The problem is that it acts as a membrane to such a small degree that it is not considered a significant pressure based absorber anymore than a diffuser is considered to be an absorber - but its nice to see that one has the ability to recognize that no device is absolute in its nature and that all are characterized based upon the preponderance of their behavior and not upon the totality of heir behavioral characteristics. Just as all absorbers ofter a bit of reflectance and that all reflectors/diffusers offer a certain amount of loss
I don't really want to pour fuel on this fire (I enjoy watching from the balcony); however, I do take a bit of exception to this (not personally mind you).
1. To suggest a membrane layered on top of a substrate (OC703 would count) or between layers of substrates has "minuscule" affect as a pressure absorber is simply a "glittering generality". There are products which utilize this method which in lab testing and field use have pr oven to be very effective. Just buyer beware.
2. When vibratory energy (sound for example) conflicts with anything (air molecule, wall, alternate universe engineered diffuser, or 180 lb. salt water bag .. ie, carbon based unit) three things* will occur: (1) energy absorption, (2) energy reflection; and, (3) energy transmission (sound passes right through that *$%( wall!). Just exactly the extent to what combination of these effects occurs is subject to a rather large set of variables. These variables can be "reasonably" predicted in order to significantly reduce the fiddling required post construction; however, I would point out that an unexpected helium enriched environment or an oxygen enriched environment can screw the pooch with respect to our best laid plans o' mice and men. I'd guess if we need to get into the nits and bits, we should demand constant temperature and humidity. Actually, if we really think we need to get down into the nits and bits to this degree, it is important to *only* allow a very specific number, make and model of salt water bag in the room and exactly the same number, makes, and models of those salt water bags must be present in exactly the same places every time the OnOff switch is placed in the On position.
So, after a few hundred rooms, sound stages, submarines and rubber duckies, let me suggest (to the consumer):
1. Models and the application of prior experience is very effective in nailing the low hanging acoustical fruit during the design process;
2. No, you do not need to measure the room before it is built to determine what your major (and many minor) problems will be;
3. Allocate, spend your money wisely ... don't be buying a super cool, friends will be blown away, 747 before you determine if your current runway will handle it or you can afford to build the runway it needs. (Both the JetA and airplane sales people do not have your best interest in mind.) There's a vice-versa part to this.
4. What is CDO? It is OCD in alphabetical order. So, ok, if you're on this bandwagon, do plan on measuring the room once it is completely done, plan on some "tweaking", adding, subtracting, moving stuff around and plan that in your budget. Also plan in your budget to have a "real" pro do the measuring and analysis (yeah, yeah, yeah. You want the challenge of learning, doing it yourself, figuring it out ... I understand that. Students are great ... just be prepared to pay for your education in time, money and "do overs".)
5. Understand that the conventional wisdom (what your buddy sez ... what Mr. Goldenears sez, what you read on the internet) is most likely absolutely wrong, incorrect, and misleading with respect to your project. Ethan, Dragon, Local, Bigmouth aren't wrong ... in any given circumstance one may be more right than the other. If you obsess over the nits the elephant in the room is often overlooked and if you don't get rid of that dadburned elephant, the nits ain't going to matter (and in the over all scheme of things, they likely won't matter anyway.)
6. If, when you're done, in the absence of measurements and laser interferometers, if you throw your priceless ming vase on the floor and it sounds like a priceless ming vase breaking on the floor, you've got it right.
7. Somewhere in the process, don't forget the process, and the end result, is supposed to be this elusive thing called "enjoyment".
8. I have to get back to work.
*I had to come back to this because some CDO lad is going to jump all over it. Also, frequency dependent delays and frequency shifts will occur; but, those effects are directly related to the three primary activities (absorption, transmission, reflection) and, unless, you're No Such Agency or the U.S. Navy, you likely don't care about those little devils.