Treatment for ceiling just above console.

longsoughtfor · Post by **longsoughtfor** » Thu Mar 04, 2004 2:49 pm

Hey folks. One of the last remaining pieces of construction I need to get to is treating the area just above the console in our control room. The area in question is essentially the bottom of the heating ducts that run the length of the basement. As show in the picture, floor to ceiling in this area is about 7ft. From the ducts down, there already is in place; 2x3 framing, 3/4" particle board + 3/4" fiberboard. That works great for the isolation side of things.

I am looking for input on what the best treatment for this spot is... I guess I'm leaning towards putting up an inch of wrapped fiberglass but wanted to hear from others if there may be a better solution.

Cheers
Kevin.

giles117 · Post by **giles117** » Fri Mar 05, 2004 1:50 am

That is one possible solution, however, I;d go with 2" instead of 1"

Other option is to slant the ceiling to reflect the sound towards the back of the room.

longsoughtfor · Post by **longsoughtfor** » Wed Mar 10, 2004 9:55 am

Is the standard helmholtz formula as given in the Knightfly's spreadsheet valid down to very shallow "depth from wall" numbers?

I'm wondering if a couple of H-absorbers (4 ft wide) on either end would do much good. From the spreadsheet, I can get down to 250HZ with a 1" depth (8in wide 3/4" slats, spaced at 1/10").

Somehow that seems non-intuitive. Anyone here with experience using absorbers this shallow?

Thanks
Kevin.

barefoot · Post by **barefoot** » Wed Mar 10, 2004 5:59 pm

longsoughtfor wrote:From the spreadsheet, I can get down to 250HZ with a 1" depth (8in wide 3/4" slats, spaced at 1/10").

Somehow that seems non-intuitive...

Other than perhaps some acoustic viscosity corrections that might need to be made for such narrow slots, this is a perfectly valid tuning. The problem is that such a resonator would be next to useless.

If you think about a slot absorber, what part of it initially absorbs the sound waves? The slots, right? So the absorption efficiency can never be any larger than the percentage of area occupied by the slots (in fact, the efficiency is only a fraction of that percentage). With 8" wide slats and 1/10" wide slots the slot area is only 1.25%. Therefore, the absorption efficiency must be less than 1.25%. Not a particularly effective absorber.

I would go with a heavy fiberglass covering like giles suggest.

Thomas

longsoughtfor · Post by **longsoughtfor** » Thu Mar 11, 2004 8:10 am

Thomas - I am going with the wrapped fiberglass in that area but so I understand this better, your saying there are some practical physical constraints to consider when you use the spreadsheet.

It looks like the only way to lower the frequency is with depth and that a shallow depth is impractical for anything below a khz.

How about surface area? Is there a practical limit (lower end) to how big an H-absorber needs to be? If it is the slot that absorbs, would a wall with one slot 1" wide and a 48" slat above and below absorb down to 200hz?

I'm just curious.
Thanks
Kevin.

Eric_Desart · Post by **Eric_Desart** » Thu Mar 11, 2004 1:25 pm

Thomas,

I don't want to be difficult here.
I've seen you say that several times already (efficiency slat type absorber related to % slots).

I say this clear now. You are wrong.
I have several designs here from 5 to 20 %.
All of them exceed 0.80 (to ca 0.95 à 1.00) at the resonance frequency (reverb room measurements).

Warm regards
Eric

barefoot · Post by **barefoot** » Thu Mar 11, 2004 5:29 pm

Eric_Desart wrote:I don't want to be difficult here.
I've seen you say that several times already (efficiency slat type absorber related to % slots). I say this clear now. You are wrong.
I have several designs here from 5 to 20 %.
All of them exceed 0.80 (to ca 0.95 à 1.00) at the resonance frequency (reverb room measurements).

Eric,

I'm glad that you are so open minded to say that I am unequivocally wrong.

You need to make a distinction between the reverberant field and individual incident waves. Yes, over the course of many reflections the time integrated absorption efficiency can be high. However, for any given wave incident on the resonator the absorption efficiency cannot be higher than the slot percentage (barring direct absorption by the slat material itself). If you disagree, please explain the physics to me because I find it difficult to imagine otherwise.

Incident wave absorption is critical in Kevin's situation because he clearly needs to worry about early reflections from this ceiling feature. A typical slot resonator will be very ineffective at reducing early reflections - even within its bandwidth.

Thomas

Eric_Desart · Post by **Eric_Desart** » Thu Apr 01, 2004 1:07 pm

Hello Thomas,

I can't exactly answer your question.
That's why I asked for info to you (with some background about my question marks) in this thread:
http://johnlsayers.com/phpBB2/viewtopic.php?t=1351
http://www.johnlsayers.com/phpBB2/viewt ... =9996#9996
If I'm not mistaken you didn't reply here.

So while having question marks and lacking related data, I know from a lot of empirical data and own measurements that this open area ratio does not apply.

I also don't see a difference with a first or early reflection (except for the angle of incidence effect a Helmholtz can show) or a reverb room measurement.

In a reverb room the absorption is related to RT 60.
Assuming a pseudo linear decay in pressure it means that for the individual sound incidences the absorption ca equals any other material with comparable RT60 (read absorption).

Kind regards
Eric