Originally Posted by mark1974
Is it always mandatory to use an absorptive at the first reflection point? Is there any advantage to not making this point absorptive?
The one advantage I've read about is when your speakers have superb offaxis response, that it may benefit from leaving the sidewalls untreated to expand stereo width. The reflected information is similar to the onaxis information. If OTOH the speakers have poor offaxis response, you'd want to absorb the "inaccurate" information. However, it may be that this is more for stereo than mch, sorry I really don't know much.
For the thickness of the material chosen, it boils down to your target frequencies. 1" or even 2" will do either nothing at all to very little for your bass, and it's typically the bass that is the most pressing issue in most rooms.
Corner trapping is a good goal, and the front corners are the bang for buck spots. There are many here who go nuts and do the "superchunk" traps, floor to ceiling.
Basically, it seems to me, the closer that either the listener or speaker is to any boundary, the more imperative it becomes to treat that area. For instance, if you are up against the back wall, it becomes imperative to treat that wall. If the speakers are too close to sidewalls, ditto. Too close to the front wall, ditto. Many people treat the front wall directly behind speakers to reduce SBIR.