Multiple Small Subs - Geddes Approach

[Pan]

Originally posted by cap'n todd
It's about the intelligent use and setup of multiple subs, i.e. optimization.

And that Swedish article was about unintelligence and suboptimum placements of multiple subs? :-/


/Peter

[breez]

Originally posted by cap'n todd
I do not average my measurements. I look at each individually, thus I can talk about seat to seat consistancy. In fact it is the cornerstone of my approach. You only have one measurement (i,e. one response curve), once you average the six. It does not take a PHD in statistics to figure out that you cannot say what the seat to seat consistancy is in your optimization, since you only have ONE measurement. If you are looking at the six measurements separately then thats another thing.

I agree with this, but there is probably some correlation between smooth average response and spatially smooth response. If you have some data on hand, could you check what kind of correlation we have between rankings of MSV and rankings of the other metric (the one for smooth average)? It could be interesting.

Another question regarding metrics, does it make more sense to use dB values or just raw magnitude values? dB would emphasize nulls more, with say 0 and -20 dB averaging to -10 dB while 1 and 0.01 would average to about 0.5 (-6 dB).

[john k...]

This is probably known my many here. But stating at the beginning might help. The thing about low frequency response in acoustically small rooms is that the modes are widely spaced. The result is that there are deviation form the sources free field transfer function which arise from two different mechanisms 1) peaks arise due to energizing under-damped modal resonances. 2) dip and/or nulls arise due to cancellation between out of phase modes. The key here is that the peaks are primarily single mode phenomena where as the dips are multi-mode. Eliminating single mode behavior is fairly easy form a theoretical point of vise, by locating sources at position where modes have nodes and by locating multiple sources which cancel modes (equal amplitude but inverted phase). When a mode is so canceled it will not contribute to the SPL at any location in a room, except when the position is very close to one source. Eliminating the dips is a little harder because of the multi mode dependence. To remove a dip at low frequency requires that all the offending modes be canceled. Simply moving sources around so that the response looks smooth in one position or another can not guarantee that the response will be smooth in another position because the phase relationships between modes, as well as the amplitude contribution from the different modes, is position dependent. SO, again, to guarantee smooth response over a wide area the requirement is the eliminate the contribution of all the modes which make significant contributions to the SPL. Todd's,s 4 woofer array is such an approach. In Todd's approach all odd number modes in the L and W directions theoretically cancel as does the first even order mode in the L and W directions. Along with this a number of tangential and oblique modes are also canceled. However, as Todd note in his paper, modes in the vertical direction are not addressed. If you look at simulation of Todd's approach, changing the vertical position, then there is significant change in the response as the vertical position changes. If listening height is constant this isn't a big deal, but there would certainly be a potential to hear different bass response when sitting or standing. The DBA basically take an array such as Todd's and places it of the front wall, canceling modes in the W and H directions. The second, identical array on the back wall, with inverted phase and delayed, is an attempt to remove low frequency reflections from the L direction, thus eliminating L direction modes. It goes the next step and yield, in theory, very uniform and smooth response independent of position, except when very close to a source. Both Todd's method and the DBA take advantage of symmetries in the modal behavior that exists in a rectangular room. With a 3 woofer array in a rectangular room there is a lack of symmetry. Once the first woofer is placed, say in a corner, when the second woofer is placed , if it is against a wall (doesn't matter if displace vertically or horizontally from a corner) the odd modes in the across room direction (source to source) will theoretically cancel. However, when the third woofer is place, It will re-excite the modes canceled by the first two woofers unless it is place at a node associated with the canceled modes. When simulation of the 3 woofer array is attempted it is possible to find position where the response is fairly consistent over a moderate listening area, but it is not very smooth. The only way to effective smooth the response is to add room damping and/or equalization. However, since many modes are still excited the damping or equalization can not correct for the position dependent variation in phase between contributing mode. Todd's approach and the DBA are not directly translatable to non rectangular rooms. Multi subs with arbitrary placement, whether 1, 2, 3, ..., n may certainly prove more effective in room of odd geometry but I see no scientific method here. It is ad hoc pushing source around until you get something acceptable. While an acceptable result is likely obtainable, there is little of a method here. It's like saying the way to grandma's house is through the woods. But which path to take is left as a mystery.

Breez said, "but there is probably some correlation between smooth average response and spatially smooth response. " Actually there is none at all. To have a correlation you need to know the number of measurement, the standard deviation and the mean, at least, before you can say much about the response. You also need to know something about the measurement. A long length MLS or impulse response may yield a different result than a swept or stepped sine and certainly a different result than a 1/3 octave pink or white noise measurement.

[pos]

Originally posted by john k...

Todd's approach and the DBA are not directly translatable to non rectangular rooms. Multi subs with arbitrary placement, whether 1, 2, 3, ..., n may certainly prove more effective in room of odd geometry but I see no scientific method here.

Any real room (with furnitures, open areas, leaks, boundaries with different properties, and with people in it...) will act to some degree as a non rectangular one.
So what it the interest of using simulations if you cannot accurately model the room in the first place?
In such a situation Geddes' approch seems to be the most sensible one.

[breez]

Originally posted by john k...
Breez said, "but there is probably some correlation between smooth average response and spatially smooth response. " Actually there is none at all. To have a correlation you need to know the number of measurement, the standard deviation and the mean, at least, before you can say much about the response. You also need to know something about the measurement. A long length MLS or impulse response may yield a different result than a swept or stepped sine and certainly a different result than a 1/3 octave pink or white noise measurement.

By correlation I meant between the rankings generated by Welti's SFM for different configurations.

http://en.wikipedia.org/wiki/Spearma...on_coefficient

High correlation would mean that the best configurations for spatially smooth result are also among the best for smooth average and vice versa.

[David Gatti]

How about this for a compromise :

- 2 large main floorstanders with 12" woofers, and good low frequency performance, i.e good enough to be considered 2 subs.

PLUS

- a single large subwoofer placed at other location

Effectively 3 subwoofers in "random" locations, but much simpler and a more efficient use of space & resources.

[breez]

Originally posted by David Gatti
How about this for a compromise :

- 2 large main floorstanders with 12" woofers, and good low frequency performance, i.e good enough to be considered 2 subs.

PLUS

- a single large subwoofer placed at other location

Effectively 3 subwoofers in "random" locations, but much simpler and a more efficient use of space & resources.

3 LF sources = good. What is missing are the phase/delay and gain settings unless the woofers are actively crossed low (say < 150 Hz). Although I guess the gain could be tackled with a relatively simple shelving filter if the crossover is more conventional.

[Pan]

And maybe make those large mains WMTW or WMTMW so that the main pair effectively has four sources.

One source close to floor and the second about 1/4 down from the roof (if possible) to deal with 1st and 2nd vertical axial modes.


/Peter

[gedlee]

Originally posted by cap'n todd



Earl, I think you misunderstand. I do not average my measurements. I look at each individually, thus I can talk about seat to seat consistancy. In fact it is the cornerstone of my approach. You only have one measurement (i,e. one response curve), once you average the six. It does not take a PHD in statistics to figure out that you cannot say what the seat to seat consistancy is in your optimization, since you only have ONE measurement. If you are looking at the six measurements separately then thats another thing.

No Todd, I undersood completely, I always have, and again, I think that it is you who do not. It makes no difference WHEN one averages the signals in a linear system, first, or in the end. When one seeks the minimum spectral and spatial variance - seat to seat - this can be gotten with an average of the seats first and then fit the parameters or fit the parameters and then average the seats. Its all the same since in the edn its a single sound field that results and all paths to this result will be basically equivalent.

You are worrying about small details that end up making no difference. Its like "Do I average the dB or do I average the linear values?" Well, it turns out that in the limit of a number of measures this too makes little to no difference (been there done that). Maybe what you are missing is that spatial variance, when taken over a number of spatial points, becomes a spectral variance when the data is averaged. Hence I can now solve the problem as simply a spectral one.

What we are both seeking is the same thing, but we get there in different ways. Granted yours is precise, accurate and yields LOTS of data, its also very expensive and time consuming. Mine less accurate, not as precise, yields little data but its VERY inexpensive and easy. If they both end up with "good enough" ... well I think that choice is obvious.

You are right that I "could" do a scientific study of this technique, but thats not going to happen. I don't work in research any more and there is no time for academic studies. Lidia and I are planning (when we get the time) to do some psychoacoustic studies, but they will be on the perception of early reflections, which, I believe, is the more important aspect of the audio problem. I don't completely agree with Floyd on this topic. You and I agree on the goal, just not how to get there. Floyd and my differences are more fundamental.

[dlr]

Originally posted by gedlee What we are both seeking is the same thing, but we get there in different ways. Granted yours is precise, accurate and yields LOTS of data, its also very expensive and time consuming. Mine less accurate, not as precise, yields little data but its VERY inexpensive and easy. If they both end up with "good enough" ... well I think that choice is obvious.

I've been following this thread with great interest. I have no input on the technical aspect other than to say that when I used a sub, I found that running the mains full range and crossing the sub at 45 worked best, just the old trial-and-error method and for sub placement in the room as well.

I would like to make a few comments, however. I for one am interested in both approaches being presented here, the "good enough" approach of optimizing for limited subs and placement as well as the effort to analyze the room and optimization from the theoretical perspective. The former is how one eventually must finish a scheme to be used, but I see no reason to minimize or trivialize the latter approach of looking for a theoretical basis to find an optimum.

It seems to me that both should be part of the process, if one has access to software to further it. Were I to do multiple subs, I would much prefer to use the software for the initial phase, that being to select the most likely placement that is optimal. Then I would move on to the next step to optimize the driver responses with the necessary EQ.

My point is that the optimal is the goal, in all aspects, placement as well as the EQ afterwards, with whatever constraints on placement being set for whatever the reasons. This is no different than how I design mains. Each one has different requirements for driver placement with EQ applied afterwards. Using CAD for diffraction analysis for baffle dimensions and speaker placement is all done before building it, then optimizing the EQ of each driver after measuring. This is how I would prefer to address room subs and EQ as well. Software that could be used as a guide to optimal placement should come first if it's available. The followup EQ methodology is also indispensible. They should go hand in hand.

I really don't see why there's any animosity between the two camps here. The goal is the same. I'm learning from both sides and would not likely follow this if only one aspect were to be addressed. It would be rather boring, to be honest, and not nearly as informative.

Dave