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Old 30th January 2009, 06:34 PM   #501
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Quote:
Originally posted by MartinQ
I did a quick scan of the previous pages but could not find what SFM stands for.

???

SFM = Sound Field Management
A multisub approach with goals and results similar to Dr. Geddes's, but involves a lot of computation in attempt to optimize sub placement and settings. Like Earl's, it works well for the not-a-shoebox room shapes** many of us have to deal with.
The computation required for SFM is out-of reach for many/most of us.

**(You probably already know, but the Harmon/JBL symmetrical placement approach and the DBA approach both require shoebox rooms).
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Old 30th January 2009, 06:34 PM   #502
gedlee is offline gedlee  United States
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Quote:
Originally posted by john k...

I think Earl misspoke a little. You can not recover the direct sound just by increasing the number of modes.
John

I did not misspeak, you are incorrect here, since you can recover the direct field with more "terms" in the summation, i.e. more modes.

I also don't accept your example as proving anything.

Todd

In Morse he shows how the Greens function series solution, what everyone uses in their room simulations, can be modified by subtracting off the singularity at the source point. This term, when subtracted off represents the direct field since it is basically the free field Green's function. The summation that remains after you subtract off this direct field term is dominated by the lower order modes and converges very quickly, but the series is now different than the usual one.
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Old 30th January 2009, 06:36 PM   #503
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Quote:
Originally posted by john k...


I think Earl misspoke a little. You can not recover the direct sound just by increasing the number of modes. What you recover is wider frequency response. The direct sound is only recovered when the listening position is very close to the source position just as you would recover the direct sound with a near field measurement.

Perhaps we misunderstand. For me, anyway, I'm not trying to "recover" the direct sound per se (i.e. separate it out). I'm just interested in whether or not it is included in my simulation (and yes it's true that that only matters when you are close to the source, but still...). And as for bandwidth, it's not really an issue. I only need to go up to order 7 or so to get the BW I need for subwoofer investigations in most rooms I work with.

I'm intested in your simulations. Are you then adding the "direct" term in? the one we seem to agree is a fudge? I did simulations where I went from order 10 to order 35 with the source and receiver 4" apart and saw no significant difference. So i conclude that you really need to go to high order to have the direct show up. Of course you are even closer to the source and going up to order 100. Is that sufficient to include the direct (which your plots show), or are you adding the "fudge"?
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Old 30th January 2009, 06:48 PM   #504
gedlee is offline gedlee  United States
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Quote:
Originally posted by cap'n todd



Perhaps we misunderstand. For me, anyway, I'm not trying to "recover" the direct sound per se (i.e. separate it out). I'm just interested in whether or not it is included in my simulation (and yes it's true that that only matters when you are close to the source, but still...). And as for bandwidth, it's not really an issue. I only need to go up to order 7 or so to get the BW I need for subwoofer investigations in most rooms I work with.

I'm intested in your simulations. Are you then adding the "direct" term in? the one we seem to agree is a fudge? I did simulations where I went from order 10 to order 35 with the source and receiver 4" apart and saw no significant difference. So i conclude that you really need to go to high order to have the direct show up. Of course you are even closer to the source and going up to order 100. Is that sufficient to include the direct (which your plots show), or are you adding the "fudge"?

Todd

By "order" do you mean total number of terms or the value of the index for each direction? 35 terms would never show up any direct field, it would take thousands. I've done simulations with more than 1000 modes considered.

But as you and I have discussed this near field thing is irrelavent, so lets not get hung up on it.

Johns simulation is unrealistic in that one can't actually do what he is simulating, so it shows nothing useful.
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Old 30th January 2009, 07:01 PM   #505
Pan is offline Pan  Sweden
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Hi guys!

Are you still debating this?? :-)


Quote:
Originally posted by gedlee
As you say, with a sub in the corner all modes are excited and that adding more sources can only change the amplitude of those modes. But this is exactly what we want to do right? - change the amplitude of the modes such that they are uniform - smooth - in both space and spectrum. Thats what correctly setup multiple subs does.
One corner placed sub excites all modes (all axial modes at least) but adding sources in all corners (8 in total for a rectangular room) we side step the first order mode excitation in the first place. It's not there.

Also I can' see how you strive for "big room bass" in a hifi listening room. The recording have ambient signatures which would be masked if you imposed a "big room sound" at the time of playback. The decay time in the listening room need to be short enough not to mask the signature of the recorded room or the artifically added room in the recording.


Well that's how I see it anyway! :-)


/Peter
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Old 30th January 2009, 09:09 PM   #506
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Quote:
Originally posted by gedlee



Todd

By "order" do you mean total number of terms or the value of the index for each direction? 35 terms would never show up any direct field, it would take thousands. I've done simulations with more than 1000 modes considered.
index
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Old 30th January 2009, 10:10 PM   #507
gedlee is offline gedlee  United States
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So the number of modes would be 35 x 35 x 35? Or did you just do 2-D? 35 x 35.

At any rate, I think that you and I agree that the direct field does not influence the results and that getting a spatially smooth response allows for global EQ correction, if EQ is used at all. This, to me, is the crux of the situation and I think that we are in complete agreement. Correct? How one achieves this may be different and I am sure that there are a myriad different algorithms, etc. that one can use, but in the end the goal is the same.

To me, personally, once the bass response is smooth, you are 90% of the way there. This spatial bass stuff may well be valid, I think that the jury is still out, but one has to have a smooth bass response first and formost before anything further can be considered.
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Old 30th January 2009, 10:13 PM   #508
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Todd,

My code does not have the direct source term. However, I believe the result I showed without the low frequency modes was fortuitous. It is a correct result to the equations solved for the position of the source and listener, but changing the position of the source or listener greatly effects the low frequency level. I'm not clear on why that is happening. I always recover the direct response at low frequency but at reduced level. I need to look into this further. I still think adding the direct term is a fudge, and I still think Walker added it for the reasons I said earlier, the modal expansion of the Green's function is not valid in heavily damped rooms. Obviously, in an anechoic chamber where all modes are damped to close to 100% the modal expansion would yield no SPL at all, clearly incorrect.


Earl,

Quote:
In Morse he shows how the Greens function series solution, what everyone uses in their room simulations, can be modified by subtracting off the singularity at the source point. This term, when subtracted off represents the direct field since it is basically the free field Green's function. The summation that remains after you subtract off this direct field term is dominated by the lower order modes and converges very quickly, but the series is now different than the usual one.
Yes, agreed. The solution to to the wave equation in a reverberant, rectangular room is posed as two problems using superposition.

G(r|ro) = g(r|ro) + x(r)

where g(r|ro) is the free space Green's function ans x(r) is the solution to the to the homogeneous wave equation with appropriate boundary conditions. I have no argument there. But the way I interpreted Todd's remark was that he was expecting to see the free field SPL reappear is low frequency modes were eliminated, as would be the case in an anechoic chamber.

So certainly you can subtract off the contribution form the singularity, g(r|ro), and what remains is the contribution from X(r) which is the reverberant part of the solution. Likewise, once x(r) is found by that subtraction it could be subtracted leaving the free space result, i.e. the direct sound.


I also agree with Pan that as far as big room bass goes, the big room effect is captured on the recording. I believe this is very different than needing reverberation in a listening room above the Schroeder frequency to provide an illusion of "space".

Now, back to multiple subs. Are we placing them to smooth the response or at locations where modal contributions either cancel or are null? You claim your is the former. Todd's 4 woofer array with 1/4 L positioning seems to be the latter. If X and Z represent the plane of the floor, then such placement cancels odd order axial modes in the X and Z directions and the positions are nulls for 2nd , 6th 10th 14th... order even axis modes, and a bunch of tangential and oblique modes get eliminated as well. But what ever the approach, the modal behavior at the listening position have the final word as to which otherwise excited modes affect the SPL at that point.
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Old 30th January 2009, 10:36 PM   #509
gedlee is offline gedlee  United States
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Quote:
Originally posted by john k...

Earl,

Yes, agreed. The solution to to the wave equation in a reverberant, rectangular room is posed as two problems using superposition.

G(r|ro) = g(r|ro) + x(r)

where g(r|ro) is the free space Green's function ans x(r) is the solution to the to the homogeneous wave equation with appropriate boundary conditions.

John we've had this agrument before and I still don't agree. Yes you can seperate off the singularity part, which ends up being the free space Green's function, but what is left, the x(r) is not the same modal solution as the homogeneous solution to the wave equation. It will meet the boundary conditions, however it has a different set of coefficients in the series. I directed you to Morse the last time that you said this. I guess you didn't read it.
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Old 30th January 2009, 10:38 PM   #510
gedlee is offline gedlee  United States
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Quote:
Originally posted by john k...
changing the position of the source or listener greatly effects the low frequency level. I'm not clear on why that is happening. I always recover the direct response at low frequency but at reduced level.
Clearly the code has some problems since it is fundamental that this cannot occur. I've mentioned before that we don't get the same results doing the same problem.
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