to be addressed too. This is something the flanking sub approach deals with that really isn't addressed in any other multisub configuration, so I felt it should be included alongside the Welti approach.

Earl said he thought Welti mis-assessed the situation entirely by not studying random arrangements. He though that since all of Welti's configurations were symmetrical, an important piece of the puzzle was overlooked. I reminded him that this wasn't really the case - that Welti had used a random structure of subs as a baseline, that it was sort of the target, but it was also a huge number of subs. Welti was seeking the performance of a massive number of randomly placed subs using a small number of subs placed in an organized and repeatable fashion.

The conversations always ended up sort of spiraling around with no real conclusions except that we all agreed once you get past four subs, it doesn't really matter where you put them. This was an important take-away for me.

Another important take-away for me was that everyone was looking at response below 100Hz, and nobody even considered the (more important, IMO) 100-200Hz transition region, where not only room modes live but also self-interference from nearest boundaries. That's why I always recommend flanking subs as first priority, and more distant distributed subs to smooth lower frequency modes, if they are troublesome. Flanking subs will smooth the range down to about 60Hz to 80Hz, and that gets the most objectionable modes as well as the lower midrange notch(es) from nearest-boundary self-interference. More distant subs can be setup per Welti or Geddes, I don't care, nor should anyone else because as we all agree, once you get to four subs, it doesn't matter where you put them.

But anyway, for historical perspective, see the "Sub Placement" thread on AudioRoundTable.com. To reach it, go to the Pi Speakers FAQ, and click on the "Room modes, multisubs and flanking subs" thread. Go to the end and click on the "Geddes configuration" link. There you'll find the original Geddes setup instructions. You'll also find "Welti configuration" and flanking sub setup instructions.

KG; sorry to hear about the big D.
I went thru that in 1994, at the time I saw my world fall apart.
Time heals everything.
best regards.

Aren't Welti and Geddes seeking different "performance"? All of Welti's papers have to do with seat to seat consistency, even if the frequency response doesn't improve, since he is approaching it with the idea that room correction/EQ will be used later. Every time I read about the Geddes method, or its refinements, it is always about improving frequency response in/around the main listening position.

Guess I don't see them as one vs the other, just different techniques based on what you're trying to accomplish. For example, if my priority was a single seat in a 2-channel set-up, I would use the Geddes method. If I wanted consistent bass across my entire sofa or across a couple rows of seating in a dedicated home theatre, I would use Welti's approach.

They're after the same thing - We all are. The goal is seat to seat consistency and smoothest response. It's not terribly hard to get good response in one spot, but the goal of uniform directivity and of multiple subwoofers is to make the whole room (or a large part of it) to be the sweet spot.

I bet most women do not have a clue as to the name or model of the equipment they seem to be fighting for. I feel for you man. May be good if you can show serious value for the stuff you have and give that to her instead of some of the other stuff. Make her think she is taking all $50,000 worth of stuff or more and you keep the house and send her on her way. Theb you can get some nicer stuff.


Good luck,

Keep the faith,


Robert

Waaaaahhhh?!?!

Ahhh man, I'm sorry to hear about this.

Sorry to keep interrupting the divorce condolences, 'cause I know that's difficult. Seems like everything goes under the bus.

I hope a little bit of interruption to get back on topic helps as a useful distraction.

On that note, I want to provide a few more historical links, so readers won't have to dig through posts to find them.

• Sub placement
• The Subwoofer thing
• Computer Simulation of Room Acoustics

The second thread listed here is the one I regularly link to as "Room modes, multisubs and flanking subs". I link to a particular post near the end of that thread where I've summarized and made some conclusions.

You will notice there was a wager mentioned in the simulation thread. Geddes was trying to get some traction with his "random" subwoofer placement scheme, which at the time did not use measurements to optimize. So he proposed a wager that he could prove his random arrangements were always better than Welti arrangements. Geddes lost that bet, and has since revised his strategy to include measurements. Some Geddes placements were as good as Welti placements, but some weren't.

I think now that the Geddes approach is fine, as it has morphed into a procedural placement that includes measurements to find good subwoofer positions. But I think that's a no-brainer too. Of course you can find the good placements when measurements are taken to guide the way.

The original discussions were directed at finding ordered placements that could be done without measurements. Welti found standard configurations that worked pretty well, one being four corners, another being four wall midpoints and a third being two wall midpoints. Geddes had an alternate strategy, which was one corner, one wall midpoint and one random. He actually changed it from time to time, sometimes saying the random sub needed to be elevated off the ground. In any case, Geddes first recommendation was an ordered placement, much like Welti's. But in fact, this original Geddes proposal was not as good as Welti's in most cases.

I say this because I think people should really give Welti more credit for the multisub philosophy. Geddes has good credentials, and it's easy to be impressed with that, but I think Welti is really most responsible for coming up with the important aspects that define the multisub configuration.

Of course, Geddes has become pretty visible in audio message boards in the past five years or so. He has also done a lot in the DIY community over these past few years. And I think it's generally pretty useful stuff, at least interesting food for thought.

With measurements being brought into the mix, I think Geddes recommendations for sub placement are good. But I also think that we all agree that once you get to four subs, placement doesn't matter much. So how much more analysis is needed?

The one thing that always seems to fall through the cracks in multisub discussions is the transition range. With four subs, you can easily get the bass below 100Hz to be smooth. Put 'em just about anywhere as long as they aren't grouped together. But what about the midbass and lower midrange, the region just below the Schroeder frequency? What do you do to smooth the 100-200Hz range? To me, that's even more important than the deep bass range.

My answer, as you can also see evolve in those early multisub discussions, is the helper woofer or flanking sub approach. I find this to be more important than the position of the distributed subs. Best if there are two flanking subs and two distributed subs, whether you want the distributed subs to be placed as Welti would have them or where Geddes would have them. With four subs, it doesn't really matter.

• Helper Woofer Location

Edited by Wayne Parham - 1/11/13 at 11:18am

Not sure I agree, depending on what exactly you mean.

Welti's four-sub configurations are very specific; are you saying that they can be placed randomly?

No, what I'm saying is that most experts agree that once you get to four subs, it doesn't matter very much where you put them, as long as they aren't grouped together.

But you're right that Welti advocates specific positions. He recommends four corners, four wall midpoints or two wall midpoints.

What I'm essentially saying is that Geddes and Welti arrangements yield pretty much the same results, provided four subs are used in each case.

If you look back through the links I posted, you'll see that Geddes has written the same thing, so he agrees with this statement.

when I had only two subs, I got much better results using Geddes approach compared to the Welti mid wall positioning.

FWIW


(never mind the db scale, the audyssey curve is from different measurement/REW SPL meter calibration)

Gold - midpoint front and back wall, per Welti.
Blue - Geddes approach. First sub is in the front left corner, second - 1/3 from the left back corner. The LPF on the second sub is set at approx @ 50Hz and volume is trimmed down compared to the main one.
Purple - after Audyssey XT over geddes

I will have a chance to test the 4 sub claim fairly soon, but with three I still had to spend time and effort finding good position and setting for each sub.

Don't forget this is a spatially averaged metric. You are looking for not just smoothness of response in one place, but smoothness of response over a large area. You can approximate it by taking several measurements in the room and averaging them together.

I think it's reasonable to limit measurements to the seating area, maybe do just a grid of four or sixteen measurements that cover the middle of the room. When you do that, it gives a better picture of the spatially averaged response throughout the listening area. What you really want to know is how much deviation there is from point to point, and the sub positions that give the least variation between all measurement locations are most desirable.

It's kind of like how a speaker can be measured on-axis, but if that's all you measure you don't know the speaker's directivity or its power response. By taking off-axis measurements too, you can get a better picture of the power response, the total energy radiated in all directions. Likewise, the multisub approach is one that provides smooth response throughout the listening area, not just in one spot.

You can find a position for a single sub that makes response reasonably good when measured in one spot, but it won't sound good in other areas of the room. By adding subs, you make the sound better in more places, and the overall coverage is more uniform. This is the gist of the multisub configuration.
Edited by Wayne Parham - 1/11/13 at 7:17pm

Hey Wayne, have you noticed that larger rooms tend to just plain sound (maybe not better but more open) at lower sound pressures given the same system in each. I tend to go for the more airy sound of a larger room even though it might not be quite as loud or flat for that matter. I have listened to quite a lot of systems and I really did not get the ear to ear smile on the really flat ones. I happen to enjoy the Mickey's Philharmagic in Magic Kingdom It is not extremely loud, but when the bass hits, it really feels like a sledgehammer. The sound is really nice for a theater for the masses. I wonder how smooth the response is in that room? Would you happen to know?


Thanks,

Keep cranking,

Robert

I second this. I use a Welti-Parham sub arrangement, because I have to. My mains only dig to 150Hz, and having subs separated far from them would cause problems. The subs are at the 1/4 wall points on the front wall, and cancel all odd order and the 1st even order mode in that direction, meaning each seat in a row has the same response to nearly 100Hz. For the 100-200Hz region, absorption on the front and rear walls as well as 1st reflection points FTW. My front and rear walls need more, btw.

KG,

Sorry bro. Hope you can get this past you as soon as possible.

JSS

Larger rooms definitely have less modal problems. The larger the room, the lower the Schroeder frequency, where the room transitions from modal to statistical. So you don't need multisubs in a really large room. But most homes have rooms small enough they benefit from multisubs.

That being said, A very capable sub system that sounds really really nice in a an auditorium (3000 square feet with 12-15 foot ceilings or higher) might sound ok by itself in a smaller room- (250 square feet with 8 foot ceilings). I have a tendency to believe that some of the good stuff results in the low Hz bass actually having enough room to propogate in the room itself. I did a little work with water and waves in college- I live in New Orleans and I knew the levies were a joke before Katrina. When the wave was given room to go through a whole cycle the resulting movement through that glass trough was very impressive and very powerful. When the wave was formed and we put a barrier in the way, the wave was less impressive as it seemed to break up and not get "moving" so to speak. Do you see sound the same way?

Keep cranking,

Robert

I wanted to add a couple things for everyone to think about.

The first thing has to do with single-seat performance verses seat-to-seat consistency.

After re-reading some of the comments made here, I realize that some have started thinking the multisub approach can be used to smooth the response at a single listening spot, which, of course, it can. But I do want all reading to remember that the whole purpose of the multisub configuration is to smooth the response over a large area, similar to the goal of uniform directivity from a waveguide.

If you just want smooth subwoofer response at a single listening spot, sit on the sub. Easy-peasy. The closer you are to the subwoofer, the more the direct sound will swamp the room modes.

There are also subwoofer positions further away that give reasonably smooth response at a single listening spot. If you just want to optimize single-seat response, you can do it with fewer subs than are required for seat-to-seat consistency, dialing them in with measurements. As I said above, you can even get one sub to provide good single-seat response if you put it in the right place.

Not to trivialize the matter, but it is, in fact, much less difficult to find subwoofer placements that satisfy the sole condition required to achieve smooth response in one spot than it is to satisfy the simultaneous conditions required to achieve smooth response in multiple spots, over a wider listening area. This is the goal of the multisub configuration.

I don't mean to diminish anyone's efforts that have used measurements to direct their placements to satisfy the single-spot requirement. That's always been the way to optimize single subs - Put the microphone in the listening spot and move the sub to find the place where response is smoothest. So adding a sub or two more can be used to make the single-seat response even better.

But again, this is not the goal of the multisub configuration. It is suggested for satisfying the simultaneous goals of seat-to-seat consistency and smoothness of response. Optimizing this requires multiple measurements taken from multiple positions. One can also use simulation software (like CARA) to analyze the room and optimize the layout, taking measurements afterwards for verification.

The other thing I wanted to say was that whether of not you choose Welti or Geddes recommendations for distributed subs, I suggest that you use flanking subs to mitigate self-interference from nearest boundaries and higher-frequency modes. This is actually my biggest focus, not so much whether the distributed subs should be symmetrcially or asymmetrically placed, ordered or directed by measurements or whatever. My biggest focus is on bringing to attention the problems in the 100Hz to 200Hz region, because I think those are often as bad or even worse than the lower-frequency room modes.

It is extremely important to consider the problems of self-interference from nearest boundaries, and this is often overlooked in multisub discussions. People often talk about the response below 100Hz, but then overlook the response between 100Hz and 200Hz, which usually as bad or worse than the lower frequency modes. These are caused by the interaction between the direct sound and the reflection from the nearest boundaries, and they are not actually room modes at all. But they do cause severe response anomalies, and mitigation can be done in similar ways, using flanking subs.

Another way to deal with self-interference from nearest boundaries - the best way, actually - is to either mount the speakers in a soffit or use constant directivity cornerhorns. If the source lies right on the boundary, it will not create self-interference notches and there is nothing to mitigate. That's the best solution.

But for most people, these aren't options and more traditional loudspeaker installations are used, floor-standing or on risers of some sort, pulled a few feet from the wall. In that case, a helper woofer or flanking sub is very effective at smoothing the anomalies that are caused by nearest boundaries.

Good and easily digestible post, Wayne. Thanks.

Hi Wayne;
What about floor and ceiling bounce? Ceiling bounce null usually comes at a frequency within the sub passband while (at least in my room) floor bounce is over 200 Hz and isn't overlapped by the subs. It seems to me that multi-subs should be effective agains ceiling bounce but not floor bounce. Even large horns don't control vertical directivity down low enough to be effective agains floor bounce - with some exceptions e.g. a synergy horn using a paraline to achieve a very narrow vertical response or a line array. But then the floor bounce null is above the Shroeder frequency so does that mean its not all that audible because of the dense room modes in that range?
Jack

Reflections from nearest boundaries are what flanking subs address. The reflection from the wall behind the speakers, floor bounce and ceiling bounce generally being the closest ones.

The most troublesome anomalies are those in the 100Hz to 200Hz range, because they're hardest to deal with any other way. Distributed multisubs are too far away to blend in this region, they operate below that band. Absorbent materials are fine for higher frequency anomalies, but they're generally ineffective down low, in that 100-200Hz region. So it's a tough transition region, too high for distributed multisubs and too low for absorbent materials.

That's the purpose of flanking subs. They are close enough to not be localizable, yet far enough away to fill in "holes" from self-interference from the mains. At positions where the reflection from a boundary is 180° after the direct sound, the flanking sub has a different phase relationship and so does not cancel. One drops out, but not both.

Wayne, please correct me if I am wrong, but is not the problem flanking subs are meant to address is the self interference caused by reflections from nearby boundaries, aka SBIR? This is different than floor and ceiling bounce which are listening position depended and should be treated the same way any first point reflections are?

I found the info and calculators hosted at the following URL very useful.

http://tripp.com.au/avfcbm.htm

The flanking sub approach smooths problems caused by both high-frequency modes and by self-interference from nearest boundaries. Either kind of anomaly in the 100-200Hz range is addressed by flanking subs, because the distances involved work at those wavelengths.

But you're right to highlight the differences. Room modes do not shift frequency with listener movement. Self-interference notches often do, depending on the distance relationships between source, listener and boundary positions. These self-interference notches are caused by a 180° phase shift between the direct sound and the reflection from the boundary. Room modes are standing waves, and there is a difference.

So flanking subs mitigate both problems, provided the anomalies fall in the frequency range where the wavelengths are approximately equal to the spacing between mains, flanking subs and boundaries.

That would be ideal, but I don't see Geddes talking about using subwoofer placement to achieve seat to seat consistency, instead choosing to measure/optimize at one location (unless he's changed his mind, yet again). From a post on his own forum:

"I am now not so adamant that spatial averaging is necessary at these LFs. The differences over a small area are just not that great and it now seems that a single median point is a good representation of the nearby area - at least up to 150 Hz or so."

By comparison, Welti's research is primarily concerned with using subwoofer placement to minimize spatial variation, only mentioning flatter/smoother response as something to be done with an equalizer after the fact (and outside the scope of his papers).

"A fundamental precept of this paper is that optimization is achieved when the responses at multiple seats are as similar to each other as possible. Making them flat is then simply a matter of global equalization. This consistency of amplitude response is a fundamental goal for the current investigation."

Based on that, they each seem to have different performance goals for what they consider subwoofer optimization. While they would both likely be happy achieving smooth bass response and having it consistent across the seating area (wouldn't we all), their approach to subwoofer placement appears to be in service of different results.