Influence of thin plastic foil wrapping in acoustic panels

Hi, I am just about to build some panel absorbers using mineral wool. I read many discussions about the possible health risk of mineral fibers and wether it is a risk or not, it would definitely give me a better feeling (personally) if I could keep the fibers from getting into the air. I read that some people use a thin plastic foil to wrap the mineral wool, which is a bit contradictory to the advice to use a fabric for the panel cover that you can easily breath through. On the other hand, I could imagine that a really thin plastic foil (like the cheapest one I find in my local hardware store) just has some reflective effect on fairly high frequencies, but not in the mid/low frequencies. And I wonder if that reflectivness might be compensated anyway by the fabric that covers the panel? What do you think about that "theorie"? Will it make a difference in the end if I wrap the mineral wool in a thin plastic foil and then cover the frame of the panel with fabric or not? Many thanks for any advice!

I don't know about plastic, but I did read somewhere that you could spray the wool with some watered down glue... Can't remember which kind, but it shouldn't be too hard to figure out. That should keep the particles from flying about. Actually I think that a decent fabric does that as well... But I could be wrong...

Hi eyvindur, thanks a lot for your reply. Some glue would probably be an option. However, it seems easier to wrap some plastic foil around the insulation and I just would like to know from the people that have a far superior knowledge of the physics involved than I do if that foil would make a noticeable difference (acoustically). You are right that the fabric should keep the fibers in place as well. But from my understanding, if the fibers can be dangerous at all than it's not the material itself, but the extremely small structure of the fibers that might stay in your lung. So, I guess it's hard to tell if the fabric is dense enough to hold these small elements inside the panel. So, if anybody could shed some light on that plastic foil issue or has any other tips, I would be very happy. Thanks a lot.

Hi folks, could anybody let me know if the assumptions below are correct or (even better) correct me: Let's say I use a PE foil usually used by painters to cover furniture and other stuff with a thickness of 0.01mm = 1*10^-5m. I also asume that a sound wave with a wavelength four times the thickness of the foil will easily pass through it (maybe that's where I'm wrong already). Then any sound wave with a frequency below f = 1/4 * 10^5 Hz = 25 kHz should easily pass through. Or put the other way round only frequencies above 25 kHz will get reflected. The good news of this would be that nobody would need to worry about such a foil on porous absorber (at least if the foil is not stretched so much that it starts to resonate, no idea what would happen then). Any comments on my thoughts are highly appreciated. Many thanks!

Mineral fibers are not really dangerous for the health, although they aren't the nicest thing to encounter. Placing a normal fabric in front of mineral wool will prevent fibers to get into the air. Even a 0.007 mm thick foil in front of the mineral wool will start to reflect highs, from memory about from 4-5 kHz up, but I have to measure that. A fabric in front of the foil might cure that, but agaian I have to measure that. But you don't need a foil when you use fabric.

Sorry for all the nonsense in my previous post. Of course it is still and always will be f = c / λ, so a wavelength of 4*10^-5 m would correspond to a frequency f = 340 * 0.25 * 10^5 Hz = 8500000 Hz. However, there's obviously more to it than just the thickness of the foil, otherwise such a foil would no have any acoustic effect at all (8500 kHz not even being in the hearing range of dogs :-)) and, of course, that's just not the case. As a simple test I placed such a foil over a speaker and there was a quite noticable loss of high frequencies in music and even speech, so I decided not to use it in the absorber and went for some sturdy fabric as others had suggested. So, no more worries about that in real life. It would still be interesting to know above which frequency such a foil starts to be reflective. If anyone has ever meassured that (or knows how that could be calculated), I would be happy to learn about it.

Angelo, http://www.johnlsayers.com/phpBB2/viewtopic.php?t=7278 Read this thread, about halfway through or so