You probably missed my question in previous post - was I correct in assuming the 3-pin non-PWM fans are now fully controllable with modern motherboards?
Thanks for the semantic heads-up - I must say I was always slightly uncomfortable using "dampening" in this context (something felt wrong) but always discounted the feeling.
I see your points - regarding the foam, so you're saying my good experiences with pyramidal sound foam are really just placebo from pluggin the hole on P183 topside? So I am better off just buying cheap mouse pads and cutting them in order to plug any direct line-of-hearing case openings? That may very well be and looking at the facts you present, I can certainly see why.
I actually would not put damping of that kind on the panels. Like I said, it is useless. If I thought they needed it, I'd cut 1/4" thick MDF board and glue it to the large panels. Or thicker, if it would fit. Better yet, old fashioned particle board, which is heavier than MDF. Doesn't have to go to the edges of the panel, only cover about 3/4" of the main area.
You mention you couldn't apply enough of the really effective stuff in the case - I know this may be an outlandish idea but did you ever consider constructing a "porous absorber bass-trap case", say, using steel exterior frame (for mass and sturdiness) and using isolation panels with material like fiberglass or mineral-wool with porous coverings? It is just a thought experiment - do you think it would be possible to build a case using such materials that would be effective at blocking LF noise (spiking around 200 Hz) and still maintaining a modicum of acceptable size? Or simply using the skeleton of an existing case and substituting the panels with fiberglass/mineral-wool insulation boards?
Short answer: No
to all the questions in this paragraph. But I did do something similar to the thing you describe in the first question. After I made a PC as quiet as I could, and it still didn;t seem quiet enough for me, I built a steel frame in which to put the whole PC, then put walls of 1" thick MDF on the steel frame. Top portion was a tunnel lined with heavy foam for exhaust toward the back, bottom portion was a tunnel lined with heavy foam for intake from the back. Additional fans for in/out, but super low speed, selected by ear, and tweaked to the nth degree. It was maybe 32" tall. Weighed well over 100 lbs with the PC in it (I'm guessing 125 lbs) The end result was virtual silence..... but not quite.
The only remaining sound was a tiny hum, probably from the 7200rpm drive(s) I had suspended in there. I doubt the hum was higher than maybe 12-14 dBA@1m. At first, I thought this was total success, but over time, I realized that hum was more annoying than the noise I had before the ridiculous box. It was a tonal noise, much like CFL light buzz, but at 120Hz, and I could not block it out of my mind. after a few weeks, I ended up turning up the in/out fans so I could hear a broadband woosh. Lived with that for a while -- it was much easier to tune out even though now the noise level was probably 18~20 dBA. A few weeks after that, I dismantled the brute box and repurposed the parts for speaker boxes and stands.
To block low frequency noise, you need concrete -- 3-4" thick will do for noise that comes from a PC. Of course, 125 lbs will seem light compared to that box.