Question on M-A-M vs. A-M stc rating

i'm in the process of builiding my vocal booth and i have to apply the inner layer of 2 x GB to the wall facing the control room i tried to find info on the possibility of leaving the leaf on the inside of the booth off. in that way the rockwool in the wall doubles as acoustic treatment, basically turning it into the inside out wall idea, but not facinganother wall. Also leaving that layer off gives me GB for the ceiling and that way i can save some money. Question is if leaving the inner leaf off compromises the stc too much, and how much? also would adding a 3rd layer of GB to the leaf thats already been done up the STC rating? i cant get the the outside anymore since thats finished, but i can add a layer of GB from the inside , between the studs.. hope my questions are clear. thanks, Jaye

There has been some controversy here as to the health affects of keeping an open faced insulation in an area that you will be using frequently http://www.johnlsayers.com/phpBB2/viewtopic.php?t=1742 Personally, I would cover up the rockwool because I wouldn't want to breath it in all the time, but I am not telling you what to do. I do know that adding 2 layers to the inside of the booth is going to raise your stc rather then leaving it open. 2 is great 3 is better. Green glue between each layer and you will have an stc in the high 50's to 60's with 3 layers Does that answer your question? One of the guys on here told me, more mass is better. The more mass you can cram in there the better isolation you will have.

thanks for your quick answer. i 'm not gonna leave the rockwool uncovered, a fabric is gonna cover it up so acoustic properties stay intact. you sure i can get those stc's with 3 layers and GG on ONE leaf? that sounds good, thats actually a lot better than i hoped for. its actually on par with a double leaf system.

You wont get that with one leaf. You would need to have 2. Sorry if I wasn't clear on that. I assumed you already had one leaf and were adding a second to the inside of the booth.

eightamrock wrote:

Personally, I would cover up the rockwool because I wouldn't want to breath it in all the time, but I am not telling you what to do.

Care to explain that? Andre

AVare wrote:

eightamrock wrote:

Personally, I would cover up the rockwool because I wouldn't want to breath it in all the time, but I am not telling you what to do.

Care to explain that? Andre

Its a personal thing, I am really paranoid about airborne particulates... this in no way is founded with any real data. The link above outlines the safety concerns with fiberglass based materials, I am just overly cautious..

But Jaye is using rockwool, not fiberglass, for his build. Does the same concern still apply to rockwool, I wonder?

yes

i'm not really worried about the particles being airborne, i staple some some thin plastic foil before stapling the fabric. i ve done this before even without the plastic and never had problems. but i'm still puzzled about the 1 leaf wall, anyone any STC data on that? i know the rating will be better with 2 leafs, but how much better will it be? thanks, Jaye

Question is if leaving the inner leaf off compromises the stc too much, and how much? ... but i'm still puzzled about the 1 leaf wall, anyone any STC data on that? i know the rating will be better with 2 leafs, but how much better will it be?

I've been looking around the Internet, and what I've found is that a single leaf of 1/2" drywall has an STC of 28. I haven't found data for two layers of 1/2" drywall, and I'm no expert, but I'd guess that the STC would maybe be around 30 to 34 STC. (A two-leaf single-stud wall with a layer on each side is STC 36, so I doubt that single-leaf two-layer could be better than that.) So your single-leaf wall at best is going to give you something around STC 30, give or take a couple of points Compare that with a two-leaf double stud wall with one layer of drywall on each side, which gives you an STC of 56, or a two-leaf double stud wall with two layers of drywall on each side, which gives you an STC of 63 at least, and more if you use GG in between. So I'd say that yes, the second leaf makes a big difference, probably in the region of a couple of orders of magnitude, at the very least! I'd definitely go with a two-leaf double-stud wall if isolation is important to you!!! No doubt about it.

also would adding a 3rd layer of GB to the leaf thats already been done up the STC rating? i cant get the the outside anymore since thats finished, but i can add a layer of GB from the inside , between the studs..

Adding mass will always help, according to everything I've read. Double the mass and you get another 6 dB of isolation, but that still isn't much in your case. But unless you have a full Mass-Air-Mass sealed system, then you you'll NEVER get really good isolation. I'd really think hard about leaving off that inner leaf. You'll be loosing a LOT of isolation. - Stuart -