New HT Build -Cement Wall Treatment - AVS Forum
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post #1 of 8 Old 08-22-2014, 02:07 PM - Thread Starter
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New HT Build -Cement Wall Treatment

Hey guys,

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.

Main questions:
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!
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post #2 of 8 Old 08-22-2014, 05:35 PM
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705 frk is good and cheap if you have a local distributor. Also, I'd worry about treating after the Sheetrock, not before. Just use regular insulation
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post #3 of 8 Old 08-22-2014, 06:04 PM - Thread Starter
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Quote:
Originally Posted by roccoleach View Post
705 frk is good and cheap if you have a local distributor. Also, I'd worry about treating after the Sheetrock, not before. Just use regular insulation
The purpose of the board/fabric faced furring is to eliminate having to sheetrock the cement walls. From my understanding sheet-rocking will just start me with another reflective surface and doesn't add much in terms of absorption. Unless it is thin enough to allow transmission to the insulation behind. However, I understand there are conflicting views on this?

Thanks
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post #4 of 8 Old 08-22-2014, 06:50 PM
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I thought I wanted to do this too, but everyone convinced me otherwise. It's as much to keep sound in as it is to keep sound out. People upstairs, cars, neighbors law mowers, etc. also, satisfy local fire code incase of a fire
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post #5 of 8 Old 08-25-2014, 06:46 PM
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I'm no expert but I thought it was code to insulate and vapor barrier exterior cement walls in a finished space? Also might not be comfortable in extreme weather conditions. I think I would Sheetrock to at least make the room more symmetrical as a starting point, creating closet space for your rack and stuff like that.

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post #6 of 8 Old 08-25-2014, 08:30 PM - Thread Starter
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Yeah, I'm now thinking of finishing the cement walls with another wall. Can I get away with normal insulation between a wall and the cement? Also how important is a rectangular room, is it worth sacrificing quite a bit of space? Also, are square superchunk corner traps effective if walls are attached to their sides (half the length from wall) ?
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post #7 of 8 Old 08-25-2014, 09:19 PM
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I live in an apt in an old brick building in NYC with poured concrete floors and ceilings, and cinderblock walls. 2 walls of my theater are framed/sheetrock over block. My personal experience has been that simply hanging stuff on the sheetrock(sound absorbing panels, pictures, rugs, whatever) for some reason has a magnified effect in the room. I have no idea why, it doesn't make any sense, and by rights you'd think their effect would be lessened not magnified. Yet, in this building at least, that is simply not the case.

Sound is a very tricky thing, and also VERY subjective, so before worrying about sound treatments, get the theater done(or at least to a movie watchable state as most theaters are usually never really done LOL). Then it's simply a matter of trial and error. Why go looking to add things before your even sure you need them? OK, sure, yeah, you probably will need some. BUT you never know, you may not. You may love the way it sounds without anything, and being in a windowless basement to begin with, who knows, the ground might give you all the sound 'keeping out' insulation you need.


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post #8 of 8 Old 08-26-2014, 07:20 PM
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Quote:
Originally Posted by NamesLucky View Post
Can I get away with normal insulation between a wall and the cement? Also how important is a rectangular room, is it worth sacrificing quite a bit of space?
My house builder attached insulation and vapor barrier directly to the cement walls in the unfinished basement. The insulation looks and feels just like the pink stuff but its yellow. Not sure if that means its regular insulation or not though. I don't think you'd be sacrificing much. My room has a similar shape and dimensions to yours. I ended up turning the L shaped bump out into a small enclosed rack closet 2 feet by 4 feet and a wet bar. In addition I have 2 rows of seating and a half wall bar that take up about 20 feet of the length of the room.

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