What I REALLY want to find out is if I still need to put panels behind the heavy velvet curtains. It looks like I do not need to based on the numbers that Basement Bob posted regarding cotton curtains.
Those examples were the best and worst absobtion figures for curtains that I found. And even the best, the heavy cotton ones that are way out from the wall and huge drape, still absorb a lot more in highs than in lows.
OK, let's talk about what's going on here.
We've got modes, flutter, and SBIR (first reflection).
Curtains will do wonders for flutter.
Curtains will do nothing perceptable for modes.
Curtains will do a bit for side reflections (mid and high), but nothing for SBIR reflections off the front wall (which tend to be louder lower).
DE designs seem to have about 1" of linacoustic or InsulSheildBlack for the front wall. ( 0.09 0.29 0.67 0.89 1.03 0.99 ).
If you have lots of couches and thin walls then some of your modal issues may already be handled.
In the stereo control room world there are two designs for front speakers - baffles and thick absorbtion on the front wall on either side of the speakers. Either handle SBIR.
LEDE may not be appropriate for small rooms, unless you're doing a dual purpose room which is also for playing musical instruments.
OK, you want a bit of a dead room from all sides for 7.1
But you don't want to absorb everything because it has three bad effects:
1) it's expensive
2) it requires you to turn up your amplifier possibly to the point of distortion
3) it removes the reflected sound which is needed for spaciousness
So what you're really after is
a) absorb first reflections and SBIR for the best imaging
b) a flat RT60 (even absorbtion at all frequencies)
c) Any reflection path less than 20ms you might want to absorb or diffuse so that it hits the listener area down about 30db or more relative to the direct sound.
If I do put panels behind the curtains, would I still need to go through the expense of putting acoutically transparent fabric over the panels? I would think that since the curtains are NOT acoustically transparent that it would be a moot point.
You got it. Curtains over absorbers are fine.