Target deflection for guitar tops

[dsutherland]

Hi everyone, I’m on my 12th guitar and I’d like to start a more scientific approach to top thickness and voicing. I’m thinking of the target deflection method. Was pondering the following:

1. Obviously larger guitars need thicker tops as they are longer. I’m Approaching tops as simply supported beams with a consistent weight or a moment applied at the bridge location. Should I put my two supports at the length of the guitar I’m building for the deflection test or keep the same distance for each test no matter the guitar size?

2. Longitudinally, does the top span from the neck to the bottom or from the waist to the bottom? Im thinking with small wasted guitars and considering the location of the x brace joint it seems that the top would be spanning from the waist down. Any thoughts?

3. Thinking of developing two target deflections through experimentation, one for the initial top thickness, then again with the two supports at the ends of the x braces after voicing. Has anyone tried this?

Appreciate any input or suggestions

Cheers,
Dave

[TallDad71]

My preferred technique was to join the two halves and cut the board to a specific sized rectangle and thickness it until it sounds musically perfect. I then support it at the top and bottom edges and place a 2kg weight in the centre and measured the deflection.

I would repeat this with a 1kg supporting the board on each side. Be very careful it has broken the joint before now.

Once I have braced the soundboard and got it to where I want it, I support the board on its X brace at each edge and measure again.

The measurements are all arbitrary and don’t mean a fat lot until you have a bunch of them and finished guitars to compare against

[GregHolmberg]

I applaud any effort in guitar building of measurement and being more scientific!

The book covers how to determine the thickness of the top panel based on the properties of an individual piece of wood. You have to measure it, weigh it, and find the frequencies of its natural resonances. It takes just a few minutes. Enter the numbers in a spreadsheet and calculate the thickness.

A larger top doesn't necessarily require a thicker panel. In a steel-string flat-top, most of the top's stiffness comes from the braces. So it may require taller braces, but not a thicker panel. A thicker panel only adds mass and reduces the volume of the guitar. The book also covers how to calculate the strength and stiffness of the braces based on the torque that the strings put on the top. Although the authors appears to just use the same brace height all the time with their carbon-fiber re-inforcements.

It's not so much the absolute deflection that matters (although I think that can be useful in comparing guitars of identical dimensions--these guys do it), as the force per unit deflection (Newtons per meter). And from there, to measure the natural frequency of the top and calculate the monopole mobility, a measure of the responsiveness of the guitar. There are some discussions here about how to build a simple jig to do this.

Of course, I can't go into the same level of detail as the book, so I recommend getting a copy.


Greg

[kiwigeo]

It's not so much the absolute deflection that matters (although I think that can be useful in comparing guitars of identical dimensions--these guys do it), as the force per unit deflection (Newtons per meter). And from there, to measure the natural frequency of the top and calculate the monopole mobility, a measure of the responsiveness of the guitar. There are some discussions here about how to build a simple jig to do this.

The Portland guys use falcate bracing on their backs.....I'd like to see the explanation for this and why it is a better option that Trevor's simpler back bracing...ie three transverse braces plus four extra short radial braces on the lower bout?

[dsutherland]

Thanks for all the comments and apologies for the late response, I didn't think anyone had replied and just recently discovered the answers going to my junk mail