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Old 15th August 2008, 07:04 PM   #1
steve71 is offline steve71  Australia
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Default Ply Wood - how importaint is void free?

I can get 3/4" 5x5 BB ply wood thats advertised as "void free". It's $65 per sheet.

My local Lowes has Arauco Ply in 3/4" 4x8 for $40. It's considered "almost" void free and looking at the end cut it only has a few small match head voids (but who knows what lies within). Would this be acceptable, or would it be too much of a gamble?

I'm entirely new to this so any advise is much appreciated.

BTW the ply will be used to construct a conical midbass horn with 1.5" wall thickness
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Old 15th August 2008, 07:07 PM   #2
badman is offline badman  United States
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Some people will tell you to go with the more expensive stuff, as it'll sound better and have better working properties.

But is this a 'final' build? A cabinet you'll be putting a lot of work into finishing?

If it's a 'test', then it's likely worth the cost savings. I've made some excellent enclosures with the 'premium' ply from lowes and home depot, and with care, it can be perfectly okay, just not as nice as the 3x pricier stuff (natch).
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Old 15th August 2008, 08:44 PM   #3
GM is online now GM  United States
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Default Re: Ply Wood - how importaint is void free?

Quote:
Originally posted by steve71

Would this be acceptable, or would it be too much of a gamble?

BTW the ply will be used to construct a conical midbass horn with 1.5" wall thickness
Having gone through the tedious affair of tracking down/filling spurious buzzes in so-called high grade plywood, I avoid it like the plague for any high SQ app if it's not guaranteed void free, especially for a high pressure one such as a mid-bass horn. Indeed, some folks prefer to use MDF when the horn's Fhm design spec is below MDF's Fs (~250-400 Hz depending on a number of variables, so in your case, doubling up would be going in the wrong direction if it's used. Instead, you would add/increase bracing to push its Fs higher.

GM
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Old 15th August 2008, 09:59 PM   #4
Andy G is offline Andy G  Australia
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using cheaper ply with voids in it can add some nice box resonances to the sound, particularly if the plys get a bit loose around some of the voids.

I wouldn't use anything else for quality boxes or PA speakers.
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Old 15th August 2008, 11:26 PM   #5
steve71 is offline steve71  Australia
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Default Re: Re: Ply Wood - how importaint is void free?

Quote:
Originally posted by badman
Some people will tell you to go with the more expensive stuff, as it'll sound better and have better working properties.

But is this a 'final' build? A cabinet you'll be putting a lot of work into finishing?

If it's a 'test', then it's likely worth the cost savings. I've made some excellent enclosures with the 'premium' ply from lowes and home depot, and with care, it can be perfectly okay, just not as nice as the 3x pricier stuff (natch).
It's going to be the first and hopefully the last build. although that might be completely naive thinking.

I used hornsrep to come up with the design, but basically I'm just modifying Erik horn (http://www.volvotreter.de/pics_plans.htm) to work with a GPA 515G driver.

Quote:
Originally posted by GM


Having gone through the tedious affair of tracking down/filling spurious buzzes in so-called high grade plywood, I avoid it like the plague for any high SQ app if it's not guaranteed void free, especially for a high pressure one such as a mid-bass horn. Indeed, some folks prefer to use MDF when the horn's Fhm design spec is below MDF's Fs (~250-400 Hz depending on a number of variables, so in your case, doubling up would be going in the wrong direction if it's used. Instead, you would add/increase bracing to push its Fs higher.

GM
OK GM, I think you have me convinced. That makes the decision easy. The expensive ply is going to be too expensive (I need one 4x8 sheet per horn per layer to build to these horns).

MDF is cheap and void free so it looks like my best option.

So you're saying the MDF's Fs is 250-400hz and that since my horns upper frequency is 500hz, I'll need to add bracing to raise the horns resonant frequency out of the bandpass. Doubling the wall thickness will stiffen the horns (good), but the added mass will lower the horns Fs (bad). So bracing is the best solution since it adds stiffness without adding too much mass, right?

Now I just have to come up with some elegant bracing ideas since the outside of the horn will be visible.

Quote:
Originally posted by Andy Graddon
using cheaper ply with voids in it can add some nice box resonances to the sound, particularly if the plys get a bit loose around some of the voids.

I wouldn't use anything else for quality boxes or PA speakers.
Thanks Andy, I'm getting the picture. Cheap(ish) ply isn't worth the risk for a final build.



Cheers to all! Thanks for the help.

I posted up the Hornsrep data here if anyone wants to have a gander.

http://www.audioasylum.com/forums/hu...13/135852.html
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Old 15th August 2008, 11:40 PM   #6
pjpoes is offline pjpoes  United States
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higher quality void free is also typically stronger, in my experience, and thus will also last longer. I've had cheaper standard grade plywood rip apart with sub boxes.

There are actually different grades of void free, and the different construction styles, materials etc. While going nuts and buying something like say mahogany plywood might be a bit excessive, a hardwood plywood with pva adhesive (or better) is not, and is a good thing to look for.

I would not recommend standard grade marine grade plywood as its not typically void free, and I've had some problems with it. However this is some higher quality void free marine grade plywood, even some with teak surfaces, which I do find to be a great working product.
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