New HT Build -Cement Wall Treatment
I'm putting together a home theater from (almost) scratch in a new home I am buying. The walls are already framed in the basement by previous owner. Which leaves me with a decent sized (17.5W x 22L x 7.5H), windowless room for my home theater, unless I want to significantly cut into a framed bedroom or remove the downstairs bathroom (neither of which I want to do).However, it is unfortunately not rectangular, and has 2.5 cement walls. crappy paint drawing of room is attached, with cement walls in grey. I understand a symmetrical room is desired, and my dimensions are not perfect for modes, but I would "like" to make this room work given my other options.
I will likely do a double drywall ceiling and remaining non-cement walls, but sound isolation is not my largest concern. I am primarily asking advice on room acoustics. After years of reading home theater forums and a solid week spent on cement wall treatments alone I believe I have an idea of what I want. However input is greatly appreciated.
First, the front of the room will be sand filled stage, 2 floorstanders, sub and my projector screen. I have not determined the dimensions of the stage, but expect it to come about about 2-3ft into the room. carpet and pad will go down on the rest of the floor, and an 8" insulation filled riser will occupy the extended "L" in the bottom right corner, on which another couch will sit.
Current acoustic treatment idea:
Cement is highly reflective, especially with low frequency, so all corners will have superchunk bass traps. I would also like to add a fabric faced soffit around the ceiling to house recessed lighting, and act as additional bass trapping. These will contain corning 703/5 or similar performing material.
The cement walls will have a 6" furring built out from them. This is where I would like specific advice. To calm the low frequencies further I planned on using rock/mineralwool insulation in these 6" sections, with a 1/8"-1/4" wood/board front to allow low frequencies through and reflect high freq. Then on some sections use a fabric front to absorb first reflection highs and mids. non cement walls will employ acoustic panels for mids and highs.
1)Is 6" insulation in front of cement better than nothing? Will it absorb any bass? conflicting thoughts on this from what I have read so far.
2)what 1/8-1/4" board is best material to use to transmit lows and reflect mids/highs? does thin sheetrock perform similarly? Is this approach recommended? I don't want to completely deaden the mids and highs morso than the lows.
3)How large of expanses (floor to ceiling) of fabric would be too much for this room. Would a couple 3ft x 7.5ft x 6" deep sections absorb too many highs/mids?
Thanks for any help or input!