Multiple Small Subs - Geddes Approach

[ro9397]

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.

[cap'n todd]

Originally posted by john k...
Todd,

Can you give me any information of where you specified the mic or listening position in you calculations? I can get results very similar to yours but only at very limited mic position. If I move the mic around I see wide, deep dips in the response.

I have checked my code against an FEM simulation and for similar setups I get very similar results. I do not include a term for the direct sound.

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.

[gedlee]

Originally posted by ro9397
I vote Earl for the world's greatest audio savant.

Thanks - thats very kind. All in a days work.

[gedlee]

Originally posted by TRADERXFAN
Would you think a more enclosed room would be better for the bass performance?
Would damping still be important if it is this open?

Earl, I think you mentioned that dipole bass you heard was smooth like the multiple monopole sub setup but fell off 25-50 range. -Tony

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.

[john k...]

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.

[cap'n todd]

Originally posted by john k...
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.

What about the grid locations I used? Did you add the damping?

[DorinD]

Originally posted by ro9397
Tony
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.

I subscribe to this statement; same things happened to me --same feeling-- just I'd changed Polk with other mass brand. Thanks Mr. Geddes!

[gedlee]

No problem!

[john k...]

Originally posted by cap'n todd



What about the grid locations I used? Did you add the damping?

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....

[cap'n todd]

Originally posted by john k...




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....

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.