Tony
IMO don't even think of spending more than the $100ea for the Polk subs mentioned above. Earl has stated many times that the quantity & siting of the subs makes the greatest impact; in comparison the type of sub is almost meaningless unless it's junk. Just don't employ matched subs all w/ the same high-Q. If matched subs w/ high Q are employed, the Qs can be staggered by stuffing ports in one or more sub &/or adding mass to the cone of one or more subs. This is a no brainer. Get the number of Polk subs needed & forget about subs forever. I've been doing this forever & this sub advice is the single greatest advice I know of, probably greater than all other advice combined. Plus it's practically free to implement. It's insane. I vote Earl for the world's greatest audio savant.
IMO don't even think of spending more than the $100ea for the Polk subs mentioned above. Earl has stated many times that the quantity & siting of the subs makes the greatest impact; in comparison the type of sub is almost meaningless unless it's junk. Just don't employ matched subs all w/ the same high-Q. If matched subs w/ high Q are employed, the Qs can be staggered by stuffing ports in one or more sub &/or adding mass to the cone of one or more subs. This is a no brainer. Get the number of Polk subs needed & forget about subs forever. I've been doing this forever & this sub advice is the single greatest advice I know of, probably greater than all other advice combined. Plus it's practically free to implement. It's insane. I vote Earl for the world's greatest audio savant.
john k... said:
Hi John, I believe in those sims the receivers were on a 4 x 4 grid, spaced every 2 feet, and centered in the room. Height was 4'. And dont forget, I had a damping factor, should be .05 or perhaps .1, I'll have to dig it up. Let me know. i also have a BEM program (NADworks) which I might try on it.
TRADERXFAN said:
Hey Tony
The smaller and more closed the room the more critical damping is. My room is darn near sealed - has to be for noise reduction, and it takes very little in the way of LF energy to excite the bass. So thats a real plus. A very open room will have a higher modal density and damping will not be so crititical, but the bass will all leak away to all those other spaces - whether they want it or not. So you will need a lot more LF power.
I can't speaker to generic "dipole bass" but the Orions that I heard had very good bass, but seemed to lack the very deepest notes.
Todd,
None of the simulations I do, using the modal expantion technique or FEM, show results which are similar to your unless I am in very specific locations. As I move around the listenign area I see very significant variations in the response resulting from variation of the even order axial modes or order 4 and above (and tangential and obleque modes based on those modes) as the listening position moves.
None of the simulations I do, using the modal expantion technique or FEM, show results which are similar to your unless I am in very specific locations. As I move around the listenign area I see very significant variations in the response resulting from variation of the even order axial modes or order 4 and above (and tangential and obleque modes based on those modes) as the listening position moves.
cap'n todd said:
I use reasonable damping and your placement. The only way I can get results which behave as yours do is if I 1) eliminate the contribution of the (0,0,0) mode (DC mode) and 2) add in the direct source term. I don't know how you handled the DC term in the expansion, and I am not suggestion that you eliminated it, but damping alone won't do it since, as shown in you paper the damping term is 2 x wn x Dn where wn is the frequency of the mode and Dn is the damping. For the DC mode wn = 0 so there is no effect directly from Dn.
As was discussed earlier this leads one to question the results on the grounds that the modal analysis already includes the direct term but the analysis is valid only for small damping. Eliminating the DC term, as I did, is like making the room very leaky at low frequency, like a ported box below its resonance. It might be argued that in such a room the response might look like the free field below the first finite resonance and be reverberant above it, again, like a vented box. The problem is then what happens in the transition between the two regions. Is simply adding them together reasonable?
I would also think that the DC mode, while not contributing to the response at very low frequency should still contribute at higher frequency. So maybe a reasonable model of the DC mode in a leaky room would be to make it look like a band pass filter with center frequency somewhere between DC and the first finite mode, perdaps higher. Perhaps something like that would also apply to all modes. Just talking out loud here.
Now this would be a rather ad hoc model of a leaky room, certainly not what a purest would accept, but it might yield predictions that were more reasonable than expected.
I think that part of the problem in the Walker paper is that Walker refers to the modal expansion as the reverberant response. It is not. It is a representation of the SPL at some position, r, in a reverberant room when excited by a point source at a location, ro.
Anyway, to duplicate your results would require knowledge of exactly how you treated damping and the DC mode or leakness or what ever....
john k... said:
Hi John, well usually in my results the "modal" and "direct" are ploted separately in the top plot and the (complex) sum at the botton. It is this "total" plot on the bottom I assume you are referring to. Also, as I mentioned, there is a near field sub response included.
Thats strange that you would find better agreement by removing the 0,0,0, mode. In my plots you can clearly see it if you look at the very lowest frequencies. you see the dropoff of the sub response, then at lower frequencies the room gain kicks in and the level goes back up. I've also plotted it without the sub response, and you can clearly see the room gain. I've also set N=0 and plotted to see what the 0,0,0 mode looked like. it was as i expected. I also dont understand your statement about "I would also think that the DC mode, while not contributing to the response at very low frequency should still contribute at higher frequency. " I thought it was the other way around. It drops off at 12 db/octave as freuqency goes up does it not? assuming no damping.
As for my model, I pretty much just coded the Walker equations. I could send you the (Matlab) code if you like.
Also, i only ran it at N = 5, but up to 80 hz there wasn't much difference if I increased N.
Hi Todd,
I include a high pass response in the sims to represent the woofer TF.
In the paper the 1/4 spacing 4 woofer array result does not show separate direct and total responses.
All modes roll off 12dB/oct. But it is necessary to consider from what amplitude. The form is 1/(Kn^2 - k^2). For DC mode Kn = 0 and the roll off is from an amplitude of infinity. For all other modes, below Kn the level is proportional to 1/Kn^2. Here Kn = eta - i x (damping function). Thus at low frequency the DC mode can contribute more to the response than a higher order mode does.
Anyway, I have looked again at the results and I find that while my sims are not exactly comparable to your, probably due to differences in how we treat the damping, the variation with position is similar to your result. Apparently I am impaired with regard to typing input, or at least getting it in the correct order. Fixed that and things look much better.
I don't need you code but could you provide me with your damping formulation?
By the way, my results show very little difference if I include or exclude the direct term.
I include a high pass response in the sims to represent the woofer TF.
In the paper the 1/4 spacing 4 woofer array result does not show separate direct and total responses.
All modes roll off 12dB/oct. But it is necessary to consider from what amplitude. The form is 1/(Kn^2 - k^2). For DC mode Kn = 0 and the roll off is from an amplitude of infinity. For all other modes, below Kn the level is proportional to 1/Kn^2. Here Kn = eta - i x (damping function). Thus at low frequency the DC mode can contribute more to the response than a higher order mode does.
Anyway, I have looked again at the results and I find that while my sims are not exactly comparable to your, probably due to differences in how we treat the damping, the variation with position is similar to your result. Apparently I am impaired with regard to typing input, or at least getting it in the correct order. Fixed that and things look much better.
I don't need you code but could you provide me with your damping formulation?
By the way, my results show very little difference if I include or exclude the direct term.
john k... said:
Right, but it sounded ike you were saying the opposite in your last post.
Thats good, at least not too different. I'm not sure what you mean by damping formulation. As I said it's just the equations from Walker. I implemented a series of nested loops. Perhaps this is part you refer to:john k... said:
if nx == 0;ex = 1;else ex = 2;end
if ny == 0;ey = 1;else ey = 2;end
if nz == 0;ez = 1;else ez = 2;end
damping = c*((ex*ax+ ey*ay + ez*az))/(16*Vm);
john k... said:
I get the same. I cant remember if any of my published plots show the direc and modal, but it seems that usually the direct only affects the dips of the modal response (either adding or cancelling, but usually adding).
When I say low frequency I did not mean to limit that below the first finite mode. For example, the (0,0,0) mode can contribute more to the response than the (4,0,0) mode at frequencies just below the that of the (4,0,0) mode. If you plot out the contribution form each mode this is pretty obvious.
What I was referring to is the a's. These are related to the absorption coefficient, alpha:
ax = 2 x Ly x Lz X alpha, etc.
So I was wondering about how you arrived at the alpha, which is frequency dependent. Earlier you said something about sheet rock.
I also look at an order of magnitude analysis of the damping which indicated that the requirement for alpha is that
alpha << n x Pi.
Hi John, about the contribution of modes, I have plotted modal versus 0,0,0 only and based on observation concluded that the 0,0,0 had a 12 dB/oct rolloff and everything else was riding on that. You say that all modes have the rolloff. I'll have to scrutinize the equation a bit more, but my plots don't seem to show that. Here's on where i added the 0,0,0 (blue). If the modal resposne for > 0,0,0 modes was also rolling of at 12 dB/oct, the entire resposne would be. I must be missing something (wouldn't be the first time!)
As for alpha, yes I'm using fixed per frequency (but I'm only looking below 100 Hz or so anyway):
ax = (2*Ym*Zm)*alphax;
ay = (2*Xm*Zm)*alphay;
az = (2*Xm*Ym)*alphaz;
I used 0.05 for all alphas
As for alpha, yes I'm using fixed per frequency (but I'm only looking below 100 Hz or so anyway):
ax = (2*Ym*Zm)*alphax;
ay = (2*Xm*Zm)*alphay;
az = (2*Xm*Ym)*alphaz;
I used 0.05 for all alphas
Attachments
oops
Wondering what the practical goals of all these simulated responses are? What with the assumptions about, for instance, eigenmode amplitude approximations (seeing as the theory implies infinite impulse and zero width), room boundary condition compromises, room dimensional constraints, all conditioned by the model, what are the real world applications, or is this just an academic exercise.
Seems Dr. Geddes has been doing the math, as well as empirical work for over 2 decades, developed award winning speakers as well as an eminently usable semi-empirical method for subwoofer placement, and tirelessly worked to explain to others his rational based on theory.
So I'm not sure what the use of these sims in real home listening environments is, seeing that this is Geddes subwoofer approach thread.
John L.
Wondering what the practical goals of all these simulated responses are? What with the assumptions about, for instance, eigenmode amplitude approximations (seeing as the theory implies infinite impulse and zero width), room boundary condition compromises, room dimensional constraints, all conditioned by the model, what are the real world applications, or is this just an academic exercise.
Seems Dr. Geddes has been doing the math, as well as empirical work for over 2 decades, developed award winning speakers as well as an eminently usable semi-empirical method for subwoofer placement, and tirelessly worked to explain to others his rational based on theory.
So I'm not sure what the use of these sims in real home listening environments is, seeing that this is Geddes subwoofer approach thread.
John L.
Re: oops
If you read the thread, you'll see that in my case, the abosrption is is not zero, but approximatley equal to the absorption of sheetrock at low frequencies. Earl has also done simulations, and John and I have also done real world measurements. They are both are useful if you know what the limitations are. That's what much of these discussions is about.
auplater said:
If you read the thread, you'll see that in my case, the abosrption is is not zero, but approximatley equal to the absorption of sheetrock at low frequencies. Earl has also done simulations, and John and I have also done real world measurements. They are both are useful if you know what the limitations are. That's what much of these discussions is about.
cap'n todd said:
Neglecting damping the form of any mode is 1/(Wn^2 -W^2) which is a 2nd order lowpass response with corner frequency Wn and pass band amplitude proportional to 1/Wn^2.
An externally hosted image should be here but it was not working when we last tested it.
below 55 Hz or so the (0,0,0) mode contributed more to the response that the (0,4,0) mode. Both ultimately roll off 2nd order.
Re: sims
Hi Todd..
Not trying to be difficult or contrary (and I've been trying to follow the thread)... just trying to figure if the math can be manipulated and or massaged to help with improved music quality in my setup.
I currently have 3 12" sonosubs in various ported configurations, a dual 10" isobaric BR, plus dual 6" mid subs in main 6' dipoles, plus 10" woofers in old Advent 6002's for sides in 7.1 configuration.
After moving subs around per (more or less) Gedde's type arrangement methodology, I've achieved reasonably smooth (by ear and rudimentary measurement in ARTA) bass response at SOME locations in the room, but the primary listening (for both music and HT) doesn't have the same quality of DEEP bass as the perimeter areas (roughly a horseshoe shaped region 16' back and maybe 20' wide of high modal intensity below maybe 40 Hz or so) with an additional punch and/or palpable attack that's kinda hard to describe except that it sounds like what deep bass sounds like in a large rock concert arena (maybe the 0'th mode room pressurization?).
So I'm trying to perhaps de-cypher the math well enough to see if modifications of the transfer function for the room would translate to physical location and/or subwoofer/room parameter changes to somehow move the modal behavior I like to the area where I want it. I really don't want to watch movies or listen to music from the sides where the bass is best, but outside of the "sweet spot" for imaging/directivity, etc.
hence the questions as to how to apply the series reduction of Green's function to the real world situation. Unfortunately, of all my present and former professional affiliations, AES isn't one of them, so I don't have access to peer reviewed articles on the subject.
maybe mathematical modal behavior
Not to mention my calculus is 30 years rusty...
D
John L.:
cap'n todd said:
Hi Todd..
Not trying to be difficult or contrary (and I've been trying to follow the thread)... just trying to figure if the math can be manipulated and or massaged to help with improved music quality in my setup.
I currently have 3 12" sonosubs in various ported configurations, a dual 10" isobaric BR, plus dual 6" mid subs in main 6' dipoles, plus 10" woofers in old Advent 6002's for sides in 7.1 configuration.
After moving subs around per (more or less) Gedde's type arrangement methodology, I've achieved reasonably smooth (by ear and rudimentary measurement in ARTA) bass response at SOME locations in the room, but the primary listening (for both music and HT) doesn't have the same quality of DEEP bass as the perimeter areas (roughly a horseshoe shaped region 16' back and maybe 20' wide of high modal intensity below maybe 40 Hz or so) with an additional punch and/or palpable attack that's kinda hard to describe except that it sounds like what deep bass sounds like in a large rock concert arena (maybe the 0'th mode room pressurization?).
So I'm trying to perhaps de-cypher the math well enough to see if modifications of the transfer function for the room would translate to physical location and/or subwoofer/room parameter changes to somehow move the modal behavior I like to the area where I want it. I really don't want to watch movies or listen to music from the sides where the bass is best, but outside of the "sweet spot" for imaging/directivity, etc.
hence the questions as to how to apply the series reduction of Green's function to the real world situation. Unfortunately, of all my present and former professional affiliations, AES isn't one of them, so I don't have access to peer reviewed articles on the subject.
maybe mathematical modal behavior
An externally hosted image should be here but it was not working when we last tested it.
Not to mention my calculus is 30 years rusty...
D John L.:
john k... said:
Oh, I was thinking the entire modal repsonse, not a single mode. Separating out one mode like that is even more theoretical than some of the other stuff we have been discussing. No physical standing (even just two walls) wave would ever have that response, since there would always be standing waves at harmonics of the first on (95 hz in your plot). but anyway I see what you are saying.
I think there is some confusion between a mode and a resonance (even to me). I hold that a mode refers to the spatial standing wave, which will never look like the plot you showed. That would more properly be called a resonance.