Fiberglass panels sealed air-tight?

Anto · Post by **Anto** » Fri Sep 10, 2004 12:21 am

hi,

i'm going to mount fiberglass panels across the corners of my room.

should the panel absorbers be sealed air-tight, or this have to be done only for the "panel traps" made with a wood front panel?

you know, i could seal them air-tight to the wood strips screwed to the wall, but it will be a serious problem with floor or ceiling...

thanks in advance.

cadesignr · Post by **cadesignr** » Fri Sep 10, 2004 2:19 am

No, since fiberglass is porous, there is NO way to seal it. In fact, you could simply lay the fiberglass across the corner and it will do what is intended to do. Frames for this type of absorber, are usually intended for wraping fabric and fastening the panel to the wall, if they have a back. Remember, the sound wave moves THROUGH the fiberglass, and reflects off the boundary behind. It is the movement of air molecules at the given frequency of the wave that allows fiberglass to "absorb", which is actually placing a resistance to this movement via the "interstices" of the fibers, which transforms energy to heat. Voila! Absorption! Also, since there is zero motion of molecules at a boundary, the fiberglass is set off the wall or panel as it is a waste of material to place it directly against the boundary. Therefore, for a given thickness, you can lower the absorption frequency by leaving a airgap between the boundary and the fiberglass, which in effect, gives a greater distance between the face of the fiberglass and the boundary, as this distance is equal to 1/4 wavelength of a certain frequency. In a corner, this distance varies, which in effect gives a wider frequency band of absorption.

And actually, peoply who place a frame around the edge of fiberglass panels, are removing some of the incident angle perfomance of the panel, as it absorbs even better for wavefronts entering at an angle to the panel. Tests have confirmed that panels whose edges are exposed , have a higher total absorption
than those with edges covered. So, for those people who champher the edges of 3" or 4" thick panels, to make them flush at corner placement, or cover these edges with a frame, are actually reducing the performance of the absorber. Well, thats my .02 on this anyway.

fitZ

cadesignr · Post by **cadesignr** » Fri Sep 10, 2004 2:25 am

One more thing. Tests have confirmed, that for a given square footage of absorbant material, say 100 square feet, the total absorption is greater if the material is cut up and distributed on a boundary such as a wall, than if used as ONE piece. This is called the "edge effect".
fitZ

Anto · Post by **Anto** » Fri Sep 10, 2004 2:53 am

THANKS FOR YOUR GREAT ADVICE.

i've got another question to ask you:

in corners i'm going to use panels with a face covered with a sheet of aluminium.
is it a good choice?

i bought those ones 'couse i've read somewhere that it is better for low frequencies.

knightfly · Post by **knightfly** » Fri Sep 10, 2004 4:28 am

Facing on absorbent, such as the ones Owens Corning refers to as "FRK", do improve LF absorption by acting as a panel trap - if your room is already too dead sounding at higher frequencies, this would be a way to lessen that problem, since high frequency absorption of these membrane panels is not as much as an un-lined insulation board would have.

You might experiment before doing anything permanent - try placing your aluminum side toward the room, then away from the room, with the panel placed diagonally across your room corners - you should hear a difference in brightness, but probably not as much change in the low frequency absorption... Steve

z60611 · Post by **z60611** » Fri Sep 10, 2004 9:07 am

knightfly:

This is what I think, not what I know.

Although it's true that "FRK" does make it more of an LF absorber, it's a membrane so it has a center frequency and a Q, and this may absorb a frequency that you don't want.
The definition of 'low frequency' may be relevant here. For example with 4" of 703 FRK wall mounted, I believe the resonant frequency is near 250hz. If you do RT60's I believe that 250hz is one of the hardest spots to reduce absorbtion at (furnature etc tends to hit this frequency, as does 1" absorbtion, etc).

703, FRK 4" (102mm) on wall 3.0 pcf (48 kg/m3) 0.88 0.90 0.84 0.71 0.49 0.23
703, plain 4" (102mm) on wall 3.0 pcf (48 kg/m3) 0.84 1.24 1.24 1.08 1.00 0.97 1.15

Let's say you have room mode frequencies at 75hz and 125hz, and the 4" FRK corner mounted out from the wall absorbs resonantly at 90hz. Unfaced 4" 703 would have relatively 1.0 absorbtion throughout most of the frequencies doing a fair job at taiming modes and flutter, whereas the FRK would be half as effective at the higher frequencies ( > 500hz), and also half as effective at low frequencies ( < 75hz), and also less absorbtion at 90hz than unfaced.

I would think that the decision to go with FRK would be very room dependant, because basically it's a tuned absorber. (Hence your advice to try it and listen)

Do you have any numbers as to what frequencies FRK absorbs when mounted across a corner ?

z60611 · Post by **z60611** » Fri Sep 10, 2004 9:10 am

cadesignr:

Good overview.

Multiple absorber pieces also have a difusion effect.

Champhering the edges for corner placement is pretty, but I would agree it's not for dollar/benefit nor installation ease.

knightfly · Post by **knightfly** » Fri Sep 10, 2004 9:40 am

Z, I don't have any info on corner response of FRK - for one thing, not having a sealed trap as part of the deal, conventional panel trap formulas don't seem to apply (at least not predictably) any more; plus, the variable depth would (I think) broaden out any panel resonance anyway - now you know the REAL reason I suggested experimenting, and why John didn't ask me to be the ACOUSTICS mod

...Steve

cadesignr · Post by **cadesignr** » Fri Sep 10, 2004 1:44 pm

Hello Z, thanks, but the fact is, the overview is compliments of Steve. Most of what I THINK I know is due to Steves unending patience with people like me who THINK they know about this stuff.

I just take a chance once in a while when my acoustics withdrawalls get the best of me. The most important thing he has taught me is to not spread "net fact" simply cause I read things that may or may not be true. In this case, I think its either true, or a good many acoustics resources are guilty too. So I took a chance. Risk makes life interesting......unless you end up on Steves shitlist. :

I heard he carrys a BIG hose. :shock .

fitZ

Oh, btw, an acoustician told me once, that the best diffusion for a small room was multiple small absorption patchs. No wonder so many old bars with live music had walls covered with checkerboard
mirror tiles and acoustic tiles or red carpet tiles.....ever see those?

UGLY