type of absorbers for first reflection - chose one?

[yoga]

I have the following choice for first reflection bass traps?
Which should I use?
- 4mm plywood -> 12mm airgap -> 90mm rockwool -> 200mm airgap ... all boxed in a 1.2m by 1.2m mdf box

- fabric -> polyester baffling -> 90mm rockwool -> 200mm airgap ....
all boxed in a 1.2m by 1.2m mdf box

these are the only two choices I have, besides using none. So if you were on a deserted island in a room of 13 foot by 20 foot room with the typical room wall plaster and ceilings. Has corner traps, front absorbers, rear absorbers and you need now to put first reflections absorbers and they're the only 2 types of absorbers to chose from, what would Gigligan the sound engineer use.

[bpape]

The soft faced one. The one with a plywood face will do nothing to help with reflections.

[yoga]

What frequency build ups or troubled frequencies usually occur on first reflection points in my room or a standard room of that dimension. The rest of the room has absorbers and mostly acoustic foam tiles. So do I really need to absorb high frequencies at early reflection points. Is that an important point in the room to absorb high to mid frequencies.

[AVare]

What frequency build ups or troubled frequencies usually occur on first reflection points

At first reflection points the purpose of the absorbers is to absorb ALL frequencies. The thicker the better for lf absorption.

Andre

[knightfly]

Andre's correct; however, for the really low frequencies (say, below about 300 hZ) the only thing that will make much difference is overall room mode treatments to minimise modal peaks and nulls; other than that, before you even START to treat a room you need to find out where the peaks and nulls below 300 hZ are; you do this either with swept sine waves (while moving around the room with a sound level meter) or (better) with a program like ETF and an OMNI mic - check out this for more -

http://www.etfacoustic.com/

Steve