Acoustic Guitar Construction

I realize that whatever the top taps like or measures by other device, that once it it glued onto the rim - things change.
This setup is easy enough to construct, but I have no idea what weight to use, nor what amount of deflection at the bridge, with this setup, would be optimal.
Got any ideas?


Edit: I sent the picture to David Wren, whose fixture it was. If he's still on the perch and answers my request, I'll pass it on to you. I'm that kinda guy..

Cannot help you here Dave. Sorry. I use eyes, ears and fingers.
Herman

Me too Dave. I lay my ear on the top and see if it deflects.............HAHA HA
sORRY Herman I couldn't resist.
Dave, you are getting technical with your building now?

There is not a magic number for deflection, but if you record the deflections for all of your tops and you get a really good guitar then you will have your number. Another step is to be consistent with the stiffness of the top plate. This can be from deflection, using tap tones or for steel strings the shaking sheet metal method. The combination of a properly thicknessed top, braced to a consistent deflection should tighten the results of guitars.

Thanks John, intelligent advice as always. Where have you been ?

Kevin - I've gotta go a bit left-brain since my hearing, she ain't so good....

David Wren got back to me.

Hi David ... sorry for the slow response. Regarding deflection "tuning" ... to tell you the truth, I gave up on using this technique because of lack of consistent results. It worked in a general sense when thicknessing a batch of different top woods and then testing cross grain deflection and long grain deflection (basically to quantify stiffness and then use the less stiff woods for smaller guitars) ... but for testing strutted tops (as in this photo) the results were ultimately too inconclusive.

As for weights and deflection numbers ... the big metal block is just my fret-buck (the block I hold under the fingerboard extension when I'm hammering in the frets) and is heavy enough that my little digital scale won't register it. I started using two weights ... a lighter one to just make sure the top registered on the supports properly (ie if the top was slightly twisted, this initial weight would get things sitting properly ... then the heavier fret-buck to deflect the top.

If you want to up your game, maybe start tracking the main resonant frequencies on your instruments. A free program by the name of "Audacity" is the simplest way to do this. Here's a brief video I made for a friend on how to use Audacity for tracking the resonant frequencies https://youtu.be/gxb4guV6lPs

Trevor Gore's books are good for diving into the physics of the guitar ... they're worth it just for the summaries at the end of each chapter.

Hope this helps.

David Wren

Dave Bagwill wrote: ↑Mon Oct 04, 2021 1:33 pm Thanks John, intelligent advice as always. Where have you been ?

Kevin - I've gotta go a bit left-brain since my hearing, she ain't so good....

I spent a month out in Utah and Arizona in my RV.