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The easiest way is to get some thin plastic and stretch it tight over the front. You can also use FSK scrim if you can find it and bond it to the mineral wool with spray adhesive.

FSK when bonded will reflect about 50% starting at around 400Hz and more as you go up.

Bryan
 

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FSK is 3 layers, one like heavy brown shipping paper. The other side is heavier foil. In the middle, it's a mesh of nylon. It's all bonded together.

Adding the membrane will give you a hump in absorbtion at a given range of frequencies depending on what you bond it to and how thick it is.

To get lower, you simply need more thickness.

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1. It depends on what you want. If it's going to be bonded and hence a damped membrane, then it's not a big deal. If you're trying to do more of a tuned absorber, then it should be tight.

2. On the sides that face the walls, that's fine, just not on the angled face that's going toward the room.

3. That's pretty much correct to a point. For a typical sized triangular chunk absorber (17x17x24"), something around a 3lb density for fiberglass or 4lb density for mineral wool is usually optimal.

Bryan
 

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You're addressing several different things there. The FSK is used as a facing and bonded directly to the absorbent material. This gives a small hump in absorption at a given frequency depending on thickness and what it's bonded to. It will not extend the bass absorber capability. It will however, limit the amount of upper mid and high frequency absorption occurring.

A taut membrand stretched over an absorber is also more narrow in scope whether sealed or not. Sealed ones can be tuned pretty low without a great deal of thickness but then the 'membrane' you're talking about is more something like plywood over a sealed air cavity of 4" or so and some 1" absorption inside spaced about 1/2" back from the plywood.

Bryan
 

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Generally in the rear of the room is the best place so you allow the surround field to be propogated and diffuse. It also gives you a good place to deal with the length modes of the room. The hump will not be terribly wide. If you want it relatively low, I'd use 4" of 703 with the FSK bonded to it.

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The thicker and more massive you go, the lower things will start to be reflective. 3 mil is pretty thin. Think something more like the thickness and density of heavy brown wrapping paper that's used to wrap packages to mail. Some people have even used thin poster board.

Bryan
 
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