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Old 26th August 2004, 09:51 AM   #1
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Default What MDF thickness is acceptable?

It seems that I can't find 3/4 '' (19 mm) MDF here, no wonder, is 16 mm ok for a 1mx20cmx20cm enclosure, with a 17 cm, 50 W driver? Theoretically it will go down to 30 Hz (-3dB). It won't be used at very large power, will it be rigid enough? Should I do bracing?
I could extend the question and ask what is the rule of thumb for choosing the thickness of the MDF based on enclosure parameters.
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Old 26th August 2004, 10:55 AM   #2
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Maybe if you put in just 1 brace on the longest walls it will be fine.
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Old 26th August 2004, 02:23 PM   #3
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If you can buy/afford 19mm Baltic Birch, this would be the preferred material. If not, a double thickness 16mm cab will be slightly more rigid/damped than the 19mm BB. In either case, a single brace near the driver is sufficient.

If 16mm is used, then since the driver should ideally be ~35.92cm down from the top, a brace above and below the driver will provide sufficient rigidity for it. If the driver is placed near the top, then a brace below it and another one ~2/5 of the way down from it is sufficient.

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Old 26th August 2004, 04:02 PM   #4
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GM,

I don't understand what you mean by "double thickness".

Since plain 19 mm MDF isn't available, so much for birch. At least I suppose, I'll ask around.

The driver will be placed near the top (about 25 cm).

And concerning the bracing itself, since I haven't given it too much thought until now, how should it be done? Is there a preferred way of doing it? How should the material be chipped out? Should the holes be round or square? Oh, I sound like a little boy boring his parents with too many questions, sorry
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Old 26th August 2004, 05:21 PM   #5
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Build the cab, then glue another layer of MDF on top of it. I recommend making the driver hole larger in the base cab, with the proper size cutout on the top layer, or vice versa if you want the driver flush or inset. Better still, make them the proper size and after the second layer is installed, bevel the inner 'baffle' out at ~45deg to allow the driver to 'breathe'.

For such a small cross sectional area (CSA), I would just use some relatively small diameter dowels or similar sized scrap hardwood to form a 'X' brace, glued or screwed together where they cross. Or you can make a 'shelf' brace cut out to mimic these in ~total open area, though using free form irregular shapes are preferred to reduce any standing waves of any amplitude from forming/summing as happens when four equal squares/rectangles or all the same size diameter holes are used. If other than a round material shape is used, then I recommend covering them with some damping material.

"I could extend the question and ask what is the rule of thumb for choosing the thickness of the MDF based on enclosure parameters."

I see I missed this Q on the first response and do not have time right now to go into all the considerations. Anyway, it deserves a thread of its own, though before posting one, please search the archives, I bet it has been covered more than once.

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Old 26th August 2004, 06:03 PM   #6
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OK, cleared the bracing issue, next question.
Here comes a translation problem. In Romanian they call this PAL (translation is something like wooden agglomerated board), but I'm not sure it is the same thing as plywood. And you can imagine that those who sell it don't know either. If PAL = plywood, it is strange, because MDF is supposed to be harder and heavier than plywood, but this PAL thing isn't. It is quite weak and light. I tried to break a 5 cm scrap and it broke in an instant without much resistance. It doesn't seem quite right for speaker building, doesn't it?
Since plywood is recommended over MDF (at least I think so, judging from what I have read here), hmm... Sorry, my question is: how can I tell if this PAL thing is the same with what you call in English plywood? How can I tell for sure?
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Old 26th August 2004, 06:51 PM   #7
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When viewing the edge/end of plywood you'll see distinct layers of material glued together. Standard plywood may have 6 layers, higher quality plywood will have 9 or more layers all made of real wood, not sawdust or chips. It is very difficult to break by just applying pressure and will splinter rather than break.
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Old 26th August 2004, 08:59 PM   #8
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Default brief translation

Mr. push-pull

MDF = PFL dur (placi fibrolemnoase)
Plywood = placaj
Particle board = PAL (placi aglomerate din lemn)

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Old 27th August 2004, 06:25 AM   #9
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Plywood is preferred (at least by me) over MDF.

I'd tend to build a well braced single layer box over a non-braced double-layer box. bracing should be non-symetric and run the long direction (dividing the 20 cm panels into even narrower sub=panels (ideally not rectangular). If the bracing also ties the box walls togther that is good too.

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Old 27th August 2004, 07:43 AM   #10
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Default Re: brief translation

Quote:
Originally posted by Ioan
Mr. push-pull

MDF = PFL dur (placi fibrolemnoase)
Plywood = placaj
Particle board = PAL (placi aglomerate din lemn)

Ioan
Placuta surpriza Deci de asta mi-a spus vecinul ca n-a auzit de MDF in viata lui. O sa reformulez intrebarea cu PFL...
Sorry about that, I said "that's why my neighbour hadn't heard about MDF in his life" (the local acronym seems to be PFL). Some use MDF, some PFL. OK, so that scrap wasn't plywood, neither MDF, it was some weak agglomerated (or is the right word sintered?) board.
OK, I'll search for plywood too.
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