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Triple-leaf effect with limp-mass vinyl?
Old 16th May 2008
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Triple-leaf effect with limp-mass vinyl?

I apologize, this is probably a very dumb question, but I've learned in the past that when it comes to pro acoustics, NEVER assume anything.

I understand the triple-leaf effect... if you have a wall system with say three sheets of sheetrock each spaced say 3.5" from each other, this is "triple-leaf" and will harm STC performance, at least verses two-leaf of same total mass or less, etc.

Sheetrock resonates and thus it makes sense that when arranged in "triple-leaf", would cause problems.

Now, let's say you took three sheets of limp-mass vinyl... something like Auralex SheetBlock, etc... and spaced the 3 sheets 3.5" from each other in classic "triple-leaf" fashion. Will this be a problem? My initial assumption is that it would NOT be a problem because the limp-mass material should theoretically not resonate at all, and I believe the triple-leaf issue would not occur with non-resonating material. I could be wrong of course. heh

Why would I want to hang three sheets of limp mass vinyl spaced apart? Well, I don't want to do this, but I have come upon a certain situation where there are a bunch of small gaps in a wall that need to be filled.... about 1'X1' square (but each and every gap is a bit different). The rest of the wall will be covered with a layer of 5/8" sheetrock which is 2.5 pounds per square foot in mass.

In a perfect world I'd fill the small gaps in the wall with 5/8" sheetrock so the entire wall is of the exact same material and PSF. But the gaps are up high and hard to reach. As well, if I wanted to cut squares of sheetrock to fit in the gaps, I'd have to specifically measure each gap perfectly and then trim, etc... and there are over 40 gaps or so... trying to fill the gaps with sheetrock like this would take many hours of laborious work.... oh yeah, and I'd need to add some furring strips as nailers to each gap also... might take days.

My solution... I happen to have a few rolls of limp-mass vinyl laying around... I found that cutting squares of the vinyl approximately the size of the assorted gaps (a little larger actually) and then stapling the vinyl into the gaps is a much quicker solution for filling the gaps. Cuts need not be exact as the vinyl bends / overlaps at the edges. I bend the edges of the vinyl and then just staple right to the wood that surrounds the gaps. Takes a fraction of the time verses adding nailers and then trimming sheetrock perfectly to an exact size then screwing in.

However, the limp-mass vinyl is 1.0 pounds per square foot... and my goal is 2.5 PSF to match the sheetrock of the rest of the wall. So I plan to add TWO squares of the vinyl to each gap for a total of 2.0 PSF... close enough, I'm happy with this.... it ultimately would be very impractical to get it any better.

Each of the two squares of vinyl per gap would NOT be touching each other but are about an inch apart. Why? Because it would be hard or impossible to make them touch for certain reasons.... very hard to explain, but trust me... the easiest quickest and perhaps only possible way to install it is to have the two pieces per gap be about an inch apart. This is a difficult location, very limited space to work, up on a ladder, surrounded by electric wires, AC tubing, very limited site, elbow room, etc.

By adding two sheets of the limp-mass stuff spaced apart, next to another sheetrock wall (only 2 inches away from that), makes a "triple-leaf" technically. I am ASSuming that this is not a problem because the two vinyl layers are "LIMP"... but figured I'd ask before I staple in another 40 pieces or whatever.

Another thought... because the vinyl is limp, perhaps it performs way better than sheetrock per PSF anyway... thus maybe 1 or 2 PSF of the vinyl is more than enough to match the overall performance of the 5/8" 2.5 PSF sheetrock...?

And I'll be sure to caulk around all seams of the vinyl just to make sure it's all air-tight.

One thing I've learned, when building a wall, you cannot have any "weak" spots anywhere... so... I'm just trying to deal with these gaps in the best manner possible, short of spending many back-breaking days worth of work cutting a bunch of squares of sheetrock, falling off a ladder, getting electrocuted, etc A few of the people I'm working with suggested just forgetting about the gaps and leaving them open to save time... but, in the name of overall STC, I can't see leaving gaping 1-foot square holes in a wall that is meant to be a part of a sound-blocking wall system.

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