I believe that a specific interpretation of Andrew's statement might indeed hold water- just giving it a shot, the interpretation goes like this:
"..regardless of any configuration, no matter how many leafs - adding mass
to any existing leaf will improve sound reduction."
The report from custom audio designs clearly shows that in some cases, going from
zero leaves in a specific part of the wall to
one or more leaves can in fact
reduce the transmission loss, something you definately don't want.
But
beefing up any
already existing leaves, even if there are more than two, should increase the transmission loss, shouldn't it? Again, that's adding mass to leaves that already exist, as
opposed to adding that mass in the form of
creating additional leaves.
I'm disappointed they didn't show what would happen if you add layers of sheetrock to some of the walls on the left, at the bottom of that report. Moot? Maybe not..
In the example of that report with the wall sections at the bottom, the 3rd example shows a wall with sheetrock on both sides next to a wall with sheetrock only on the farthest side from the first wall. The Rw rating is 50dB. Say that the wall section on the left with sheetrock on both sides is an existing wall in a rented house and cannot be disturbed, so you're stuck with it. Let's say it has antique wallpaper on it with original teak chair rail molding. You can neither touch that wall nor attach any mass to it in your quest for better isolation (this almost describes my situation, btw). Putting up a wall frame with only insulation (absorption) and
no sheetrock is not going to give the folks on the other side of that wall much isolation from my noise. Extrapolating from the first two examples at the bottom of that page of that report, a wall of insulation alone may add maybe 3dB of additional isolation, whereas adding another sheetrock leaf onto a new wall section with insulation will add 14dB of isolation from just the original insulated wall rated at 36dB. And, as far as adding mass, that report tells me that if my new wall had sheetrock on
both sides, it will
not isolate as well as a new wall with only
one leaf of sheetrock on it, on the inside of the control room. But wouldn't adding mass to that one leaf, making, say, a total of three sheets of drywall instead of one, sandwiching one 1/2" layer between two 5/8" layers, wouldn't that have to increase my TL even more?
Is it ever possible that adding mass to a leaf that's
already there can cause a loss of TL or even a
perceived loss of TL? In other words, with that extra third leaf beefed up (as OPPOSED to adding mass in the form of an
additional leaf), would it still improve blocking only
part of the frequency spectrum while not affecting the other part of the frequency spectrum (i.e., in the "
already unwanted third leaf" scenario, could the extra loss of higher frequencies due to extra mass of that leaf
exceed the extra loss of lower frequencies. thus giving an overall
perception of
lower TL from the other side simply because one narrow part of the frequency range leaks through more than the rest of the frequency range)?
My situation is actually even
more complicated than that, and because it is, I need to solve some of that here before refining the design stage of the project.
My design so far (which has now been changed to one single control room taking up the whole room space, but to be built as a room within a room):
http://johnlsayers.com/phpBB2/viewtopic.php?t=1862
And the complication is this: starting from the inside of my neighbor's kitchen, the layers are: plaster, wood slat backing, stud cavity, sheathing, exterior wood siding, exterior wood siding (our houses touch), sheathing, stud cavity, wood slat backing, plaster. Effectively three existing leaves, I guess.
NOW what do I do? I need maximum isolation without touching the walls or ceiling of my present (haunted) dining room!
This post proves it's time for me to get some sleep..
Perry
