Acoustic Guitar Construction

I find Trevor Gore's methods intriguing, but the price of the books hold me back. And I don't need it for finding the right thickness of my plates. But, I found out that my best guitars have a top- and back resonance frequency that are 3-5 semitones apart. And I think that is the way to go for avoiding dead- or wolf notes.
So what I am trying to find out now is the way to approach the plates before the box is closed. Not for stiffness, flexibility or weight, but for resonance frequency.
So question ( John P?): Can a braced and shaped top predict its final resonance before gluing to the rim?
Herman

Short answer if one can get the top close; then build the back a bit stiff with a way to tune it down to 3 to 4 semi tones above the top. An active back affects the top frequency. So when tuning the back keep an eye on the top as well. I like to keep the top between note values.

Longer answer
One needs a way to get the top plates thicknessed to normalize for stiffness, and have a way of tuning the top braces. By ear or deflection one can get the top in a good frequency range. Post build there are ways of bringing down a resonance removing stiffness or adding mass to fine tune. With proper selection of back wood, stiff but not real dense (EIR, Mahogany, Koa ...) one can get the back light enough to be acoustically coupled to the the top (active back). Gore has a bracing pattern he likes for an active back. The lower bout has one tunable lateral brace. His build process starts with the back pitched high and tuned post build (shaving the center of the backs lower transverse brace) to 4 semi tones above the top.

With the top plate thicknessed to normalize for stiffness/mass, I can pretty much nail top frequencies when using CF and spruce for bracing. The carbon fiber takes some of the variability out of the brace stiffness. I control the top resonance with brace height. I tune (shave braces) my xbraced tops by ear. I do not tune to a frequency I just tune until the top sounds right. It really helps that I start with a properly thicknessed top plate. It really helps to start bracing on a top plate normalized for stiffness and mass. The sheet metal shaking test is a good way of doing this for a steel string. I use Gore's thickness formula for classical guitars as the sheet metal test leaves the plate too thick for a classical. Other methods are deflection or even weight.

OK John I see that, Thanks.
But has the top, that has a resonance of lets say 190 Hz on a finished guitar, a similar (or related) frequency when it is about to be glued to the rim and still "free". Or there no relationship and has the tuning be done when the box is completed?

Herman

Herman wrote: ↑Sun Oct 16, 2022 11:09 am OK John I see that, Thanks.
But has the top, that has a resonance of lets say 190 Hz on a finished guitar, a similar (or related) frequency when it is about to be glued to the rim and still "free". Or there no relationship and has the tuning be done when the box is completed?

Herman

I know of no formula between a free top and the top as part of a completed box.

For me anyway it is a matter of consistency. If on a previous build I ended up with the right finished frequency I would try to replicate that or if too low or high I would change what I was looking for. I use my ears, deflection and at times taping on the top to the rims with the back glued on. I think a top's free resonance could also be one of the attributes, I have not used it. When I tap a free top for tuning I am listening for the quality of the tap and not measuring a frequency. It may be the same thing.

Gore stuff is more about consistency such that one can tweak a process build to build with some confidence of the outcome.

Right John, that is an answer. I already expected this, but since you are so deep in this material, I thought I ask.
Thanks mate
Herman