Best back of panel material for covering windows, ply, particle or what?

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johnbono

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I'm about to cover a few windows in my home studio. I don't need total isolation just a bit of it. I have a 3 1/2 " depth to work with. I plan on using 1"x4" (3 1/2) frame with 2" mineral wool face coved with fabric.

From what I've read it appears there should be a back to trap the sound. I'd like to use something "lightweight" and I am concerned about aesthetics from the outside.

Being I'm not looking for total isolation would you guys think that 3/4" pine (sealed seams) or 1/2" plywood would be okay or would fabric all around be okay?
 
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The most important thing is mass a good seal, so pay most attention to the caulking and weatherstripping. So absolutely you want a backing, a heavy one, because the mineral wool is getting you just about zero in terms of isolation.

As for appearance from the outside, I would pick an attractive fabric so that it just looks like the curtains are closed. You might even think of putting pleats in it for that purpose.

Remember it is mass and sealing that makes for isolation. Absorbtion is not isolation.
 
Thanks for the info and link. WIll follow all the good advise.

I'm curious as to why mineral wool has no isolation properties . I mean if it absorbs so much, isn't that a form of isolation? Anyone?
 
johnbono said:
Thanks for the info and link. WIll follow all the good advise.

I'm curious as to why mineral wool has no isolation properties . I mean if it absorbs so much, isn't that a form of isolation? Anyone?
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Absorbtion is not isolation. Absorbtion is how little sound bounces off of the surface of a barrier. In order not to bounce off it has to let the sound into and through it. It is then expected to hit the wall behind it (and have much of the energy go into the wall!) and then have to come back through the absorbtion material a second time. That is why it is mostly useless for isolation. It deliberately allows the sound in to hit the wall and has only performed half of the round trip when it has done so.
 
Interesting! Then I take it if I decide I need isolation rather than a a combo absorption/iso panel I can just build a solid multi layer 3 1/2" panel with no insulation?
 
No, for best isolation in a 2-leaf sound barrier, you need two masses separated by ABSORBED air space - this improves the performance over using just air between masses. Optimum density of porous insulation between wall panels is around 2.5 pounds per cubic foot - this can help a sound wall by as much as 8-9 dB overall. Lighter weight insulation improves low frequency performance by a few dB, heavier weight hurts low frequency isolation but helps higher frequency performance. The 2.5 PCF value gives a balanced result between lows and highs.

Absorption INSIDE a room does nearly nothing for sound isolation, because the overall sound PRESSURE is still there and acts on wall surfaces, which will still pass the same amount of sound through to the other side - also, when you absorb some frequencies within the room, it tends to (usually) sound better and more even, so you tend to "crank it" which negates whatever you did.

Absoption inside a WALL, breaks up air currents, turns some of the sound energy into heat, and dampens vibrations of the wall panels - all of which improves the performance of the wall, and almost NONE of which changes the room acoustic (other than making the walls stiffer, which causes more bass trapping to be necessary to balance the room acoustic response)

Hope that helped... Steve
 
No, for best isolation in a 2-leaf sound barrier, you need two masses separated by ABSORBED air space - this improves the performance over using just air between masses.
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Steve, is this assuming the 2 leafs are DECOUPLED from each other, or simply a 2 leaf assembly such as a wall? I thought the BEST isolation was 2 decoupled leafs, which would be very difficult for a window enclosure. Hmmm, actually, I have to think about this for a minute. :D

fitZ
 
Yes, decoupling helps, but in a case like this where the two leafs are probably joined only at the edges they are fairly decoupled already, but could be increased by some sort of decoupling, like maybe a layer of foam or rubber where they are attached.

Also knightfly is correct about needing to have some absorbtion in the air space, otherwise your two leafs just have endless slapback echoes just like parallel hard walls in the room.
 
but in a case like this where the two leafs are probably joined only at the edges they are fairly decoupled already,
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Hmmmm, from my understanding, either something is decoupled or it isn't. How can you partially decouple something? One screw through sheetrock on RC into a stud frame can negate all the work intailed in using RC. From a discussion with an expert in this, I was informed that a small wooden wedge inserted into an airgap between an exterior concrete shell and an interior concrete room within, where it was allowed to touch both shells, was enough coupling to hear a small portable radio place on the inside shell, from outside, through both shells, although when no wedge was inserted, a 9mm pistol blank could NOT be heard when fired within the interior. That tells me something. Although, in this case, I admit it is probably next to rediculous to float a leaf as the wall it resides in is a residential wall. But, I just wanted to make a point. Regardless if it was ludicrous. :D Thats just me.
fitZ
 
RICK FITZPATRICK said:
Hmmmm, from my understanding, either something is decoupled or it isn't. How can you partially decouple something? One screw through sheetrock on RC into a stud frame can negate all the work intailed in using RC.
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But they were already touching through the RC, right? The screw is much more rigid than the RC, which is why it constitutes a more effective flanking path for sound, but it is not like RC is some magical material through which sound waves cannot travel. So it is a matter of degrees of decoupling.
 
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