One should define 'sounds good' a bit better.
I had my room like this
and when I played the end credits to We Were Soldiers DVD, with the men's choir, I was in acoustic heaven. Room support was fantastic. I litterally stopped what I was doing and couldn't stop listening. I played that bit over and over, turned the volume up, showed that bit to all of my friends and relatives. It was fantastic. Easilly the best sound I've ever heard, anywhere.
But that layout wasn't what I wanted.
And the imaging was off and speach inteligibility seemed low.
I tried different speaker positions and treatment layouts.
and although the last one above sounded really great,
what I ended up building was a little different:
That's 4" thick rockwool, with a 4" airgap behind it.
And I can tell you without a shadow of a doubt, it's not that heavenly room support I had before. It's dull and lifeless.
(for more about this and some more detailed acoustic observations regarding each picture please seehttp://www.bobgolds.com/LivingRoomPl...ction/home.htm
at 2006/12/01 about 60% of the way down)
Speach inteligibility is pretty good though.
I had some imaging problems, pod race from Star Wars I couldn't tell left from right. Room geometry takes care of left speaker off the left wall, and right speaker off the right wall. But the problem turned ot to be left speaker off right wall, which a right side wall absorber took care of and solved the imaging. Pod race is better with good imaging.
I've been meaning to do some ETF tests and tweaks, but I haven't gotten around to it.
That room will never be optimal acoustically due to other constraints -- and I haven't tried more or less surface area, nor more or less absorbtive material, in that front wall yet. Although I built it so I could fiddle.
A year ago I had the same speakers in the basement (five concrete surfaces walls and floor) and even with two subwoofers never had any bass. Upstairs in the living room (above pictures) I've got bass to burn. I'm hearing things I've never heard before and really enjoying them.
My point is 'sounds good' can actually be a bad thing.
With my lifeless room now, I've tried a bunch of the DSP modes on my receiver (DSP pretend to be a church, DSP pretend to be a concert hall, DSP pretend to be a rock studio, ...) and I've found that while I can get some of that heavenly sound with that particular spot in We Were Soldiers (although not nearly the stop dead in your tracks as if you've fallen under a spell like the first time you fall in love and think that all's right in the world that I had with the two other layouts), if I leave that DSP setting on there's some DVDs that I can't understand speach with -- but I turn off the DSP modes (output 'STRAIGHT') and it clears up immediately.