Doubbled up foil faced board question..

[mixsit]

Just wondering what happens acoustically to foil faced fiberglass boards when you double them up foil sides together? I.e. if fiber of one at 1.5" deep is effective down to 'x frequencies, are the two similar or the same as glass 3" deep?
I should know this by now..

 

[c7sus]

Two layers of foil would make the second layer less effective.

But then again the foil might act like a membrane if you spaced the entire absorber off the wall a bit, allowing reflections to reach the second layer.

 

[mixsit]

Two layers of foil would make the second layer less effective.

But then again the foil might act like a membrane if you spaced the entire absorber off the wall a bit, allowing reflections to reach the second layer.

Thanks. I need to make some more gobos to help isolate amps but larger than the std 2x4' fiber boards.
That's kind of what I figured. If one side were against the wall -foil in the middle, that side would surely be wasted basically. Free standing- I'm not sure what effect the foil would have -other than include a 'barrier effect which is what I want for most of them.
So I was thinking of using plenum board which is available 4x8' sections. One side being reflective is good (for some of them). If I do decide to double some up I realized there's also the option to strip off one -or both of the foils actually.