Best Density?!?

[RICK FITZPATRICK]

What´s the best way to layer them up in a frame?

Don't use spray adhesive, as that closes off some of the "interstices", which defeats the purpose of layering, which is to increase the 1/4 wavelength depth. Thats why spacing a panel off the wall an inch or so INCREASES low frequency absorption.
There are many ways to do it. Heres one. Cut some 1/8" or 1/4" plywood into panels the same size as the absorber panels. Cut styrofoam or wood cleats, about 1"x 1" or 3/4"x 3/4". Hotglue or tack these strips to one of the plywood panel FACES, along the perimeter. Lay out some fabric on a table, face side down. Place 2 or more layers of rigid fiberglass panels on the fabric and align the edges. Lay the ply panel with the face that the strips are on face down on the fiberglass and align the edges with the fiberglass. This creates an airgap the thickness of the strips between the fiberglass and the ply. Pull the fabric AROUND the whole assembly on 4 sides and staple the fabric on the back side of the ply. Then build frames 1/8" larger in lenth and width of the completed panel, and 1" wider than the THICKNESS of the completed fabric coverd panel.(Actually, depending on your finished design, this can vary) Again, nail or screw 1x1(3/4"x3/4" ) cleats to the REAR inside face of the frame. Pre drill pilot holes in these cleats to screw the frame to the backside of the plywood. Or you could simply nail or screw the frame to the spacer cleats on the plywood but in this case don't use styrofoam cleats.
Then use any wall mounting method you please. One method is a simple wire like a picture frame. There are others such as a wood HOOK CLEAT. Use your imagination

And actually, you don't even need a frame if you don't want to. With no frame, you actually get MORE absorption, as the edges are exposed(under the fabric.) A 2'x4' panel, 4" thick, will have 288 square inches of exposed edge. This equals 2 more SQUARE FEET of absorber material exposed to sound waves. Of course, these edges absorb sound at a different incident angle than the face. Not only that, but when panels are used in checkerboard patterns on walls, the OVERALL absorption will increase, not only from exposed edges, but diffraction effects as well. There are many ways to do this though. Even sewing a fabric "pillowcase" and inserting the panels into it will work. Remember though, the fabric must BREATH to work. Blow through it to test it. There are many fabrics that work well. Also, it is recommended to treat the fabric with a spray on fireproofing product such as 3M makes, prior to assembly.

And I was told that layer up several panels to get 100mm, does not make the same acoustic effect as get only one panel with 100mm!

True. You get BETTER absorption at lower frequencies when you INCREASE the thickness(depth). This is because resistance absorbers work on the !/4 wavelength principle whereby molecular velocity is higest at 1/4 wavelength from a boundary, and pressure is highest at the boundary(backer panel or wall),
fitZ
PS, here is a vertical section detail of what I was telling you.

 

[filete]

...And actually, you don't even need a frame if you don't want to. With no frame, you actually get MORE absorption, as the edges are exposed(under the fabric.)...

Well, i dont know about rigid fiberglass,
but using rockwool panels without frames, in my opinion, can be a little tricky.
Even with an high density (around 4.4 pcf /70kg m3), its easy to break or tear the wool apart if you mess with the panel alot.
So having some kind of frame makes the panel more resistant and you have a better place to grab the panel.

Anyway, I finally received my panels today,
and because in my country there´s only one factory that makes semi-rigid rockwool,
and buying small quantities can take almost 2 months to deliver,
I had to choose from what was ready in stock.

So, I´ve just got:

12 panels of 3.5 pcf (56 kg/m3) with 1.5"/40 mm of thickness.
16 panels of 4.4 pcf (70 kg/m3) with 2.3"/60 mm of thickness.

Now, some questions for you, gurus

1. Should I use 2 panels of 4.4 pcf (70 kg/m3) to make each bass trap
with 4.7"/120 mm of thickness or one panel (60 mm) should be enough?

2. My idea is to use the panels with 3.5 pcf (56 kg/m3) for the first reflection
points. I was not able to get 2"/50 mm of thickness, so using just 1.5" will
it make much difference?

3. Is there any problem to join these 2 panels beside having
different densities?
Lets say, for instance, to make a bass trap with 4"/100 mm of thickness.

Also I would like some help with links, where I can see some pictures, of other members building rockwool panels...
I only find rigid fiberglass pictures, that seems more easy to build, there´s even members who dont cover the back of the panel,
but like I wrote on the start of this reply, I believe with rockwool is another story...
because of the irritate skin danger and fibers can floating around...

Thanks in advance!