# Basement HT with soft walls



## artsci2 (Sep 14, 2015)

My basement is 24.5x39ft with a 39ft, 6x10 beam down the middle. A dedicated HT is going into one end of the space (18.5ft wide and 24.5 deep). The ceilings are 8ft with 2"x8" floor joists. The 6"x10" beam is supported at the middle of the space by a floor jack.

I hope to make a soft wall enclosure for the HT. The screen will be on a soft wall spaced 2ft from one of the 18.5 ft cement walls. The left. center, and right speaker will b recessed into the soft wall. The R-L spacing is 8ft and all the speaker distances will be 8ft from the reference listening position.

The back wall will be a soft wall that is spaced 5ft from the cement wall and have the surround back speakers on it, The 5ft space between cement wall and soft wall will be a walk in closet.

The left wall will divide the HT from the rest of the basement.

The right wall will be made up of free standing bass traps. The corner traps will be 4ftx4ftx7ft. The rest of the wall will be lined with 2ftx2ftx6ft traps. 

The ceiling joists will be filled with sound adsorbing and covered with black heavy moving blankets.

Floor bounce will be addressed.

Eventually, most of the visible interior will be moving blanket black.

Extensive work is going on to silence the heating ducts. 

I am also building a computer to run REW and have a mini-DSP mic. 
(replacing the broken mic of my omnimic and the old system it was running on)

The seven channels are handled by KRK 10-3 triamped speakers rated 31-20kHz and 113dB
Four Dayton ULM18-22 subwoofers in sealed cabinets are driven by two Crown XTi 1002.
With the Crown DSP I was able to get 16-160Hz +-3dBb frequency response.
The signal goes from an inexpensive bluray player to an AV preamp with xlr outputs.

Currently the design of the soft walls is 2x4 spaced 16" and filled with ground denim batt insulation then moving blankets to cover that.

I will start construction on mid January 2016.

Omni-mike tests showed a high and wide peak in the 40-50Hz range. Some of which was an axial mode in the 39ft direction of the basement. 

How should I build the 4ftx4ftx7ft traps so that they adsorb 40-50Hz?


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## artsci2 (Sep 14, 2015)

8ft distance for all channels.


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## bpape (Sep 14, 2006)

I would not unbalance the room like that with all the bass traps on one side. Symmetry left to right is very important. If you did 2x2' traps floor to ceiling in the front corners, provided appropriate core material, they will get down to lower than 40Hz assuming both front and side are open to absorption.

Side walls I would just use 4" thick panels in the reflection zone.

Rear wall - try 7-8" thick with a membrane on them like pond liner or preferably MLV to address non-modal bass cancellations off the rear wall but without killing the surround field.


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## artsci2 (Sep 14, 2015)

bpape said:


> I would not unbalance the room like that with all the bass traps on one side. Symmetry left to right is very important. If you did 2x2' traps floor to ceiling in the front corners, provided appropriate core material, they will get down to lower than 40Hz assuming both front and side are open to absorption.


Thanks for the response. I am also concerned over the left-right symmetry. It seems a tmoohy problem, 9 ft to the right of the center line is a cement wall. The cement wall to the left of the center line is 30 ft away.

Should I Build a block wall on the left side to create this symmetry? That would create two very rigid side walls.



bpape said:


> Side walls I would just use 4" thick panels in the reflection zone.
> 
> Rear wall - try 7-8" thick with a membrane on them like pond liner or preferably MLV to address non-modal bass cancellations off the rear wall but without killing the surround field.


I am hoping to tame the 40-50Hz range and then add high frequency diffusors after that. Maybe use the QRD type to get temporal effects too?


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## bpape (Sep 14, 2006)

If you have that option, it would help certainly to create better symmetry and have a more uniform, smaller sized (though not too small space that would be easier for the subs to pressurize and also easier/less expensive to treat.

If you are going to build the wall, do not count on any of the measurements you currently have taken.


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## artsci2 (Sep 14, 2015)

bpape said:


> If you have that option, it would help certainly to create better symmetry and have a more uniform, smaller sized (though not too small space that would be easier for the subs to pressurize and also easier/less expensive to treat.
> 
> If you are going to build the wall, do not count on any of the measurements you currently have taken.


A full block wall on the left side is a real option. What do you suggest for a door?

Yes, Roger that on the need to re-measure the acoustics.


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## artsci2 (Sep 14, 2015)

The giant walk-in closet at the back of the theater will act as a bass trap and hopefully kill all front to back axial modes. What thickness and construction should the back wall be? Would the "7-8 inch thick with a membrane on them like pond liner" still work down to 40Hz?


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## bpape (Sep 14, 2006)

7-8" won't do 40Hz. More like 70Hz. Not sure how you figure a walk in closet outside the room is going to act as a bass trap though.


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## artsci2 (Sep 14, 2015)

bpape said:


> 7-8" won't do 40Hz. More like 70Hz. Not sure how you figure a walk in closet outside the room is going to act as a bass trap though.


I am hoping the porous wall (Blue line near SBR and SBL of post #7) will adsorb low frequencies. Maybe a full 12" thick insulation with the 60" air space behind it? Basically it would be a super sized broadband adsorber.

A panel with 1" fiberglass and 7" airspace adsorbs well down to 200Hz (Master Handbook of Acoustics 5th ed, fig 12-41). The walk in closet is basically an 8 times deeper model so it should adsorb well down to 25Hz? (assuming it is well sealed around the edges etc)


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## artsci2 (Sep 14, 2015)

bpape said:


> 7-8" won't do 40Hz. More like 70Hz. Not sure how you figure a walk in closet outside the room is going to act as a bass trap though.


I am hoping the porous wall (Blue line near SBR and SBL of post #7) will adsorb low frequencies. Maybe a full 12" thick insulation with the 60" air space behind it? Basically it would be a super sized broadband adsorber.

A 2ft by 4ft panel with 1" fiberglass and 7" airspace adsorbs well down to 200Hz (Master Handbook of Acoustics 5th ed, fig 12-41). The walk in closet is basically an 8 times deeper model so it should adsorb well down to 25Hz? (assuming it is well sealed around the edges etc)


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## artsci2 (Sep 14, 2015)

A minimalist option would be to place the speakers and then link them with a 12" thick soft wall. I'd like to hear opinions on this idea.


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## bpape (Sep 14, 2006)

Unfortunately it's not a linear thing like that. 40Hz is a LOT different than trying to do 200Hz. You're talking waves in 10's of feet long, wide, tall, etc. Plus what you are suggesting seems to be wasting a TON of space. 24" with no gap will get down to below 40Hz with no problem and save you a lot of space. Side walls generally you can get by with 6" at most or some people do 4" with a 4" gap. SBIR doesn't generally go low enough that more than that is required.


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