@Ossi Going on, yes. Coming soon, no. I tend to experiment with a lot of things but be very slow to actually finish anything.
NXT panels probably would be good for ceilings, although I haven't tried it. They're lightweight so they seem ideal for mounting. Personally I'm convinced they'd be good for all kinds of things!
As far as my own projects...
I've played with some various pieces of cardboard to see what kind of stuff would both sound good and look good. I tried spraypaint, although the cardboard loves to soak it up, so it's hard to get nice-looking color. In retrospect... duh. I've never tried painting cardboard before, but it makes sense that it needs to be sealed first.
I'm sure PVA glue would help, but I decided to try some other things: gesso, Claycrete, Mod Podge, and then finally water-based polyurethane. (From what I can tell, Mod Podge is PVA + PU, but I don't need the glue part of it and PU comes in larger containers.) I didn't try shellac as I've never used it before, and I figured PU would have a similar effect. To my ear, they all seemed to improve the sound, although my coat of PU damped the cardboard more than the Mod Podge... probably because I laid it on really thick. If I sanded it down I'm guessing it would be about the same. None of these things warped the cardboard, but they did cause buckling in the corrugations, especially the thick coat. I haven't gotten farther than that.
Meanwhile, I've been reading a couple white papers I was able to find about DML technology. They're a few years old but still informative. It's all stuff that's essentially been mentioned before, but it did help me understand the relationship between the frequency, the elasticity, the density, and the edge conditions.
The reason sandwich panels are favored by manufacturers is because thin plates separated by a less stiff core combine different mechanisms of wave propagation. Each mechanism has some weaknesses, but this construction evens out the frequency response.
Of course, professional panels, besides being made of expensive materials, are also sandwiched with the help of heat and/or pressure. For me, I think coating something less stiff (like cardboard) with something more stiff (like PU) is the most I'm going to do. Cardboard all by itself *is* a type of sandwich construction, but I like it better treated.
After reading all of this I spent one evening experimenting with XPS edges -- clamping them, holding them, etc. It certainly made a difference, although after awhile my ears were tired and I couldn't decide on anything that was "best." I also treated one side of an XPS 2x2 panel I had with a coat of PU. Interesting, it seemed to have a very similar effect to clamping, at least to my ear.
I also still have designs on making cargo-cover subwoofer in my car. I've had trouble getting very strong bass from my 2x4 1" thick XPS panels... but they were hooked up to a pretty puny amp and have a single 20W excited on each. I'm building a Gainclone, but that's still in progress. For the heck of it I decided to try using the amp and subwoofer channel from some old computer speakers I don't use anymore. I got a 2x4 2" thick panel of XPS, and I hooked up 2 DAEX30HESF-4, which are rated for 40W and just plain have more oomph than the others. I was very pleased with the results -- for the first time, I was hearing some significant bass! So, on my (increasingly long) list of things to do is to treat and shape this panel to actually fit in my car.
Also, one day when I was rambling on about panel speakers, a friend of mine asked if they could be used as the speaker for a guitar amp. Well... why not. I think we will explore this further just for kicks.
I guess a lot of things have been going on, I just haven't finished any of them!
