# Room Acoustic Advice



## Diesel 48 (May 11, 2010)

Hey everyone,
I am in the process of building a house. I plan on building a home theater in it. I have been doing months of research and am getting closer to doing actual work. I believe I have a sound acoustic plan but I wanted to confirm with the latest thoughts and theories.

The room is 9'H x 15'9"W x 25' long. The room will use stagard stud construction with insulation, whisper clips and channel, 2 layers of drywall and green glue. Something similar will be done to the ceiling. For sound proofing.

I plan on building a stage (filled with sand) and riser. I will be using an 2.35 AT screen.

The acoustic plan is the following:
Front wall: 2 - layers of 1" linaccoustic or something similar on the front wall. The front corners will have triangles cut from 2" -2'x4' OC703 or something similar going from floor to ceiling.
Side walls: coming out from the front corners 1" linaccoustic or alternative from floor to ceiling for the first 10' or so. (maybe to the riser) From there only go from the floor to about 5' up with linaccoustic. The remainder of the wall will be just be cotton behind some panels or nothing at all behind GOM covered panels.
Back wall: 2" OC703 or something similar with a kraft paper or a FSK facing. Build panels with a 2" airgap making a 4" total thickness and have them floor to ceiling.

Does that plan sound right by today's standards? I have been following multiple build threads all over and it seems to make sense to me.

I plan on using the sofit for can and rope lighting as well as looks. I have read you can put insulation in it as well. If so what do people typically fill it with?

I also read that a riser can be used as a bass trap. Are there any guidelines on how to do this?

Thanks for the advice!


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## Tonto (Jun 30, 2007)

Actually that will be too much treatment resulting in an overly "dead" room. Only the screen wall should be totally covered, with other treatments placed to cover "measured" problems. The corner traps we know you will need & if you have a riser, stuff that with fluffy insulation (air holes in each cavity, front & back).

Do you have the ability to measure the room before any treatments are applied. That will help us a lot.


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## Diesel 48 (May 11, 2010)

The acoustic plan above came from the build thread "Bacon Race" theater on another forum. I believe the the plan came from Bryan Pape. Maybe I misread though. Not sure I will be able to get measurements before starting to treat the room or not. Not sure I can hook up all the speakers first. I definitely do not want an overly dead room. I thought that was the reason treatments were only floor to ceiling for a portion of the room.


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## Tonto (Jun 30, 2007)

B. Pape is our acoustic guru, I have never seen him recomment that much treatment. I'm sure he will chime in soon. The back wall needs to be a bit reflective & some diffusers are/would be appropriate with a couple of absorbtive panels (faced).

While we can assume what common problems exist, it is always nice to know (measure) first, that way we can minimize treatment/cost of treatment.


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## Diesel 48 (May 11, 2010)

The back wall called for the 703 with facing, I am sure that was to reflect the higher frequencies.


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## sdurani (Oct 28, 2010)

Before you choose actual treatment products, get a clear idea of how you want things to sound. 

For example, do you want surround information reflecting off the front wall and coming at you from the same direction as your front soundstage? I certainly don't, since I prefer surround information stay in the surround field and not muddy up the phantom imaging between my L/C/R speakers. 

So, the front wall (at least between your L/C/R speakers) is a good location for absorption. Same with the back wall: I would cover at least the middle third or half with absorption, since your centre speaker is going to be pointed right at that location. 

How about reflections off the side walls? Ipsi-lateral reflections (on the same side as the speaker) tend to stretch out the soundstage and produce imaging past your side walls. Some people like this while others don't. Contra-lateral reflections (on the opposite wall as the speaker) tend to blur directionality. Do you really want sounds intended to come from the left side of your soundstage to also come at you (however subtly) from the right side of the room? I certainly don't. 

So, right speaker reflections off the left wall and left speaker reflections off the right wall are good locations for absorption. Centre speaker reflections and nearby wall reflections (right speaker off right wall, left speaker off left wall) are up to you. I personally like the effect, but you should go by your preference. 

IF it turns out you find ipsi-lateral reflections too strong but absorbing them too dry, then consider using diffusers at those locations. Again, you'll only know whether you like it or not by hearing for yourself. 

Corners are good locations for bass trapping, but you knew that already. 

So before committing to actual treatments and locations, figure out which reflections (i.e., from which direction) you want absorbed, which ones you want scattered and which you want left alone (if any). Once you decide on the sound you want, then you'll know which type of treatment to put where.


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## bpape (Sep 14, 2006)

Front wall is OK. Corners OK. Side walls, do up to 5' for most of it. Upper portion only enough to capture 2 bounce reflections to the primary seating. No batting in upper portion of the balance. I would prefer to see the side wall 2" if possible - some rooms do not allow for that due to size constraints. We want a well controlled front and reflection zone and a lively more diffuse surround field.

Rear wall and soffits are where you make up ground. Look at the rest of the room, furnishings, construction methods, existing treatment and see where you stand vs a target decay time curve for the particular room and usage. Likely you will have enough mid/high absorption already and be short on lower mid through bass control. 

Think thicker (much) on the rear wall with anything from FSK to pond liner to MLV on the rear wall pending how low in frequency you want to start NOT absorbing. You can also do phase grating like our Scatter Plates on top of the absorption to get a little upper mid and high but all the bass and some scattering at the same time.

Soffits are another option if you need more but the rear wall is a place where a lot of bass problems come from. 

Bryan


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## Diesel 48 (May 11, 2010)

Thanks for the information Bryan. Based on the room size I might have room for 2" but will now more once the room is built. I am assuming I can see the first and second reflection points with the mirror trick correct? 









What type of insulation is typically recommended for the sofit? Besides some extra conduit and lights my sofits are mainly for looks. I think they will be 16" wide by 10" deep. It is a dedicated room so besides carpet there will be six hometheater lazy boy type chairs. Here is a roughly layout of the room.


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## bpape (Sep 14, 2006)

You can find the reflections that way. The ones that go off the wall, then the soffit, then to you are a bit trickier

For the Soffit, with that thickness, I would do something like 2" OC703 as the layer that faces the room and then just use fluffy pink insulation for the rest. You can also use all 703 for slightly better performance. Again, pending how low you want to stop absorbing will determine what type of membrane you use on the outside.

If at all possible, I would consider going 6" on the rear wall or even thicker. 

Bryan


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## RossoDiamante (Aug 12, 2013)

Wow. 6". That will shrink your room really quickly! At what point is the rear wall "far enough" away to be able to discount or at least use minimal absorption? I think I have something like 16' between my listening position and the back wall.


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## bpape (Sep 14, 2006)

Not really any fixed distance - more a matter of where you're sitting in relation to modal problems or direct cancellations off that wall - or just plain sitting too close - I would say 4' or so minimum pending room size.

Bryan


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## Diesel 48 (May 11, 2010)

What about making the riser into a bass trap? Any guidance with this or is it not worth doing. I have seen a few opinions on it but what is yours? Any construction tips? Looks like most put vents along the back and sidewalls if they make one.


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## bpape (Sep 14, 2006)

Certainly that's something to get more bass control in the room overall, though it won't help a ton with specific issues off the rear wall - it's more for decay time control. Slots at any wall intersection will help as well as larger, irregular holes in the front plate of the riser (joists run front to back) in each cavity. All covers with carpet and the front gives you some control right behind the front row.

Bryan


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## Diesel 48 (May 11, 2010)

My room has changed slightly. I used to have a door on the rear wall going to the AV room. This door is no longer there. This will allow me to build bass traps in the rear corners now if it would be desirable. Is this something I should consider? I was going to put two in the front of the room. They are going to be triangles of OC 703/705 stacked from floor to ceiling. Should this be done in the rear or would that suck up to many high and medium frequencies? Should kraft paper or something else be placed in front of them in order to still have reflections?


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## bpape (Sep 14, 2006)

Yes that would be desirable. I would agree with using some sort of facing so you're not killing the highs too much back there.


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