So I did manage to redistribute the duct liner that's been leaning behind my sofa for the last year, and I believe it's made a significant improvement.
As a reference, here is the waterfall for the bare room:
Last year I leaned 16 inches of Manson Akousti-Liner (in 1" sheets) against the back wall, as shown in my initial posts of this thread. The result was this waterfall:
.
This week I built a frame so I could put four sheets (4" thickness total) as a makeshift cloud over the listening position:
To make the cloud, I attached 2x4 lumber across the studs on the side walls, with two 12' spans separated by four feet. The insulation sheets were just lifted in between and I screwed furring strips along the bottom. It seems very secure, and avoids having to anchor anything to my ceiling (which is three layers of 5/8" drywall on resilient channel - it's already heavy, and anchoring is non-trivial due to the thickness). The insulation sheets are not touching the ceiling.
This also removed 4" of insulation from my "pile" against the back wall:
I happened to have some 3.5" slabs of mineral wool left over from another project (not acoustic - people actually do use this stuff for insulation
). For a rough treatment of the first reflection points on the side walls, I wrapped two slabs (total thickness 7") in some breathable landscaping fabric for each side, as well as a single slab located approximately where the front reflection points would be (at least for the tweeters, and as well as my current monitor placement would allow):
I will continue experimenting with the vertical placement of these (and maybe even a more aesthetic mounting system
) - the one on the right is constrained by a window:
The resulting waterfalls after this treatment are shown below. It's also worth noting that I added a subwoofer (Focal Sub6) in the last couple of weeks, but in the waterfall below it's disabled (for the sake of comparison with earlier tests). I also can't be sure that the REW output levels were exactly the same here, although I did the 80dBC calibration as prescribed by Stuart in all cases.
The new treatment (beyond just the back wall) appears to have reduced the time-domain ringing across the spectrum, although the most severe low-frequency modal peaks and dips remain.
Somebody will no doubt notice the large speakers sitting (disconnected) in the back corners of the room. These are a pair of old Kef 104/2 speakers (of "American Psycho" fame) that I have nowhere else to put. I was toying with the idea of using the enclosures to try and make some kind of resonant absorber, but will probably just go with a slatted Helmholtz made from MDF. I suppose the enclosures might be impacting the room acoustics - would be interesting to measure this. I've read some articles on how people put RLC networks along the speaker terminals to try and tune the resonant absorption, but as I recall Stuart was not a big believer in this.
PS the three additional attachments below show waterfalls acquired
with the subwoofer, at various stages in the recent treatment. If nothing else, the side and front treatment (using my leftover mineral wool on the sides and front) seemed to have had an effect in the high frequencies.