Double Top Guitars

[billgennaro]

Originally Posted by Tim McKnight

Traditionally we adjust the stiffness of the top plate by graduating it. One is unable to do this on a Double Top because a .030" piece of spruce does not have enough material to graduate. Add to that if one dings the top, during construction there is no way to sand that ding out. Its a mind boggling and tedious process to say the least.

I was just starting to think of this as I was reading your post. And then you went ahead and confirmed my thoughts. That would be a drawback to those luthiers who like to voice their tops in that way, wouldn't it?

Bill

[billgennaro]

Originally Posted by Jim.S

It is just hard to see how that floppy nomex could be stiffer so I then guess that the epoxy is the culprit.

Jim

Jim,

Perhaps thinking in terms of how a cardboard carton is made with outer layers of cardboard and an inner layer of a cardboard mesh. The mesh itself is not very stiff but, together, they form a stronger piece of cardboard than if you just used one thick piece without any core.

The I-bean analogy that Tim refers to becomes apparent. An I-beam gets its strength from the outer layers. The inner, thick core just separates the two outer layers and also makes the beam much stiffer in the process. I believe that the thicker the core the stronger the I-beam. Someone feel free to correct me if I'm wrong.

Bill

[printer2]

While the I-beam analogy works for the wood on the top surface (inner and outer) it does not really reflect what is happening with the honeycomb, I will get back to this in a minute.

As far as stiffness goes, two sheets of wood which separated so that the surfaces make up a structure 0.100" thick using the same wood as the I-beam member will be less stiff than if you have a solid piece of wood. While the outer material contributes more to the stiffness than the inner material the inner material does add to the total.

Now the honeycomb material seems floppy and lacking in stiffness as there is no outer surface to carry the load. Looking at the honeycomb when you bend it you see that the material is not collapsing but rather it is the 'paper' that is bending. If you think of four sheets of paper that are glued together to make an open box you can easily distort the box as the sheets have little strength to resist from bending. (Bending as in place a piece of paper draped over a horizontal rod)

But once you close the box with a sheet of paper on top and on the bottom of the box the structure become stiff. The outer fibers of the nomex are not carrying a load, they are acting as an accordion and does not seem stiff. But once you close the ends you get a stiff structure, stiffer than if you had the same material made into little boxes. Even honey bees know the value of the honeycomb structure (on the cellular level).

So if the double top measures to be stiffer than a solid piece of wood with the same overall thickness it is because the nomex is stiffer than the solid wood and keeps the outer fibers of the wood sheets straighter where they can carry more load.

I hope that makes sense. Much easier to describe it in pictures or even better with the actual material in hand. Basically the nomex seems floppy because of how the forces are transferred through the honeycomb. But once it is assembled in its intended form it shows its strengths.

[jmat]

I have a double top classical made by Dake Traphagen. It is spruce on top and cedar inside. As I understood it, Dake uses nomex on his double top steel strings but not his classicals – or something to that effect. The benefits that Paul, Tim and Larry describe are what I have experienced with my classical. Generally the guitar sounds more like a spruce guitar than a cedar guitar, has incredible dynamic range and is very balanced. It is a pleasure!

Here is the build thread:

http://www.acousticguitarforum.com/f...ight=traphagen

[rick-slo]

The sound takes on the character of the outer layer more than the inner layer. The sound tends to be more uniform from string to string and up and down the neck (good things about that, not good things about that IMO). Sound does not break up when played loud.

[Tim McKnight]

Originally Posted by printer2

As far as stiffness goes, two sheets of wood which separated so that the surfaces make up a structure 0.100" thick using the same wood as the I-beam member will be less stiff than if you have a solid piece of wood. While the outer material contributes more to the stiffness than the inner material the inner material does add to the total.

With all due respect your assumption is wrong. As I stated in an earlier thread, those who have built double tops can attest to the fact that they are MUCH stiffer across and along the grain than a solid piece of wood of the same thickness and its easily measured and verified on a deflection board. Perhaps I am reading your response wrong and I don't want to start an argument but we need to keep the facts straight that a properly constructed double top with a Nomex honeycomb core is stiffer and stronger than an equally dimensioned piece of solid wood AND is considerably lighter.

[billgennaro]

Originally Posted by rick-slo

The sound tends to be more uniform from string to string and up and down the neck (good things about that, not good things about that IMO).

I agree. I wonder, if for this reason alone, whether or not I'd truly take to a double top. I've had two guitars that were incredibly uniform from string to string and also up and down the neck. Both have been sold off as I found I could not fully control the dynamics the way I like to.

Bill

[printer2]

Originally Posted by Tim McKnight

With all due respect your assumption is wrong. As I stated in an earlier thread, those who have built double tops can attest to the fact that they are MUCH stiffer across and along the grain than a solid piece of wood of the same thickness and its easily measured and verified on a deflection board. Perhaps I am reading your response wrong and I don't want to start an argument but we need to keep the facts straight that a properly constructed double top with a Nomex honeycomb core is stiffer and stronger than an equally dimensioned piece of solid wood AND is considerably lighter.

Actually I am agreeing with you. The sentence you quoted is an example of a piece of wood as a beam and an I-beam made of the same wood. Further down I give reasoning why the wood with the nomex is stiffer and why the nomex seems floppy on its own. As I said I was not sure if my explanation was enough to convey what I was trying to convey in so few word.

[Tim McKnight]

Thanks for the clarification. Its been one of THOSE days... when I am not very focused

[printer2]

Originally Posted by Tim McKnight

Thanks for the clarification. Its been one of THOSE days... when I am not very focused

That sounds like one of my normal days.