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jeepy1260

Plywood thickness USA vs Europe

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Hello all,

 

I am in the process to rebuild a Cornwall  I pair in the horizontal collector version in mahogany finition. (will be beautiful!)

However US Plywood thickness standard =  3/4'' = 19 mm doesn't correspond exactly to european= either 18 or 22 mm!

What you, Cornwall lovers and klipsch lovers,  would recommend. I think that the cabinet design , type of wood and thickness  has an influence on the quality of the sound. Paul Klipsch

did kow that for sure! . 22mm could be too much?

Thanks!

Jp from Switzerland

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18mm is perfect, 22 is just massive.
Go for good quality ply, and if in doubt add a brace.


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Just make sure you keep the internal volume the same as originals and change any differences in dimensions outward versus inward.  Otherwise, you reduce some cabinet volume.  Not a lot, but some.

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3 minutes ago, avguytx said:

Just make sure you keep the internal volume the same as originals and change any differences in dimensions outward versus inward.  Otherwise, you reduce some cabinet volume.  Not a lot, but some.

Yes it is what I will do. Start from the internal dimensions! Thanks

Jp

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  • Yes, you should keep the internal dimensions the same, for sure.
  • I'd be inclined to use the 22mm plywood.  As far as damping out resonance is concerned, the thicker the better.  When Klipsch improved another speaker, the La Scala, making the La Scala II, the only non-cosmetic change was to increase the thickness of the lumber; many listeners said the bass improved (we are now up to the La Scala AL-5, which has a new tweeter).
  • Be sure to get void free plywood.

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3/4" Plywood is never actually 3/4" thick, its much closer to 18 mm.

 

Stay away from the Chinese stuff.

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6 hours ago, jeepy1260 said:

US Plywood thickness standard =  3/4'' = 19 mm doesn't correspond exactly to european= either 18 or 22 mm

 

3/4" plywood is 18mm plywood.

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  I think the 22 mm will make a better option also.

  Hate to say it, but keeping the exterior dimensions constant will not make any difference. The internal dimensions of the Cornwall will be 8 mm less tall, wide, and deep. That is 0.315 inches. The TS parameter variation in drivers is much wider than this change in internal volume. 

   If they were mine the internal dimensions would be as close to spec as possible. I am OCD about things like this. 

  Made a much larger mistake than this in a horn cabinet once. One was per spec, one was off. They sounded the same. Best I could tell they measured the same. Cabinets are very forgiving.

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Do they have good wood plys in the Baltics?

 

😛

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Well the description of the thickness is just a guide. Working with some 18mm and 25mm Baltic Birch and measure with calipers .678" (17.22mm) in the 18mm and .985" on the 25mm.

 

  One of my ongoing projects plugged in .75 because I was assuming .75" was 3/4" in the plywood world. Using 18mm Baltic and figured it was close enough and end up having to account for lumber whose actual thickness size I should have measured before cutting.

 

  If I ever start building speakers for more than myself I will juggle the cut layout based on what that lot of plywood measures and not assume ever again.

 

 One other thing here. I had run short of wood and my local lumberyard had 4' x 8' Birch plywood. So I bought a sheet. Get it home and go to cut and see made in China on the side. Let me tell you that garbage has huge voids and the plys are folded and inconsistent and it is far lighter. It even cuts different with splintery cuts and there is just nothing I can say good about that garbage. NEVER again. I can pick up a true Baltic Birch piece and rap on it and it sounds entirely different than the China imitation of  good stuff and weighs much more.

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22 hours ago, donmark said:

18mm is perfect, 22 is just massive.
Go for good quality ply, and if in doubt add a brace.


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And 25mm is AWESOME since thicker is better. You build something for yourself don't stop at adequate when you can have superb. It does not cost much more and your big expense is your time so don't cheat yourself in this important area. At the very least make the front and back 25mm. Used 25mm on the sides of a set of La Scala style bass bins and the resonance was gone and needed no bracing.

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Tolerance has got to be upwards of +/- 1mm, anyway.  Maybe much the same on every sheet if all the material is from the same batch, maybe not.  You could always leave capping cuts a tad proud and trim with a router after assembly.

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And 25mm is AWESOME since thicker is better.

 

I don’t even think they have 25 in stock at my shop.

 

 

 

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  Try to find a cabinet supply company. Usually there are suppliers around. Cabinet shops require higher quality plywood than the HD and Lowe’s products. 

 I used some lumber core Birch about 25 years ago. This was impressive stuff. Later started using European 19 ply that was void free. But the sheets were not 4x8’. 

  

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I'm pretty sure if it's Baltic birch it's European.  Don't know how it could rightly be called that if it weren't.

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The Baltic comes in 60" x 60" sheets unless special order and then I would have to buy a bunch of it. Yes European perhaps but I thought it was Russian. I am lucky here in Nashville since there are so many custom speaker builders around. 1" or 25mm in two places at least.

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On 4/12/2019 at 11:04 AM, Dave A said:

And 25mm is AWESOME since thicker is better. You build something for yourself don't stop at adequate when you can have superb. It does not cost much more and your big expense is your time so don't cheat yourself in this important area. At the very least make the front and back 25mm. Used 25mm on the sides of a set of La Scala style bass bins and the resonance was gone and needed no bracing.

 

'fraid I have to agree with Dave whom I've never met.

 

Jubilee story as I understand it from someone who helped design it....

 

They used 3/4" ply with some form of variance.  Let's say .10" variance so anything from .85 to .65 is good.  They had some complaints/issues that the Jubilee bass bin was buzzing.  Long story short, it turned out that their plywood supplier was accurate enough to provide most of their stock at .65" which is within specs so there was nothing the factory could do about it.

 

The thinner panel was vibrating when it was used as the back panel of the jubilee bass bin.  Other panels didn't vibrate as I understand, just the back panel.  If they had a .75 piece on the back panel, it didn't vibrate.  This is why Roy came up with the little strut braces in there to help deaden the back panel.

 

He's since redesigned it to have the "shelves" in there but I'm venturing to guess (without any knowledge from Roy) that had the speaker never buzzzed in the beginning, they would have never put the struts nor shelf braces in there and the speaker would have a big fat open mouth today.  (perhaps much like my wife accuses me of having.....or was it me accusing her???  :emotion-14: )    (I'm so going to burn in hell for that one)

 

The world might be a different place if .75 really meant .75!!!

 

So I agree with Mr. Dave....  if I were doing it, I'd go the slightly larger thickness.  I'd hate to go through all that effort and end up with a buzzy box.  (doesn't mean it would happen).  I just like to remove all the odds I can that are easy to remove and that's one of them.

 

I also think that's how the factory does it with the proud surface, then they route it down.

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