A small query on wall construction.

[TerrorFiend]

I was talking to my mates dad the other day, who's been into accoustics and recording since the 70s, and he gave me lots of ideas and advice about building my studio.

One thing he said was that fibreglass batts make virtually no difference when sound insulation is concerned. That they are only good for heat/cold.

Therefore, his version of a studio wall would be the second of these diagrams, just: plaster - Wall frame(with no insulation) - air space - wall frame(no insulation) - plaster - foam insulation.

But I was previously under the inmprssion that the fibreglass batts did a lot to stop the sound getting through. (so the wall would be built as in the first diagram).

Sorry to bother you, but what are your thoughts on this?

[knightfly]

Show your dad this -

http://irc.nrc-cnrc.gc.ca/cbd/cbd239e.html

Specifically, have him read the section right after "Table 2. STC Ratings for Walls Formed From Two Layers of Wallboard*" , and ESPECIALLY the part with the asterisk... Steve

[TerrorFiend]

Yeah I see what you mean, thanks.

I don't really know what to believe now.

Would regular fibreglass batts, commonly used in house insulation (for heat and cold) qualify as a sound absorbing material?

If so, according to that table, the best wall would be as in the following diagram.

Am i correct in this?

Thanks for your help Knightfly.

[knightfly]

Any insulation (except closed cell products like most foam insulation) will improve Transmission Loss somewhat - the standard fiberglas batts will improve walls by 3-4 dB or so. The heavier ones (like SAFB's) give a more balanced attenuation - optimum density for sound walls runs from 2.5 PCF to around 4 PCF (Pounds Per Cubic Foot) - if you're on the metric system, multiply these numbers by 16 - 3 PCF = 48 kG/m^3.

Placing the batts so that they contact the center areas of wall panels from inside the wall helps damp the panel and also improves wall performance mostly on the low end.

Here's a study from USG on this -

http://www.usg.com/Design_Solutions/2_3 ... onperf.asp

unfortunately, all the illustrations they mention were stripped from the page for some unknown reason. Still, some interesting comments... Steve

[TerrorFiend]

Yeah this is an interesting page, thanks. I'll show it to my mate's Dad and see what he thinks.

Sorry to ask again, but from what I've just been shown, my conclusion is that for best accoustic performance, the 2 wall, 2 leaf(on each wall) system, with fibreglass insulation - density 2.5 to 3.0 lb/cb.ft - in each wall (touching the inner wall) is best. - As in the diagram in my previous post
(But the cost of all this wall material justifies using less leaves/insulation etc.).

Is this correct?

[knightfly]

From outside to inside, the most efficient use of materials is as follows -
2-3 layers of mass, such as 5/8" sheet rock(gypsum wallboard)
Stud frame, filled with fiberglas insulation, 2.5 to 4 pounds per cubic foot, snugly against the wallboard for damping.
NO wallboard on the other side of this first frame.
Air gap - more is better, up to decreasing return at around 8-12 inches.
Second wall frame, filled with more insulation but NO WALLBOARD on the side toward the first frame.
2 layers of wallboard on the inside of the second frame. It helps if these layers are NOT the same as the ones on the outer frame, so they have different resonant characteristics.

Almost all of this is presented in the sticky called "complete section" - there is also a thread from Ozzie titled "walls, walls, and more walls" - he posted a copy of a wall STC comparison that is used here a lot, it's an actual lab test.

There are other ways to build walls, but the two leaf, one air space method is the most efficient use of materials - in the previously mentioned STC comparison, look at the differences where there are 2 or 3 air spaces and you'll see what I mean. For example, the 40 dB wall and the 63 dB wall take EXACTLY the same amount of material and almost the same space, and the same amount of work.

If you read through that "sticky" 2-3 more times, more of this will start to make sense... Steve