as this is my first post, perhaps an introduction would be prudent. I'm a studiotips member, and my professional work crosses into the area of transmission loss. I'm not an in-room acoustics guru by any stretch, and i do not have part in any of the arguments hereabouts.
but a friend mailed me a link to a thread here, and i've enjoyed reading bits of the forum for a few weeks now... and i have a small number of thoughts that i think might be interesting (although they aren't an answer to DoeZers question in the most direct sense - i'm not the fellow to answer that)
i'll enter them in separate posts so as to not create one immense un-readable gargantuan rambling.
the first relates to the discussion of modal problem abatement stemming from walls with poor transmission loss behavior. the second relates to ETF as an absorber assessment tool.
with respect to low transmission loss corresponding to "absorption" being observed in a room. This is, of course, true - a real phenomenon. Imagine an open window or door - open into free space. That would have zero transmission loss, and it would have 100% absorption at all frequencies - all incident sound could escape. A door open to the outside might make one heck of a bass trap, perhaps, i haven't seen discussion relating to this.
But it was commented above that the transmission loss of real walls, windows, etc. is sufficiently high to render this phenomenon essentially moot with respect to room acoustics... I took the liberty of making a couple of pictures reflecting the transmission loss of windows in practice and calculated, and the worst wall imagineable.
(here's hoping that i'm clever enough to run the html)
And here are the corresponding absorption coefficients, based upon transmission loss. They were calculated based upon 10^(-TL/10) where TL is the transmission loss.
The point of all of that is that even the worst imagineable walls would not exhibit large levels of absorption DUE TO POOR TRANSMISSION LOSS. I'm open to correction if my math was incorrect above.
Now, walls do exhibit higher absorption than that at times... But think of a panel trap for a moment. It exhibits a bell curve of absorption (or a sharp peak if damping is low, i believe) centered on the resonance of the trap. And a cavity wall - like a double-pane window - will exhibit similar behavior, and the absorption around that resonance may well be vastly higher than for a single panel.
The double-glazing may exhibit a fairly large level of absorption at ~400hz, but it's a resonant phenomenon, not one related to poor transmission loss behavior. The resonance yields poor TL, but i think that the absorption may be higher than the still fairly high TL of 20dB would imply.
shows resonant dips in two walls measured by National Research Council, Canada, and a hypothetical transmission loss plot for a bass trap. In the case of the concrete block, even at the resonance point the transmission loss remains fair (20db), but one could reasonably expect the absorption in the room to be much much higher than the formula i offered above would imply. The TL remains fair due the mass of the concrete, and the in-room absorption would be high due to the resonant behavior.
the common figures for various walls that one finds here and there on the web for absorption are suspect (IMO). they vary widely, and the peak in absorption would likely relate to some resonant behavior.
i think that is what was meant above when the comment about wall absorption generally relating to spring effects was entered.
whatever that's worth, and i 110% realize that many persons offer the thought that poor walls will mitigate modal behavior, i doubt (based far more upon the general math than experience with room modes) that a poor wall or window will alleviate your peak at, say, 81.5hz. Unless it happens to resonate at 81.5hz. :)