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Old 31st May 2013, 11:58 PM   #1
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Default Passive radiator sub box thickness

Planning to put together a passive radiator box with a TC Sounds LMS Ultra 5400 and two 18" VMP passive radiators. My first thoughts were to make it half a meter thick to stop wall vibration, but wondering if perhaps this might overkill. (No I wasn't literally going to use .5m walls, but something like 50mm MDF).

Being that the passive radiators are going to absorb / move instead of a cabinet wall, how thick do I really need to go to stop resonant vibrations?

Would 32mm MDF be enough? It would simplify the build.
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Old 1st June 2013, 12:11 AM   #2
NEO Dan is offline NEO Dan  United States
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32mm cardboard is ok, ply is better and 19mm ply with a brace to the opposite side wall is best. Flex is one thing but it takes some doing to stretch and compress ply.
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Old 1st June 2013, 12:41 AM   #3
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Unfortunately we pay too much for decent ply here, and I have never worked with it before, but good point the laminations are probably going to give it less chance to do its own thing. Would 19mm ply be thick enough or would I have to double up? Doubling up would allow me to overlap the seams.

But considering the cost of the drivers etc, it probably is worth springing for ply on this one.
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Old 1st June 2013, 03:25 AM   #4
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I personally like (poplar) cabinet grade 5-ply 3/4" (18mm). There may be some voids but I don't see it as a serious issue. I'd much rather use that then MDF for the same price, lighter and stiffer.

But bracing is just as important, you can go a good 8" between braces on the walls if they are stiff braces.
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Old 1st June 2013, 04:39 AM   #5
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Quote:
Originally Posted by turbodawg View Post
I personally like (poplar) cabinet grade 5-ply 3/4" (18mm). There may be some voids but I don't see it as a serious issue. I'd much rather use that then MDF for the same price, lighter and stiffer.

But bracing is just as important, you can go a good 8" between braces on the walls if they are stiff braces.
Which leads us back to the question of wall thickness. If the walls are adequately thick enough, do I need any bracing, or possibly could I get away with less?

Don't know where you get your ply but here it is 2x or 3x dearer than MDF.

What is the best ply to use? Assume structural grade or marine?
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Old 1st June 2013, 04:50 AM   #6
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Originally Posted by NEO Dan View Post
32mm cardboard is ok, ply is better and 19mm ply with a brace to the opposite side wall is best. Flex is one thing but it takes some doing to stretch and compress ply.
Have been giving some thought to the bracing over the last couple of weeks and was wondering how a horizontal X might go in the box... Figure with driver cones on 3 out of 4 faces, an X could be secured to each of the 4 corners and not get in the way of the drivers. If I used more than 1 in the centre, each layer that is further away from the centre could have a bit more meat on it as the driver cone reduces.
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