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Old 2nd October 2008, 03:23 AM   #1
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Default 18mm vs 3/4 inch plywood

I have been lurking these forums for a long time, but just starting to post now.

I am looking at building the fostex 208sigma horn for a set of 206s,

In my local lumber yards I am able to get 3/4 plywood but none of them carry 18mm.

Will the difference between the two make a difference in the sound of the horns or are the tolerences close enough to get away with?
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Old 2nd October 2008, 04:32 AM   #2
CLS is offline CLS  Taiwan
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The difference is only 1mm.

For a bass horn, that's nothing.
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Old 2nd October 2008, 04:40 AM   #3
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More important than the mm of thickness is, how many layers in the ply? 18nn BB has 13 layers

But you can only build it out of what you can get.

dave
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Old 2nd October 2008, 09:51 PM   #4
hubsand is offline hubsand  United Kingdom
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Are more ply better, or worse?
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Old 2nd October 2008, 09:53 PM   #5
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You would think more is better to reduce noise introduced by the cabinet but there are differing theories on that. Is that what you're alluding to?
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Old 2nd October 2008, 10:00 PM   #6
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Quote:
Originally posted by hubsand
Are more ply better, or worse?
more better... makes it stiffer and gives more transition interfaces to disapate energy.

dave
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Old 2nd October 2008, 10:29 PM   #7
chrisb is online now chrisb  Canada
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Default Re: 18mm vs 3/4 inch plywood

Quote:
Originally posted by s_aloisio
I have been lurking these forums for a long time, but just starting to post now.

I am looking at building the fostex 208sigma horn for a set of 206s,

In my local lumber yards I am able to get 3/4 plywood but none of them carry 18mm.

Will the difference between the two make a difference in the sound of the horns or are the tolerences close enough to get away with?

Try Windsor Plywood - the local branch here carries Baltic Birch (i.e. the 5'x5' Russian product)

http://www.windsorplywood.com/stores/prince_george.html


There are also EuroBirch panels (4x8') with higher quality face veneers available through trade suppliers like Reimer and P.J. White Hardwoods, but they tend to be much higher cost per square foot.

As to the dimensional differences, bear in mind that "imperial" plywoods can easily vary in thickness by almost .5mm within the same sheet .
The 1mm difference generally won't affect the sonic performance of most enclosure designs, but you do need to adjust the plans for parts to actually fit together*. This will be particularly important for designs that have critical internal clearances or complex foldings wherein the dimensional difference can compounding significantly.

(* how would he know that?)
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Old 3rd October 2008, 08:46 PM   #8
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Thanks for pointing me at Windsor plywood, they have 3 sheets of 5X5 BB in stock.

They also pointed out that 18mm is basically 3/4 inch with one side finished.

Thanks for the help.
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Old 3rd October 2008, 09:03 PM   #9
chrisb is online now chrisb  Canada
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Quote:
Originally posted by s_aloisio
Thanks for pointing me at Windsor plywood, they have 3 sheets of 5X5 BB in stock.

They also pointed out that 18mm is basically 3/4 inch with one side finished.

Thanks for the help.
Not that it matters much as far as a speaker's sonics are concerned, but the second sentence makes no sense - .75"x 25.4 = 19.05mm.

Even with either a standard paper backed veneer or Post-form grade plastic laminate on one face, the net thickness of the panel would be between 18.65mm and 18.75mm (depending on adhesive) "General Purpose" grade P-lam or the wood backed veneers will add at least double that additional thickness to each finished face. (typically allow 1.5mm for material and glue)
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Old 3rd October 2008, 09:04 PM   #10
Aengus is offline Aengus  Canada
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Quote:
(* how would he know that?)
Probably looked at some stuff I've built.
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