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Say I was real dilligent about tuning the top, but neglected to tune the back.
The top is glued on and the back is real stiff.

If I want to tune the two together, should I shoot to get the back down to the pitch that the ring+ was at or do I need to glue it up and tune it after it is attached to the rims?

DL

Tops and backs are so much different that it's really hard to say in any general way how the top and back 'free' plate modes should relate to each other. I've used several different back bracing systems over the years, and have found that I have to simply work with each one for a while to get to know how to do it. Archtops are less confusing in that way.

Alan,
I'm curious what you mean by that.

I posed the question to see if my assumptions would be correct.
When I finally got back out to the shop last night, I checked the hz of the back with spectrum analysis.

It took about ten minutes to get the main hz of the back into the target zone.

If you are saying the top and back will react differently once the plates are attached to the rims, I'll understand your comment. IE, the top could drop 10 hz and the back rise in pitch by 10 hz.

I've not noticed this on past builds.

Dave

The plates will be wildly different once they're on the rims. The mode shapes will change due to a change in boundary conditions and the frequency of any given mode will go up as the structure is now much stiffer.
I do believe however that David Hurd has some information in his book "left Brain Lutherie" on relating free plates to each other.

_________________
Jim Watts
http://jameswattsguitars.com

Dave Livermore asked:
"I'm curious what you mean by that."

Which 'that'? There were a couple of statements in that post. I wrote:
"Tops and backs are so much different that it's really hard to say in any general way how the top and back 'free' plate modes should relate to each other."

Tops and backs usually start out with radically different densities and stiffness ratios, and then they are braced much differently from each other. The 'free' plate modes of tops are different from those of backs. There's really no way, as Jim Watts points out, to predict the assembled mode frequencies of either plate from the 'free' plate modes, and getting two of them to converge on some value is even trickier.

" I've used several different back bracing systems over the years, and have found that I have to simply work with each one for a while to get to know how to do it."
That's pretty self-explanatory, I think.

"Archtops are less confusing in that way."
With archtops you're tuning the 'ring' modes, not the 'ring+', and you can get the same modes on both plates,which seldom or never happens on flat tops. Tuning them so that the 'ring' modes are at the same pitch on the free plates, and well formed, usually results in the top and back having the same 'main' resonant pitch when the box is assembled. The mass and down pressure of the bridge shifts the 'main top' mode downward in pitch, and you're in the clear.

Alan,
I wasnt sure what to make of the statement about the backs and tops behaving differently.


Just measured where things ended up after attaching to the rims.

Top was at 220 before. Ended up at 207
Back was at 227. On the rims it is 194

After adding a bridge, things should still be fine.
I sure hope i dont have to go on there and shave pn the back braces. It always roughs up my forearms.

Dave