# Behind Proscenium Corner/Bass Traps?



## allredp (Feb 7, 2007)

Hey Shacksters,

In the pictures attached, I have an interesting space above my subwoofer caves in my proscenium (one on each side) that I'm considering filling with spin-glass chunks (4" thick) to the ceiling. I would slide these cut pieces in vertically and then wedge them down horizontally inside the space. I'm hoping to create something of a bass trap in these two front "corners."

Both L & R of the front have these bays that are roughly 18" x 24" x 6' high to the ceiling. Where the slit opening is from top to bottom will be filled with a theater curtain when watching movies. 

The panel on the front and top half of the column face (just above the sub cave and to the side of the L/R speaker shelves--which is brown) will be openened up and house a decorative medieval shield. This will be mounted on a simple frame in that opening covered with linacoustic (1") and covered with quilt batting and fabric for a poofy look. I'm hoping that having the suspended linacoustic in that bay will allow sound to travel back inside the proscenium and get trapped in my stack of spin-glass chunks.

So, based on that rather convoluted description, will filling that space above the subwoofers with chuncked spin glass (much like OC 703) in flat 4" thick pieces give me much effect? 

Also, I'm planning on filling the hole below the equipment cave in the right rear (see pic) with the same kind of chunks and then covereing the opening with upholstery (breathable). That space in the back R corner is about 38" high and about 18" x 18". I don't have a similar cave in the L rear corner...

Of further interest, the bottom boxed panels all around the room, including the back wall and the front below the screen (as well as some along the face and sides of the proscenium in front) will all be covered with 1" linacoustic and GOM, while the top boxes will all have quilted batting and GOM on top. 

BTW, my room is 15.5 x 14.5 x 7'10" with two risers (8" high ea.).

I really appreciate any insight/ideas!

Thanks much,
Phil


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## bpape (Sep 14, 2006)

Fill every cubic inch of that opening with insulation. The LAST thing you want is to restrict the airflow of the sub in any way. Having them in a 'cave' like that really isn't a good idea - sorry. They should be in the open as much as possible and preferably out of the corners but looks like you're stuck there. 

If it was me, I'd fill it up and strip the drywall off of the faces of that recess and just cover in a contrasting cloth. That'll give you MUCH better performance.

Bryan


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## allredp (Feb 7, 2007)

bpape said:


> Fill every cubic inch of that opening with insulation. The LAST thing you want is to restrict the airflow of the sub in any way. Having them in a 'cave' like that really isn't a good idea - sorry. They should be in the open as much as possible and preferably out of the corners but looks like you're stuck there.
> 
> If it was me, I'd fill it up and strip the drywall off of the faces of that recess and just cover in a contrasting cloth. That'll give you MUCH better performance.
> 
> Bryan


Hello Bryan--thanks for the reply!

The sub-caves (the bottom square hole in the front) are made to house one SVS PB12 NSD sub on each side. These SVS's are front-firing and front-ported, and the front of the subs will sit flush with the face of the proscenium. They will be covered by breathable cloth to match the fabric covering the linacoustic. So, the airflow of the subs will not be impeded in any way. That's o.k., right? 

BTW, I thought corner-loading the subs (especially two) wasn't a bad idea--JL Audio and SVS both suggest corner-loading. 

As to the rest of the cavity, I understand that it would be preferable to have those surfaces be as open as possible so that the stack of chunks is catching as much reflection as possible. However, can you guesstimate how useful they would be as the previous pics stand (knowing that the larger side box which is across from the L/R speaker shelf and above the subwoofer face will be open and only covered by linacoustic & fabric)? 

How significant a difference would it make to cut/alter the proscenium's wood trim (it isn't sheetrock) to be as open as possible? I know that's a difficult question, but with that long slit (floor to ceiling) on the sides next to the screen, and the front opening as described above, will these filled cavities still absorb well as bass-traps? 

If I have to I can alter (= cut!) those proscenium housings by cutting out other rectangular openings (see attached pic with white hand-drawn rectangles), but I really hesitate to do so unless it is really going to make an appreciable difference acoustically. 

I really appreciate the help!


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## bpape (Sep 14, 2006)

Corner loading gives the most sheer output - but generally the least smooth frequency response.

Even with a front loaded driver, bass radiates as a sphere. You're going to get lots of buildup and boom in that little cavity. Opening up the front and sides will negate that effect - but you'll still want to kill that corner as much as possible while leaving room for the curtain to recess in it.

It's not just about the sub response (which will be much better), it's also about allowing access to the bass absorbtion from all directions in the room to maximize the usefulness of the absorber.

Bryan


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## allredp (Feb 7, 2007)

bpape said:


> Corner loading gives the most sheer output - but generally the least smooth frequency response.
> 
> Even with a front loaded driver, bass radiates as a sphere. You're going to get lots of buildup and boom in that little cavity. Opening up the front and sides will negate that effect - but you'll still want to kill that corner as much as possible while leaving room for the curtain to recess in it.
> 
> ...


Great info! 

OK, by cutting those rectangles out as drawn on the photo above, that will help alleviate the "boom" produced by having the subs loaded into the cavity. And, from your response ("much better") it will also help reflections and absorption. Dang, I have some cutting to do!

So, would it be alright to hang linacoustic in those newly created openings, or should I just fabric cover them? Any advantages or problems to having the linacoustic in those openings as the air flows into the trap? Will the linacoustic negate the effect of the opening at all?

Also, I thought of having a bass trap along the back wall behind the seating along the floor. How helpful would it be to have either an angled 4" thick sheet of spin glass resting on the floor all the way across, or better a chunked triangular wedge all the way across? I figure I have about 18"H x 12"D x 15'L to work with behind my cinema rocker seats. 

I'm really hoping to make this room pop--not just aesthetically, but also acoustically!


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## allredp (Feb 7, 2007)

Bryan (or others)--I'm still wondering if I should or shouldn't use Linacoustic 1" in the openings I'm going to create in the proscenium as per above. 

Also, will making a corner-trap (floor-to-ceiling, which due to the 2nd riser is only 5'8" tall!) in the rear corner be more effective than making a 15' long corner trap along the floor against the back wall? 

I really appreciate the help! I've just taken receipt of my 30 sheets of 4" thick 2' x 4' spinglass and my 100' roll of Linacoustic and I'm anxious to get to work!!!


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## bpape (Sep 14, 2006)

No need to do Linacoustic in the front cutouts. Do some thicker absorbtion filling the corner and that's sufficient.

In the back, if you have the material, doing both corner (vertical) and along the floor will be a benefit. FYI, the breakeven for material is 6" thick. ie - if you do a 6" thick panel straddling a corner, you can do a chunk of the same length and face size with the same amount of material. If the one along the floor is smaller - like say only 1' wide, then I'd definitely do the chunk. If it's 2' wide, then you can slide with 4" to save material to do the other corners if budget dictates.

Bryan


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## allredp (Feb 7, 2007)

Very cool, Bryan! I can't thank you enough, man... 

I feel stupid, but I'm not understanding the 6" rule for material. :nerd:

For example, I've attached a picture below of someone's corner trap inside their proscenium framing made of what I call "chunked" spinglass (triangular or angled pieces cut and stacked to fill the corner). This is what I plan to fill my front L/R corners with above the subwoofer boxes as per the pics above). I won't need to use linacoustic in those new openings I'm cutting in my trim--thank you very much! But, I will be filling the whole space available with stacked "chunks" of spinglass as per the pic. So, that should be good.

Now, I'm assuming I should do the same stacked-chunks cut to fill the corner in my back left corner that will look like the 2nd pic, right? As well as, in the cubby hole under my equipment shelves in the opposite corner (but this time they won't be triangular, but square) for that space--right? 

However, along the floor across the back behind my seating, are you saying that if I'm over 6" thick that I could just lay the panels sideways (cut to height and length) vs. actually cutting corner "chunks" and laying them in a row side-by-side like the vertical trap is stacked? My spinglass is 4" thick...

I have about 18-22"h x 13-15"d (from back wall to the base of the chairs). I have rocking theater chairs, so the depth angles up from there to the 18-22" height. With this sized space all along the back wall/floor, do I need to cut triangular chunks as per the vertical trap (eating up a lot of panels), or can I get away with laying doubled, or tripled panels--shaved to fit the angle--back there? 

Again, thanks for the patience and help!!! I'm just about there--and will post pics, etc. that will help others who pass this way...


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## bpape (Sep 14, 2006)

You're correct on both of your assumptions re the front corners.

For the 6" thing, all I'm saying is that if you're planning to STRADDLE a corner with 6" of material, you can CHUNK it the same size and same length (height if vertical) with the same amount of material and take up less space - and get better performance.

Just cutting and laying flat and stacking up will accomplish the same thing if you want a square/rectangle instead of triangles - but will take more material - but will perform better too, just takes more space.

Bryan


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## allredp (Feb 7, 2007)

Sweet, Bryan. I get it now. If on a budget, then use it length-wise (and less performance)...

BTW, I can get mineral wool for those front proscenium corners--in the stiffer "boards". 

I heard that will be even better at LF trapping than the spin-glass. 

Is that right? It is also a little cheaper!

Big thanks for all the help--I'm anxious to return the favor somehow...


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## bpape (Sep 14, 2006)

No problem.

When building chunks, you can actually get by with a slightly less density on the material. If you're doing full 17x17x24 and want to save a few bucks you can do 3lb fiberglass or even cheaper with 4.5lb mineral wool. 

For smaller (and less thick) ones, I'd recommend the 6 or 8lb density in the mineral wool. 

Bryan


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## allredp (Feb 7, 2007)

This is great info, Bryan! I really appreciate all your help. 

Cross your fingers that my system sounds great, eh?!

Please permit me one last question: I have a 110" screen that is not acoustically transparent and I've heard that you should try to cover the entire (or as much as possible) front wall with Linacoustic (or some absorbant material). 

Considering my screen isn't transparent, can/should I still run linacoustic behind the screen? Will it give me any benefit?

Thanks big time!

Phil


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## bpape (Sep 14, 2006)

It really depends on the screen and what else is going on. Some screens don't have the space and/or will show through the color a bit. The nicer screens won't show through and will have enough space. Realistically, in most cases, just doing around the screen is fine - but yes, cover it 100%. 

The big questions is with what? That depends on a ton of things that are way beyond the scope of a forum disussion. Things like distance from speaker to front wall, type of speaker, how the rest of the room is treated, etc. will all impact that decision. Normally, I find a good generic balance to be 2" 703 or equivalent.

Bryan


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## allredp (Feb 7, 2007)

Thanks, Bryan--I was thinking of using the 1" Linacoustic (that I'll be using in the ear-level and below trim boxes on the sides/back of the room) behind the screen, but maybe I'll go ahead and throw some spinglass instead.
You've been amazing in guiding me through this!
Enjoy the day. 
PS--I'll be posting pics of the install process and finished look in the next couple weeks...


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