# Advice on basement theater



## patchesj (Jun 17, 2009)

Construction thread is here:
http://www.hometheatershack.com/for...struction/29022-bluegrass-basement-build.html

My Phase I plan is:
Cover the entire front wall with OC703 (or equivilent)
Construct 2 12x12x17 corner traps for front corners
Build wall panels to cover first reflection points on side walls and ceiling

Room interior dimensions are 13'4"x18'3"x7'6"
Screen is AT and will be mounted on false wall 20" from actual front wall.
There will be a 5'6" wide riser, 6" tall, across the width of the room at the rear.
The theater door is basically right in the middle of my wall reflections. I'm planning on attaching a panel to the door (and the opposite wall)

I'm looking for advice on treatment materials and placement. 

First attempt at FRF:







Thanks.


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## lsiberian (Mar 24, 2009)

After discussing first order reflections with Sean Olive I'm convinced that it's counter productive. I'd treat the front and rear walls first diffuse the rear corners and you should be golden. Treating the first order reflections increases the power needed to fill the room.
_Posted via Mobile Device_


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## patchesj (Jun 17, 2009)

lsiberian said:


> After discussing first order reflections with Sean Olive I'm convinced that it's counter productive. I'd treat the front and rear walls first diffuse the rear corners and you should be golden. Treating the first order reflections increases the power needed to fill the room.
> _Posted via Mobile Device_


Interesting concept on first reflections not being primary focus on treatments. The room is fairly small (at least in my mind) and I've got plenty of power (again, my opinion...). My gut tells me I'm going to have my hands full with room nodes and bass trapping. This will probably occupy most of my effort. Would it be worth it to take some initial measurements with REW? Room is totally bare, concrete floor. Sub location will not be very adjustable (must stay behind front false wall). Would the result be an overwhelming number of problems, or will my larger issues still show themselves at this point?


Side note: Mains speakers are Thiel MCS1s, should have relatively limited vertical dispersion but I don't have any test data to back that up.


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## Dennis Erskine (May 29, 2010)

Whether you'd want diffusion or absorption on the first order reflections will depend on the quality of the off axis response of the speaker. However, there are exceptions here ... one of which is the case where you have a speaker (crossed over to the subs at 80Hz) closer than 3.5 feet from a front or side wall. In this case you'd need to address SBIR, most likely with a combination of absorption and diffusion (depending upon boundry distance from the speaker).

Depending upon what mechanism(s) you plan on using to address room modal response you could find yourself in a poor power response situation.


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## Dennis Erskine (May 29, 2010)

> Side note: Mains speakers are Thiel MCS1s, should have relatively limited vertical dispersion but I don't have any test data to back that up.


...and you're not likely to get that information. Many CE speaker manufacturers don't even have that data themselves and those that do, don't publish it and will very, very rarely give it out. (Maybe there's something they don't want you to know.  )


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## patchesj (Jun 17, 2009)

Time to revive this thread... I've "finished" the room. All absorbers listed are made from 2" thick rockboard 60. 
Density 125 Hz. 500 Hz. 1000 Hz. 4000 Hz. NRC 
6 lb. 0.32 1.06 1.02 1.04 0.95 

The front wall ended up with a layer on the whole thing. 

I built 6" thick bass traps and put them in the front corners at an angle, floor to ceiling (the left front corner is slightly less than floor to ceiling, I had a vent in the way).

Rear wall has 2 - 2'x2'x2" absorbers and 1 - 2'x4'x2" absorber (horizontally placed above rear couch).

Side walls have 2 - 2'x2'x2" absorbers near the rear corners.


The light blue star is my "primary" seating position, Orange is the center of the rear row. (I'll post REW measurements in a bit...)


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## patchesj (Jun 17, 2009)

Here are 3 frequency plots. All after running initial Audyssey calibration (4 points), I'll be honest, the calibration probably needs redone in the future. I also have a BFD in there, currently running 3 very minor filters to smooth below 50. My "seating" isn't complete yet, I have a futon on the rear row and a single recliner in the front. Will be moving to a 4 seat rear, 3 seat front row later. The positions will be the same in the room, but the furniture (and seating height) will change somewhat.

There is a room node around 74, associated with the sub only. I have my LFE low pass set to 80Hz. The fronts are currently crossed at 40Hz and I think this is leading to the issues around that range. I might have some phase issues. I am fairly certain that the pre/pro is crossing correctly here (PR-SC885P), only sending info below 40Hz to the sub.

Sub only 








Sub and Mains fron the rear seating position









Sub and Mains from the "center" position


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## patchesj (Jun 17, 2009)

Waterfall and RT60 (TOPT) from rear seating position

















Waterfall and RT60 from center position


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## patchesj (Jun 17, 2009)

I should also mention, the mains and center are behing an AT screen (Center Stage XD). 

The L/Rs are just inside the edges of the screen (~6" inside the edge) with a slight toe in. I experimented with holding them as close to the screen as possible (far away from wall) vs. as far away from the screen as possible, and I've found the imaging is much much better with the speakers almost touching the rockboard. I've placed additional absorbtion materials on/around the backside fo the screen frame to help tame any rearward reflections. 

The center speaker is also backed up against the rockboard. 

The sub is front firing, on the floor between the left and center speaker. Due to box size I have a limited amount of space to move it left and right and no room front to back (can maybe move 2" further left and 8" to the right). The front of the box is 2" from the back side of the screen wall framing and up against the rockboard behind it. The sub fires out below the screen via breathable fabric.


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## patchesj (Jun 17, 2009)

I re-ran Audyssey and my REW sweeps. I've created 2 filters, one for the primary seat and one for the center of the couch (the couch becomes my spot during family movie night....). I've also bumped my front and center crossover points up to 70Hz.

Front row (primary)








Center of couch (back row)


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## bpape (Sep 14, 2006)

If you go thicker on the rear wall treatments in the wall center, they can help with the problems in the rear row somewhat.

Bryan


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## patchesj (Jun 17, 2009)

How thick should I go with those? 4"? I've been fairly pleased with the rockboard 60 so far. Seems easy to work with and works well, should I continue with this material or move to something like 705?

Thanks!


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## bpape (Sep 14, 2006)

The 60 will be fine. 4" would be a minimum. 6" would be better.

Bryan


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## patchesj (Jun 17, 2009)

I also have a bit of a hump around 4k in the RT60 plot. I'm thinking this is a first reflection issue?


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## bpape (Sep 14, 2006)

Doubt it. More likely you have something resonating at that frequency.


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## patchesj (Jun 17, 2009)

I have a fairly broad bump in frequency response in that general area. That wavelength would be < 3.5"? I would say maybe the speaker to rear of screen reflections, but the distance is larger than that. Maybe Audyssey playing tricks? I guess it's not that big of an issue, should I be worried about it? It sounds pretty good at the moment, do you see anything that stands out other than the 70Hz dip (to be treated with thicker rear wall panel)?
















Thanks


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## bpape (Sep 14, 2006)

I think you're already pretty good. The decay time looks good enough to me.


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## patchesj (Jun 17, 2009)

Thanks Bryan. I have not treated first reflection points yet, but I don't think absorbers would be good. I'm worried about making the room too dead. The room is split between music with just me, and movies with the family. They are not as picky about sound reproduction as I am (yet... I'm working on them!), I adjust my BFD filters based on where I am sitting for the movie.. :heehee:

I like the look of your smaller 2x2 diffusors, my FRF plot shows the "heaviest" reflection point to be the ceiling for the first row. The MCS1s have a failry controller vertical dispersion so I think that is going to be helpful in that regard. My side wall reflections are spread horizontally and would be more difficult to treat, I can just see a string of 2x2 panels lining my walls..... Is there any good way to determine the impact of those reflections?

The room is tight by music room standards. I'm 12 feet from my mains and they are only 9.5 feet apart (limited by screen width). My main listening position is decent, about 6 feet from the rear wall and 12 feet from the front. I've got a feeling that diffusing the sides and/or ceiling might open up the space a bit more acoustically.


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## bpape (Sep 14, 2006)

Whether you absorb or diffuse is a function of the overall desired decay time as well as the quality of the off axis horizontal response of your speakers. If typical, then the reflections are not only going to be arriving out of time, but also with a very different frequency balance in the highs.


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## patchesj (Jun 17, 2009)

So basically: Off axis response more than likely tapers off on the high end (less energy). Any diffusion is going to spread that energy out over time and frequency? Would it be possible to "aim" my difusion efforts at the 2k-6k peaks and try to spread that energy out more? Forgive me, I'm not quite as familiar with the science behind diffusion. My "goal" was something in the .35 range across the board. I figured this would be the best compromise for music/movies. It looks like I've overshot that in the midrange...


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## bpape (Sep 14, 2006)

In most speakers, as you get farther off axis, the high frequencies are lower in level than on axis. Diffusing these reflections will scatter in space and time. 

You'll want something that will go lower than 2k though. The ones you were looking at start around 600 and go up to about 3.5k with very good diffusion and then some additional scattering up to around the 6k boundary you're looking for.


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## patchesj (Jun 17, 2009)

It also looks like most diffusors are designed to work on one axis depending on placement? How do you decide where to put them and how to mount them?


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## bpape (Sep 14, 2006)

Diffusion (2D) scatters perpendicular to the orientation of the wells. For side walls, you'd want to use the wells vertically so you're scattering front to back and having more impact on the waves rather than having them horizontally. If you're using them for reflection duties, they'd be in the same places as absorbers would be at the reflection points.


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## patchesj (Jun 17, 2009)

Recalulated reflection points. This view is from below the floor to show the ceiling as well.









Perhaps 4 2x2's on the ceiling in line with the front row reflections? Would I want the wells to run left to right?
Could a pack of gridfusors work for this? I don't have a dropped ceiling, but they seem to cover the range I would need.

For the side walls, if I concentrate on the front row (my primary spot..) I think I could get 2 2x2's per side to take care of the front most points (these are for the speaker closest to that wall, and in theory the one with the most energy on the reflection?). I have a 3' door and lighting switches right in the middle of the reflections on the right wall. I'm not sure how deep I can go if I mount to the door and still be able to open it all the way.


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## bpape (Sep 14, 2006)

That would probably work just fine.


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## TDO (Mar 8, 2011)

Sorry to sneak in here, but what program did you use to plot your reflection points? It looks pretty slick.


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## patchesj (Jun 17, 2009)

A program called "First Reflection Points". Found the author on AVS forums.


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## anberg (Jan 5, 2011)

I'm just building one

Room is 18' x 13' x 7'.

Acoustically, I can turn the 18' into 20-1/2 feet by knocking out some closets and replacing with false front.

I've got Thiel 3.7 floorstanding speakers. Audio is most important (and that's where 80+% of my $ will go).

Figure got to get the Thiels away from the side walls. I've only got one row, but it's a Salamander four seater, so it extends about 10' wide. I've given up on optimizing sound for the end seats. I'll concentrate on the center. However, I would still like those in the end seats to be able to see the full screen. With the Thiels away from the side walls I think 80" Wide (92" Diag) is the biggest I can fit.

Need advice on Acoustic Treatments. Spoke to Bryan at GIK, who was very knowledgeable and helpful. 

Can't do Bass traps in corners (for various reasons), but can do a thing along the floor where it meets the wall. Will do front and back wall with some of their 4x2x2's and thinking of 2 sets of three 4x2's (one for each side wall) .

I've got a lot of $ in the equipment, so I want this right.

Any way to figure out where the side 4x2x2's (48"x24"x3-1/2") need to go before undertaking project. Want finished thing to look good and have sound treatments not in awkward places relative to decor.

I've got about 12" of play on the seating position, so I can use that to do a final adjustment

Any thoughts appreciated. Many thanks in advance.


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## patchesj (Jun 17, 2009)

I've tested some 8lbs carpet padding (left over scraps...) on my first reflections on the side wall. Seems to have tamed the RT60 peak at 6Khz.









I also finished my 18Hz HP mod on the sub amp.


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## patchesj (Jun 17, 2009)

Here are two pictures of my "absorber test". The difference in the soundstage is amazing. One of my favorite imaging test CDs is Tori Amos Under the Pink. Without the sidewall absorbers, her vocals are well centered and seem to be in a 2' wide area right between the speakers. With the absorbers in place the vocals are immediately narrowed to 6" width and you can pick up a lot of previously "smeared" detail in her voice. Not to say it was bad before, but wow. Simply amazing now.

I just need to figure out what I'm going to use for the "production" absorber material now that we are moving out of development.... I'm tempted to use the foam given the measureable improvement. The right wall is going to require 2 panels, one on the door and one to the left. The area covered now is 2'x6'.


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## TDO (Mar 8, 2011)

patchesj said:


> One of my favorite imaging test CDs is Tori Amos Under the Pink....I just need to figure out what I'm going to use for the "production" absorber material now that we are moving out of development....


Good to know, I'll have to pop that CD in when I get home tonight.
Love the terminology. :T I get my monitoring equipment in this week, so I'll be entering my "testing" phase soon.


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## patchesj (Jun 17, 2009)

I also did some sweeps with the mic taped to the wall/ceiling at the reflection zones. I didn't do a sweep on the wall without the absorber in place, but this is one with the mic taped to the absorber. About what you would expect I think as far as absorbtion goes? I should also mention that I've decided to go absorbers on the walls instead of diffusors. I have limited depth on the right hand wall because of the door. If I have something 5 or 6" deep, I won't be able to open the door all the way. I need to stay less than 1".









This is on the ceiling. Seems that I have a lot more energy up there than I thought I would. I'm still leaning towards <edit...) diffusors here.


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## patchesj (Jun 17, 2009)

I've finally put couches in the room and then added 2x4x2" absorbers on the "main" first reflection points (farther forward toward screen, right wall for right speaker, etc.). Here are the latest plots. Overall I'm very happy with the sound of the room. Front sound stage is amazing, tons of width, great focus, even a sense of depth. It's so wide I've had to get up and check to make sure there was no sound from the surrounds.... At some point I need to calculate how many cubic feet of Rockwool I have in the room. I'm a bit surprised at how low the RT60s are, you would think it would be a "dead" room, but I think it's well balanced. Odd that the RT60 is actually showing an increase in the 120Hz range now. I'm done tweaking the room, time to enjoy the music/movies!

:bigsmile:


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## TDO (Mar 8, 2011)

That's pretty flat bro. I'd like to see what it looks like with the recommended axis setting for both graphs.


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## patchesj (Jun 17, 2009)

Sorry.. Forgot I had changed them to get better visability into my HP filter on the low side..


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## patchesj (Jun 17, 2009)

It looks like I've used 21 2'x4'x2" Rockboard 60 panels (~28cu ft). 8 panels on the walls in the theater itself , 8 panels on the front wall, and then the remainder as corner traps (3 panels thick across the front corners.


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