Multiple Small Subs - Geddes Approach

[janneman]

Originally posted by markus76


Hi Jan, that someone was Earl. We discussed this in one of the Nathan threads if I recall it correctly. I built my subs (http://www.mehlau.net/audio/sub_peerless_sls-10/) accordingly with casters underneath. The gap is about 5cm.

Best, Markus

Thanks Earl and Markus, I didn't remember where I found that guidance. I have 4.5cm space at the moment. I'll measure the impedance curve, see what that tells me.

Jan Didden

[john k...]

The room length should be 24' not 26. That, plus a possible 1 Hz plotting error on my part, might account for the 85 vs 95 Hz thingy. Also, I was using N=5 at the time I did that sim, though using a higher value did not have much effect.

Yep, I should have used 24. The eyes are getting old.

I look forward to your plots using a modest damping factor. My code can also do different absorption on different walls (haven't thought about if I could put reactive ipedance in or not) but I have not tried it, since that would be a further departure from the assumptions inherent in the model.

The model I use allows for the reactive components. But I don't think it's a big deal. Besides I have no idea what I use for the data.

BTW you shoud add y axis units on your plots (for posterity if nothing else!).

5 dB/division

My plots showed "modal" and "modal" plus direct.

Yes I see that in one of the plot. What did you use for the direct sound transfer function? It appears to have a pretty steep roll off.

It may be a stoopid question, I dont understand why, even at a non modal frrequency, if there is no absorption, how can there be a steady state? doesn't the energy just build up? Or, if there is no absorption, isn't the Q of each mode infinite, or no bandwidth?!?!?

Good point. I'd have to think about it. Off hand I would say that truncating the series to a finite number of terms might have something to do with it. The contribution to the SPL at a frequency well below the modal frequency goes like 1/(wn^2 - w^2), so while wn keeps increasing the contribution for each mode doesn't go to zero until wn goes to infinity. I'm just thinking out loud here, but what's the integral of 1/(x^2 -c) as x goes zero to infinity?

[gedlee]

Originally posted by cap'n todd


My code can also do different absorption on different walls (havent thought about if I could put reactive ipedance in or not) but I have not tried it, since that would be a further departure from the assumptions inherent in the model.

Wall reactance is actually simpler than resistance because it still results in real modes. It is the absorption that presents the biggest problems, because the modes become complex and this is a lot harder to handle (fully complex code, eigenmodes and eigenvalues). Within the assumption that the modes are orthogonal, any wall impedance is handled basically the same way, but the failure of the orthogonality assumption will come arround and bite you in the a__ before very much impedance change is added to the walls. The orthogonality issue is a real problem. Wall impedance causes the modes to couple and energy leaks from one mode to another. This causes the modes to not be orthogonal (if you think about it this should be apparent). This energy leakage effect DOES occur in real rooms but will not occur in a simple simulation. You will not see energy move from one mode to another in any simulation that uses the standard Green's function series solution (like all of us use). But you will see this in real data. It can be seen in the data that Markus has provided.

Simulations can only tell you so much about real rooms, because rooms can easily violate the assumptions. Rooms like mine will clearly violate the models and its exactly those violations that I would contend offer the greatest gain. Its those things that make a room NOT act like a simple room - a modelable room - that improve it the most. You don't learn this from a model.

[cap'n todd]

Yes, reactive wall impedance data is hard to find!

The direct sound was calculated using the equation from Walker. However, I did convolve with a nearfield measured sub response. Myabe thats what you see.

I guess if/since the response at the modal frequency goes to infinity, the bandwidth is infinitely narrow at that EXACT point. Still, it's discontinuous, so hard to conceptualize. Or, as you say its because the nuber of terms in finite.

I'm not sure it makes any sense t use 0 absorption.

I'm at home so no calculus books here. Looked it up on Wolfram's Integrator.

integral is:

-atanh(x/(c^.5)) / c^.5

I get - i * pi/2/c^.5

for integration from 0 to infinity. I have pretty much no doubt thats wrong. I havent done more than one or two integral since college!

[cap'n todd]

Originally posted by gedlee

The orthogonality issue is a real problem. Wall impedance causes the modes to couple and energy leaks from one mode to another. This causes the modes to not be orthogonal (if you think about it this should be apparent). This energy leakage effect DOES occur in real rooms but will not occur in a simple simulation. You will not see energy move from one mode to another in any simulation that uses the standard Green's function series solution (like all of us use). But you will see this in real data. It can be seen in the data that Markus has provided.

Yes, I have seen this many times. Thats why looking at the fine detail of a waterfall can be bewildering. Earl, would your comments about orthagonality relate to the fact that if you have real damping, you technically no longer have a standing wave? i.e. now there is net energy flow.

[pooge]

FYI-

I ran across this product today. It's not really on topic, but I thought it may be of interest to some of you serious about room treatment and/or constrained layer damping:

Green Glue

[gedlee]

Originally posted by cap'n todd


Yes, I have seen this many times. Thats why looking at the fine detail of a waterfall can be bewildering. Earl, would your comments about orthagonality relate to the fact that if you have real damping, you technically no longer have a standing wave? i.e. now there is net energy flow.

When there is damping there is no pure standing wave there is a standing wave plus a traveling wave, just like in a plane wave tube - Standing Wave Ratio. But the damping in the wall impedance is imaginary not real. I don't see the direct connection to orthogonality though.

As far as the modal Greens function goes, the denominator term is (kn^2 - k^2) which is not discontinous (except when kn = k) although it is singular if kn is real, which it can never really be. So there is always a contribution to the pressure from all kn due to any excitation k. Its never zero. Thus there is never a true null in the response except when the numerator goes to zero, except that never really happens either because the modes are complex too and the numerator is never really zero. Your trying to give a rational explaination to an irrational problem.

The near field term from Walker is wrong, I thought that we had settled that already.

[ro9397]

Originally posted by gedlee
...While I've never tried it, It would be possible to attach a dead wall onto an existing wall. Use RC-1 channel and then mount constrained layer damping onto that. This would yield a good amount of LF damping in a space of only about a 2" depth. Thats because its virtually all structural damping and not acoustic damping, which is a big difference. Structures can disipate a lot of energy while wave motion through some porous media is only marginally effective.

A million thanks to all for your time & effort in this excellent thread.

Probably ridiculous, but I gotta ask. What about one in each front wall corner, floor-to-ceiling cylindrical concrete former (Sonotube), 16-18" diameter? Stuff the cylinder w/ insulation as tightly as possible & seal one end w/ wood. The other end is covered w/ a screen OR put a 15" passive radiator on it?

How much bass damping might the above cylinders provide vs. the recommended false wall placed over the current sheetrock-over-concrete (front wall)? The cylinders would cost less & be portable.

[cap'n todd]

Originally posted by gedlee



When there is damping there is no pure standing wave there is a standing wave plus a traveling wave, just like in a plane wave tube - Standing Wave Ratio. But the damping in the wall impedance is imaginary not real. I don't see the direct connection to orthogonality though.

As far as the modal Greens function goes, the denominator term is (kn^2 - k^2) which is not discontinous (except when kn = k) although it is singular if kn is real, which it can never really be. So there is always a contribution to the pressure from all kn due to any excitation k. Its never zero. Thus there is never a true null in the response except when the numerator goes to zero, except that never really happens either because the modes are complex too and the numerator is never really zero. Your trying to give a rational explaination to an irrational problem.

The near field term from Walker is wrong, I thought that we had settled that already.

ok. I thought we agreed that the "direct" term was a fudge, but acceptably small error. That is, assuming a (relatively low) order modal calc is made, then the direct is not included, therefor a simple additional direct term is ok.

[soongsc]

Originally posted by gedlee



You need to look at the math some more. Once the room boundaries have complex impedances, as they must if they are not rigid and are absorptive, then the modes become complex. Any solution which does not allow for complex modes is therefor incorrect. But there is a further complication. Once the modes are complex then they are no longer necessarily orthogonal. Hence one cannot form the Green's function as a series solution to any room with non-rigid walls. Basically once the walls become "sufficiently non rigid" the entire solution fails. So it IS and issue.

What does this have to do with FEA? If the analysis does not match you measurements, then of course the analysis will not be useful. But we won't know until we actually compare, can't we. I think you are just afraid that the analysis reflects the reality of you recommended setup, and thus providing more credibility to john k's analysis.