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So I'm reading up a bunch on bracing and optimal soundboard thickness (or at least how to determine it). A giant can of worms that is. So much to know about but so much you can only really KNOW by building guitars.

Anywho, I'm not sure what the luthier term is for it but I've learned that in some guitars the braces "overlap" and don't just butt up agains each other. A little notch is cut and a brace is inserted into this notch. Does this help top stiffness and how much do the braces overlap?

Also, I know the general prewar Martin spec is 0.117" but is there any way to measure top deflection to determine the best thickness for a given soundboard? There's some info out there but I haven't been able to find exactly what I'm looking for. I'm not sure if I will find what I'm looking for without experience, nor am I entirely sure people are going to give up some of their key secrets to a hobbyist but thought I'd ask anyways.

Martin notched the braces at one point. This was done often because of tradition and glue was not as consistent . This I find is not all that much stiffer. The top is more like a truss. I have done this both ways and unless my commission requests it ( add $50 ) I don't like to do it. Today's glues are superior to what they had in the past.
When I do notch I do a miter notch so I expose more face grain , end grain is a weaker glue joint.

tugboat wrote:Also, I know the general prewar Martin spec is 0.117" but is there any way to measure top deflection to determine the best thickness for a given soundboard?

This may help:

Ben-Had wrote:

tugboat wrote:Also, I know the general prewar Martin spec is 0.117" but is there any way to measure top deflection to determine the best thickness for a given soundboard?

This may help:

http://www.youtube.com/watch?v=xDDJVAaD ... -1Jjs-do1g

That's a big help! Thanks for posting that video! Ultimately when I start building it I won't exactly know what I'm doing but I do want to give myself the best chance to bring out the most potential from the guitar! Top thickness seems like too important a factor, especially since top stiffness can vary so much, to just take a spec and roll with it.

Here's another way to optimize thickness of a top:



-tommy

Don't complicate things. 3mm seems to be fine for back and top. If you have a favourite guitar measure the thickness of the topp. Ther back will probably be the same.

Having made over 150 guitars I find that the secret is there is alot of misinformation out on the web. I tried all the popular " Proven theories " and I can tell you that most are pretty much bull.
I find the best is to do a deflection test pre and post brace. Find the number when you compare the unbraced top to the braced top. This will give you a ratio and work from that.
Martin NEVER tap tuned a top and work all from a pattern.
You have to find what tickles your ear. Go with that.

In my case though I have no clue what to look for in deflection tests. I also very much doubt I'll ever get close to building enough guitars to experiment and find what works best for me, and I don't necessarily want to pick a random number and go for it. If there are reasonable enough sounding tests that don't ping my bs-o-meter that help me know when to stop thicknessing the top and when to stop carving the braces, then I'm interested in hearing about them. I know there's not one master technique that will guarantee an incredible sounding guitar every time, and that some of the theories behind tap tuning sound wacky even to me, but there has to be something out there for me to learn that will help me out. That's all I'm trying to do. :)

I'm in the midst of thicknessing Carpathian spruce for my second build and have entered that "musicality range" (where if you grasp the edges and flex the top it will make a sound similar to sheet metal) that Robbie O'Brien mentioned in that video link above. My top is around 3.25mm/0.12" (give or take) but I haven't done the rosette yet so I'll likely go a little thinner - but not much because I think I'm right where I want to be. Like others have said, I'm starting to see that it's more of a "feel" than an actual numerical figure.

Good info. In my (admittedly very inexperienced) mind I can see a benefit to tap tuning, but only as a means to know when to stop thicknessing a top and stop carving braces. Tuning certain parts of the guitar to a certain pitch sounds like voodoo to me (the whole thing will change once the back, sides, and neck are installed) but using tap tuning as a means to gauge how flexible a top is seems like there might actually be something to it.