[JohninCR]

I need advice for the best way to proceed with panel traps at the front of my room.  The rear wall is out for now as it is concrete.  I'm going with a dipole sub setup discussed in another thread, so only the front to rear axis is of concern.  I'm concerned about bass absorption, but my primary emphasis is preventing bass transmission, because the other side of this front wall is the main house.  Currently I have a layers of styrofoam, cardboard and 3/16 fiber board covering wood frame windows that overlooked a terrace which was replaced by what is now my room.  The current layering provides pretty good isolation of everything but the low frequencies.

The space I want to fill, is 3 areas separated by squared concrete columns.  Each space is exactly 4ft wide, 8ft tall and 6" deep.  I have rolls of 4ft wide by 3in thick fiberglass, 4ftx8ftx1/2" sheetrock, 3/8" plywood,  1/4" fiber and concrete based composite board, and some 1" foam rubber sheets.  I also have some epoxy that when mixed is about the consistency of latex paint, so I could use it to make a double layer of sheetrock and or concrete fiberboard very stiff using the epoxy.

With 4ft of space between the columns, construction should be super simple, which is good because the easy way is what I like best.  I'd also like to tune the center panel the lowest since it is the only one directly on axis.

My intial thought was just put a layer of fiberglass then sheetrock and do 2 similar layers to use the 6" depth available.   When I read about panel trap design, air space depth seems important.  I want to optimize blocking sound transmission and absorb as much bass as possible.  I could even use some 1"x3" wood that I have to increase the depth another 2" if it is a significant benefit.

I guess my question is should I just keep it nice and simple with 2 layers of fiberglass and sheetrock or would there be a big benefit to something more exotic?   eg  First, use the expoxy to glue to the current pressboard a layer of sheetrock and concrete fiberboard to create constrained layer damping, then just one layer of fiberglass with airspace, and lastly a layer of sheetrock (maybe a double layer of sheetrock in the center panel for lower tuning).

I don't plan to use any bracing except at the edges and will provide a good air seal for all layers.  I think full 4ftX8ft traps is my best bet since it is so easy and gives me very large panel areas.  Any advice is greatly appreciated.

[JLM]

John,

Your 6 inch deep cavity is perfect to take care of the isolation issue between this room and the rest of the home you mentioned (with a cheap/effective solution):

1.  Build a partition of 2x4s recessed 1/2 inch in from the front face of the concrete posts.

2.  Fill the space with fiberglass.

3.  Drywall and paint.

As I recall, your room is deeper than needed, so "giving up" 6 inches should be no big deal.  I've used this technique and it works very well.  Just keep a gap between the framing and the existing wall behind.

Now you have a flat wall (clean slate) to build whatever absorption/diffusion solutions you want.  Sometimes we get stuck as we try to fit preconceived answers into existing conditions.  This option would allow you to build to a more standard/perhaps ideal solution.

[ScottMayo]

I need advice for the best way to proceed with panel traps at the front of my room.  The rear wall is out for now as it is concrete.  I'm going with a dipole sub setup discussed in another thread, so only the front to rear axis is of concern.  I'm concerned about bass absorption, but my primary emphasis is preventing bass transmission, because the other side of this front wall is the main house.  Currently I have a layers of styrofoam, cardboard and 3/16 fiber board covering wood frame windows that overlook ...

Styrofoam is one of the best *conductors* of sound known to man. Absolutely useless for soundproofing.

Stopping low bass is almost impossible. The trick is to use layers, and it sounds like that's your plan. You want a heavy, largely rigid layer (pound for dollar, sheetrock is good and 2 layers of 5/8"th sheetrock is wonderful), followed by fiberglass and air. Repeat. You don't need much air as the air is just there to separate the layers, but make it at least an inch, and more doesn't hurt. If you use wooden frames, stagger the studs so they dont' face each other directly across the air gap, even if there is sheetrock between them.

It is *very* important to consider how these layers are supported. If you tie the layers to the same joists, the layer will vibrate, the joist will vibrate, and the next layer will vibrate, making the fiberglass and air pointless. Bass will go straight through.  Since joists are generally 16 or 12" apart, that's not easy to arrange without eating a lot of space.

I got very good results with:

1 layer 5/8ths sheetrock
1 layer homasote
1 layer 5/8ths sheetrock
mounted on standard 2x4 studs filled with rockwool, with short screws so 3 layers were never fully and directly connected

~9" of air

Another frame of standard 2x4 studs, staggered so the studs were not lining up with the first wall, with fiberglass insulation and
1 layer 5/8ths sheetrock

Yes, that's 4 layers of board and 2 layers of fiber. Yes, that cost something. The separate 2x4 frames were tied to separate joists (which were 12" on center in my case, in part so I could do this.) I went through merry hell getting contractors to understand this, but it worked.

There is no point in making any one part of the wall more resistant to sound than any other - bass frequencies flow like water in all directions and will always find the weakest point to flow through. Which means it will happily go up through a ceiling and back down again over a wall, something else you may want to think about...

[Ethan Winer]

John,

> I need advice for the best way to proceed with panel traps at the front of my room. <

A panel trap is a specific type of trap, and it does not resemble what you described unless I misread your post. Here's an article with plans for building panel traps:

www.ethanwiner.com/basstrap.html

Note that all bass traps work best in the room corners. You can put them mid-way on the walls, but they do the most in or near corners.

--Ethan

[JLM]

I agree with Scott, the insulated staggered stud wall with no common framing between each side of the wall is exactly what I've used before and am using in the listening room I'm building.  Works very well.  

John in your case, only the concrete posts would be common (conduct energy).

[JohninCR]

Thanks for the input guys.  No studs to stagger.  I essentially have a 4ft x 8ft x 6" box with 3 of the edge borders being concrete, so total isolation.  I'll just attach a 2"x2"s to the concrete for attaching the hard layers.  The top cap will be wood.  I'll just compress the fiberglass with the 1st hard layer to give me more air space in the final layer and double up the sheetrock on the outer layer for strength and lower tuning of the trap.

Ethan,
I've researched traps and the optimized design is narrower than what I will have.  In fact, your article was one of the first ones I ever read about room treatment.  I do believe mine will be bass traps because I've got a large heavy panel with a sealed airspace containing absorbant behind it.  A second layer behind that should just make it that much better.   I understand that typically corner placement is best, however, with my dipole subs, the concentration of SPL will occur directly on axis, not in the corners.  The effect I want to create is for bass to go thru that first layer and get dissipated within the layers.

[ScottMayo]

Thanks for the input guys.  No studs to stagger.  I essentially have a 4ft x 8ft x 6" box with 3 of the edge borders being concrete, so total isolation.  I'll just attach a 2"x2"s to the concrete for attaching the hard layers.  The top cap will be wood.  I'll just compress the fiberglass with the 1st hard layer to give me more air space in the final layer and double up the sheetrock on the outer layer for strength and lower tuning of the trap.

Some things to keep in mind:

Compressing fiberglass can make it less effective at absorption. You want the fibers to be free to rub against each other; that's where sound is turned to heat and how absorption happens. If you compress it too much, it starts to act like a rigid solid, which is bad. On the other hand, you don't need much air space to make this work, so maybe you don't need to compress too much. OC 703 (or maybe 705 for this) would probably be a good approach - it comes in a form that doesn't need to be squeezed and you can get it in 1" or 2" thick sheets. They are easy to work with.

Two courses of sheetrock screwed together will sucessfully reflect some deep bass. 3 layers will reflect even more. Mass matters. So If the goal is to prevent the transmission of bass into the space beyond, more sheetrock is good. But if you want to stop bass from echoing inside the theater room - which is often a goal - multiple layers might not be a good idea: you want some to get through, so it can be eaten by the fiberglass.

Watch out for strange effects at around 141Hz. Your box is going to react to that frequency, even with a layer of sheetrock over it. It shouldn't matter much, but...

Hey, no one said this stuff was easy.  

Finally, you can radiate bass in any pattern you like, but if you excite a room mode, it's going to resonate according to the shape of the room, no matter how you radiate it. Treating corners can help with this, and then you have less energy building up in phase, to batter into other rooms.

[JohninCR]

Scott,

Thanks for the feedback, now I feel like I'm back to square 1.  I want to try to optimize both.  Unfortunately material selection with reasonable pricing is very limited here in Costa Rica, so I have to work with what I have.  Here's my current train of thought:

At the back of my "box" use 3/4" of space for a hard stiff composite layer and use the epoxy to make it stronger and more rigid, with the idea that it will reflect bass.    

Then 2 independent layers of fiberglass and sheetrock, to act as a double layer trap.  Will that absorb more bass than a single layer trap with more air?  It seems logical to believe that double is better.  How much benefit would I get by adding 2" of depth to my "boxes" ?

Also, I'm not really concerned about the 1 room mode that could be excited.  I will be at about 28hz and that will be my XO point to my dipole mains, so  if I need to deal with in my XO.  I will check out that potential problem higher up that you mentioned.

[JLM]

John,

Back to square one is where I recommended you go in my first post here.

Take care of the isolation needs first (by minimizing any solid connections between your room and the rest of the house, loosely fill the void with fiberglass, then sheetrock).

Then work on bass traps.

IMO you must separate these two functions.  Seems you want to kill two birds with one stone, but it won't work in this case.

[ScottMayo]

John,

Back to square one is where I recommended you go in my first post here.

Take care of the isolation needs first (by minimizing any solid connections between your room and the rest of the house, loosely fill the void with fiberglass, then sheetrock).

Then work on bass traps.

IMO you must separate these two functions.  Seems you want to kill two birds with one stone, but it won't work in this case.

I have to agree. If you want to prevent sound from getting out of the room, you have to do substantial work around the room to isolate the room. It's expensive to do right and sometimes not worth it. Doing it will often increase the bass in the room, so then you tackle absorption and diffusion as needed. If your major mode is 28Hz, that's not too terrible and you'll find it managable with corner traps and so on.

[Ethan Winer]

John,

>  I do believe mine will be bass traps because I've got a large heavy panel with a sealed airspace containing absorbant behind it. <

Got it. I didn't understand that part. Yes, it will trap bass if you build it as described and it's air tight.

--Ethan

[JohninCR]

Scott,

Expensive in comparison to what?  My cost looks like they will be miniscule and I'll likely come in under my $600 budget.  I decided to rip out all of the existing walls giving me a wider room.  What I ended up with to start is a roof with exposed rafters, rear wall of concrete, a side wall of concrete with protruding rocks (some previous attempt at an indoor waterfall, which should be a pretty good diffusor), a front wall of concrete with 3 windows between the concrete columns which are spaced perfectly for me.  I've already closed the remaining open side wall with water resistant panels attached to the existing metal security fence.  This first layer of wall has an airtight and waterproof seal.  This leaves me with just over 20ft of length and 11ft of width.   I plan to construct 2 additional sheetrock walls inside the exterior wall, with all 3 damped with fiberglass, and each sealed and isolated from each other.  For the ceiling I'll use fiberglass between the rafters, then sheetrock and seal.  The second layer of ceiling will form an airspace with the shallow peak.  I'll do the math to get the best ratio of room dimensions before I start the interior walls and 2nd ceiling.  I plan to use varying stud spacing, and the end result is that all of my added construction acts as panel bass traps, so I get the isolation I want plus dissipate significant amounts of bass making additional traps unnecessary.

With my dipole setup, the construction of these "boxes" covering the windows at the front are somewhat critical.  Not only do I want to achieve good isolation, but especially in the middle box, I need to absorb and dissipate as much of the on axis dipole bass as possible coming from my front sub.

That puts me back to thinking that I should go with a hard stiff layer of contrained layers to cover my current soft multiple layers.  I'll consider that to be my isolation and if inadequate I can easily go inside and take the window panes out, add fiberglass and cover that 4" deep space with sheetrock.  The window frames would make that very easy.  The current windows don't look bad as is with the wooden window frames and white styrofoam on the other side of the windows.  The window frames make little shelves for the wifes nick-nack decorations.

That leaves me with 5" or 7" of "box" depth to create the best panel trap possible for the lowest frequencies.  My gut is telling me 2 layers is better.

Once the construction is ready, then I'll deal with diffusor and absorption to tune the room with my dipoles in place.  I doubt bass is going to be an issue and I doubt that the corners will have bass concentration with dipole dispersion, but I'll find out later.

[JohninCR]

Between my ceiling and walls I and doors I ended up with over 400sqft of panel traps of many different dimensions  I'm glad I went to all the trouble of sealing every crack and joint with silicone as I went through the different layers.

Not only did I get the isolation that I was looking for, but even though I'm going dipole I tested the room for bass with tone sweeps and bass heavy music.  Both listening and my spl meter showed no modal peaks and even in the concrete corner there is no accumulation of bass, so the traps are functioning as I hoped.

Adding a false wall or 2 with wide and varied stud spacing is so easy cheap and effective that I highly recommend it to anyone serious about their listening.  I essentially created an 11ft x 19.5ft listening room/HT/office from 2 existing walls and wood lined tin roof for a total cost of $600 .  All that's left is some fine tuning with absorption and possibly some adjustable diffusers after I get the permanent speakers built.

[ctviggen]

Are you sure there are no modal peaks?   That would be impressive.  What do you mean by "tone sweeps"?  I think that testing with music is meaningless -- how could you tell whether one part of the music had a modal peak?  Where are your speakers located?  Are you using a sub?

[JohninCR]

Yes, I was using my big EBS ported sub for testing how well the bass traps functioned and the degree of isolation achieved.  Searching for room mode problems I used sine waves from the test tone generator on my computer.  I used my ears and music to search for accumulations of bass anywhere in the
room and I was happy to find a nice even bass response even near the walls and in the corners.  Outside the room showed the bass wasn't escaping to avoid accumulating, which means it's being dissipated within my construction as intended.

[Red Dragon Audio]

Nice job JohninCR!  You ought to post some pictures here to display your handy work.

[JohninCR]

Nice job JohninCR!  You ought to post some pictures here to display your handy work.

Thanks!  Pictures to come when complete.  It's raw and quite a mess right now.  Plus I've got to build the speakers.  I've just been running prototype baffles of cardboard or plywood, trying different things at the first reflection points and planning the home stretch while I spend too much time listening to some simple OB's now that I have space in back.

Wait till you see what I have planned for my motorized remote control toe-in, which also swings the music baffles out of the way for the OB line arrays used in HT, along with my motorized solution for video masking for the occasional wide screen movie (most DVD's are pirate copies down here ripped in 4:3, so I have to be set up for both).