Steve,
> if you have an inch or two of gap behind the fiberglass built INTO the frame, you only have to have the trap another two inches off the wall via spacers <
I think you're missing Glenn's point about mid and high frequency absorption. Most panels based on rigid fiberglass can absorb through the rear surface as well as the front. But in order for sound to get to the rear the edges must not be blocked. That is, more total sound is absorbed with a panel spaced 4 inches off the wall, when compared to a panel having sides 2 inches deeper than the panel thickness and spaced 2 inches.
I also agree with Glenn that it's not possible to have too much absorption at bass frequencies. No matter how many bass traps you have, the room will still not be flat and there will still be low frequency ringing. So the best you can aim for is as much bass trapping as possible, then you accept the results. I do agree with you about listening, but more to "tune" the appropriate amount of mid/high frequency absorption.
--Ethan |