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Old 27th February 2014, 07:03 PM   #1
diyAudio Member
 
Join Date: Feb 2014
Default In wall boxes..... Questions....

Hi guys and gals,

This is my first time posting so be gentle!

I am curious about taking on a very small project compared to some of what I have seen. I plan on buying some in wall speakers to appease the wife. At the moment I have a decent marantz av receiver and from the research I have done am interested in the paradigm brand. They seem to be fairly high quality as far as in walls go.

I plan on purchasing an AMS LCR for a center. Two ams 350's for the left and right, four AMs-150r's for surround channels, and two in ceiling subs also paradigm with a 300w per channel paradigm amp.

It will be quite the purchase to be honest and in terms of application I would be using it both for music and ht.

My wife will not allow floor standing or on wall as much as the performance might be better so I am trying to get the best performance I can from in walls, plus it sounds quite fun and interesting for my proposed project as well.

My questions there in lie...

I would like to build boxes for each speaker and sub and am looking for tips. My plan is to go with 2 cu ft per box, except for left and right which will be 3 cu foot. These are roughly the sizes of the boxes offered by paradigm. But the boxes are $300 each! No way I'm paying that, and I simply can't afford it.

I want to build boxes out of 3/4 inch plywood or mdf. I planned only be using wooden dowels for prancing. In the speaker and sub boxes. Is MDF or plywood better?

Also I know that dampening is something most people do. I can tell the in wall boxes have quite a bit of what looks like polyfill or something inside them. I have heard for subs about 1.75 lbs of polyfill per cu foot is about right. Is this correct?

I was also thinking of making an outer layer around the sub boxes with mdf/plywood and putting green glue in between the two layers. Is this safe on wood to wood applications? Couldn't find anything on the green glue website. Also is this too much dampening? Was trying to prevent billed over into the wall cavity from the subs, as I would prefer an accurate sound over simply loud and muddy.

Is there a best way to dampen speakers? Is polyfill still a good idea? Or should I just buy a walmart mattress pad and use memory foam? Or maybe some cheap acoustic foam? For things like foam... How many walls inside the box should I put it on? For polyfill is there a cu ft to lbs ratio for speakers instead of subs? Or is it the same?

Also... I was just going to use screws and glue for assembly but is this bad? Or is there a certain type of glue to use? Maybe silicone to seal up the inside of the box?

For wiring I was just going to drill a small hole and then fill it with silicone, is this bad?

Unfortunately I will not be able to test my boxes prior to application considering I will have to install them with new construction (happening now) and then buy the speakers at a later date.

If anyone has any familiarity with this line of paradigm and has suggestions that would be great!

Please feel free to correct or bash any of my plan! I really am excited about the building process but I want to do it right!

Much appreciation for your time! If I can think of any other questions I'll try to post them!
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Old 27th February 2014, 09:49 PM   #2
laplace is offline laplace  Australia
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Join Date: Jan 2014
How thick/hollow are your walls? 2-3 cu.ft is a lot of volume to fit in a wall, especially once you subtract 36mm for the front and back skins of your boxes...

Damping material density is a design decision for which we can't tell you one definite answer upfront, it's a function of the box design and the drivers chosen. It's used to control box Q.

Nothing wrong with screw+glue. Use PVA or outdoor-rated wood glue. MDF and ply are both fine, what matters more is how you brace the boxes.

For the terminals, you buy a terminal block (e.g. 2x banana plugs, or a Speakon), cut the right size with a holesaw and then screw the terminal block in with something to seal it. Yes you can seal the wood joins with silicon, but don't silicon the terminal block or drivers because they'll never come out again if you do. Use 2mm-thick compressible closed-cell self-adhesive foam strips, they will compress down to nothing and seal perfectly.

Not being able to test is a Really Bad Thing.
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Old 28th February 2014, 04:17 AM   #3
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Luckily the left right and center are in a pocket wall that has quite a bit of depth. The two subs and 4 surrounds are in the roof so I have space there as we'll too.

The foam is a great idea for seals. Are dowels ok for bracing? Or should I be looking at something else?

Theboxes will be simple square design with internal bracing nothing fancy.
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Old 28th February 2014, 06:50 AM   #4
laplace is offline laplace  Australia
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sheets of MDF or ply are better for bracing, because they provide both shear and bending-moment support to the box faces by supporting along a line instead of at points. holesaw most of the volume out of each brace first.

Don't make square boxes; you want the three dimensions to have no common factors in order to avoid standing waves in the boxes.

I suggest looking at some existing designs in detail and stimulating their performance to see what is typical or achievable with a particular volume or driver.

If the boxes are permanently fitted in the walls, you might consider making them a bit oversize and then tuning them in volume by adding (very well sealed) glass bottles of water to consume volume. That would give you some flexibility of design.
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Last edited by laplace; 28th February 2014 at 06:56 AM.
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Old 1st March 2014, 09:46 AM   #5
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My advice would be to follow laplace's fantastic advice!

From my experience bracing is very important have a look at B&W's Matrix design, sheets of MDF or plywood are better than dowels although iv'e had good results from three dowels spaced out in middle of panels.

Definitely consider internal walls tilted like laplace said there great for avoiding standing waves.

Glueing 4mm bitumen to internal panels (except baffles) has gave me great benefits from less cabinet vibration yet I'd recommend 8mm felt glued on the bitumen to prevent standing waves almost altogether, especially on the driver enclosures in the roof.

Polyfill is fine and I believe sealed enclosures need more wadding in general. The wadding's thickness and location is more important than which material it is. Have a look at Troels Gravesen website for great info on dampening.
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