Official Luthiers Forum!

Yeah, definitely seems that way. I have to admit, I did think the range would be a bit tighter and that the proverbial floppy top or extra-stiff top would be outliers at the outer limits.

This is a very interesting discussion, guys! I wouldn't have been surprised if most tops were within a range of ten to fifteen thousandths and we'd see that deflection fine tunes it, but they really do seem to be all over the place. I'm really looking forward to finding out where mine tend to fall.

I've also been interested in first testing with Trevor's formula, joining the plates, then using deflection to see how they correlate together and if there's any kind of relationship there.

The Young's moduli (stiffness) of the different spruces overlap a lot, and the values range over a factor of two. So at the extremes, at constant thickness, stiff wood will deflect twice as far as flexible wood for the same load. To make up the stiffness of flexible wood, it can be left thicker, but its rigidity changes as the thickness cubed. So 26% thicker gives you twice as rigid. Thicknessing to a deflection target is a lot better than thicknessing to a dimension as it at least gives you repeatable stiffness, which constant thicknessing will not. But that's not the whole story. Usually we want to pitch the resonances in the right place, because the position of these resonances largely determine the sound of the guitar. But the resonances are dependent on both stiffness and mass (Young's modulus and density, at the material properties level) so if you want to control the resonances it helps to measure both Young's modulus and density (and then know what to do with the results!)

The "best" tops are those which resonate at target frequency and are of lowest mass. But as the Young's modulus of the wood increases, generally, so does its density. So is the low density, low stiffness top going to be better than the high stiffness high density top once they're both thicknessed properly? Nobody I know can tell me that without measuring both stiffness and density. I've flexed and measured many hundreds of sets, and I know I can't do it. If you make lots of different types of guitar the sort of analysis I do helps a lot, because I know I can get the result I want from different shapes and sizes of guitars without having to build prototypes to "calibrate" a deflection measurement.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Trevor: Did you misquote or am I down a cup of coffee this early in the morning...?
Tom

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A person who has never made a mistake has never made anything!!!

Only under southern hemisphere reverse gravity! (I forgot to add that rider!)

Must have written that when I was short of a coffee. It should been "So at the extremes, at constant thickness, flexible wood will deflect twice as far as stiff wood for the same load."

I'm please that you, at least, are awake, Tom! Thanks for pointing that out.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Eyahhh, we knew what you meant

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Steve Smith
"Music is what feelings sound like"