Limp mass membrane for bass traps

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Strainger

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Paul Roux
Ok, this is all Vaughn's fault. He mentioned "I would use porous (including a membrane for enhanced LF absorption, scattering of highs), rather than just wood panel traps".

So I found this thread on limp mass membranes: http://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/335848-need-limp-mass-membrane.html

Read the thing for detail (there are a few arguments, the usual stuff), but I'll just highlight a few points:
- Ethan Winer suggests that something as simple as cling wrap will enhance performance:
Yes, cling wrap type stuff is fine and you should spray glue it to the front surface only.
I will say that many reflecting materials can do the job. I've used FRK paper, various thicknesses of cardboard, various thicknesses of plastic, and so forth. All are better than plain rigid fiberglass.
I have some theories. One theory is that a slab of rigid fiberglass acts somewhat as a membrane all by itself. In this case I mean the type of "membrane" used in a wood panel bass trap. Rigid fiberglass is pretty heavy and massive. So adding a layer of thin non-porous material on top might help that behavior.

-He points to tests his company conducted: http://www.ethanwiner.com/density.html
Conclusion:
"Likewise, in all tests of 12 panels the FRK facing improved absorption from the lowest 41 Hz mode to the 200 Hz upper limit displayed. The tests of six 6-inch thick panels show less of a trend, and in one case - 701 plain versus 701-FRK - the plain version appears a tiny bit better at the 42 Hz mode. The other modes hardly changed at all, but this might be due to covering less total room surface. This also shows that density appears to become less important when the panels are made thicker. However, even with only six 6-inch panels 705-FRK still emerges the clear winner, if only by a small amount at 42 Hz."

-In answer to a question if it's better if the limp mass membrane does not touch the outer fabric:
Ideally the membrane will be bonded / glued to the absorber. Then it probably doesn't matter if it also touches the fabric.

On page 3, Glenn Kuras from GIK adds that even paper (like business card paper) will work, on both rigid fibreglass and mineral wool.


Now, the point of this post is... errr... I just found it interesting and may just consider experimenting a bit. I took the plunge over the weekend, and bought U Thermo 8 boards, wood for frames, fabric (according to some suggestions today), and tomorrow it is back to the Free State and my first ever attempt at doing anything myself that involves some kind of tool.  :drool:

So... err... ok, then...
 
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