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Old 3rd October 2008, 02:54 AM   #1
Pashley is offline Pashley  Canada
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Default basic queries on build details

For an 8" full range driver (Beyma 8CX20) and 10-inch passive radiator, with an internal box size around 14" (35 cm) cube, are two layers of MDF well glued strong enough to work without internal bracing? Simple construction is more important to me than weight.

One site I read http://www.vikash.info/audio/xls10/ mentions "Three coats of diluted PVA glue applied (1:1) to seal the open-pored MDF. Sanded down with 500 grit after each coat leaving at least a couple of hours between coats. The result is a glassy-like finish ready for priming."

Some sort of sealant seems a good idea but why glue? Would urethane (two coats, the first diluted) do as well? If you are painting the outside of the box, does that need sealant, or just the inside? In general, what's the best way to deal with this?

It seems to me there should be some sort of sealant under the flange where the driver mounts to the box. Do drivers come with this? Or does the metal frame just come down to the surface and provide an adequate seal? Are there standard sealing rings available? If not, what does one use? felt? rubber? a bead of silicone?
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Old 3rd October 2008, 05:53 AM   #2
Nanook is offline Nanook  Canada
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Default don't do it!

No MDF please. If it's worth spending your time and money on (good quality drivers), why cheap out on the material for the enclosures? Baltic Birch is superior in every way, except cost. And here in Canuckland it is relatively inexpensive.

Also BB is watertight, and so no "sealant" or glue is required. Properly constructed, a box that small won't need anything more than a single layer of 18mm BB ply, perhaps with glue blocks where the sides meet. You can use RTV (room temperature vulcanizing) sealant between the driver and the enclosure if you like. Just know that it may be impossible to get the driver out once "glued" in.


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Old 5th October 2008, 04:00 AM   #3
Pashley is offline Pashley  Canada
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Default Re: don't do it!

Quote:
Originally posted by Nanook
No MDF please. ... Baltic Birch is superior in every way, except cost.
I thought MDF was supposed to be better for speakers -- heavier, less resonant, something like that.

Quote:
And here in Canuckland it is relatively inexpensive.
I'm in China. I have not looked seriously yet but just peering into a few places selling materials, all I see is low-grade ""plywood" (veneer/rubbish/veneer sandwich) up to 12mm. No MDF, no decent ply, no 18 mm... Maybe I will find those eventually.

Lots of marble, tile, and hardwood or compressed bamboo flooring, though. Also chopping blocks up to 50 cm of laminated wood, ...
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Old 5th October 2008, 08:51 AM   #4
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Default Re: Re: don't do it!

Quote:
Originally posted by Pashley
I thought MDF was supposed to be better for speakers -- heavier, less resonant, something like that.
A widespread myth. heavier, less stiff, lower frequency resonances, high energy storage, not airtight, and due to its consistent density, effectively acoustically transparent at some frequencies.

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Old 5th October 2008, 11:03 AM   #5
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Good quality birch ply is better, seems to make a bit less dust when cutting or routing and takes knocks better. But MDF is not as bad as being made out!
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Old 5th October 2008, 06:41 PM   #6
Nanook is offline Nanook  Canada
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Default welll, if in China...

check out bamboo ply. It is reportedly very good, but unsure what might be available in China (even though bamboo must be everywhere). If wood working facilities are available to you, I'd consider laminating up some bamboo flooring into panels and run it through a planer or a large drum sander. Use that for your panels, finish both sides of it. Speakers would be beautiful.

I guess you could call out to members in China to see if they may be able to point you to a source....



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