Official Luthiers Forum!

As above, what's the thinnest you have gone on a top, I know the bracing is all part of it, but just for my curiosity how thin you gone?

Sent from my SM-G960F using Tapatalk

.085” on a 24” scale 0. It’s 7 years old and doing fine

2.0mm (0.079") with Sitka, 2.15mm (0.085) with a very light but stiff Lutz.
But the bracing was quite different to a "normal" soundboard.
Steel string, strung with 12's, 650mm (25.6") scale length.
EDIT - 16 1/8" wide lower bout GS

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I didn't build it but was repairing an old Rameriez and I measured the cedar top which was 0.060" thick. The top had numerous cracks but it also had a concert hall filling volume. I think of this example as a sort of far end of the spectrum in regards to how thin you can go.

.75 in on a current guitar I am finishing up. IT's a small parlor guitar. I use deflection testing and I could tell this top was very stiff but was surprised it went this low. I also used the Gore method of acoustic testing and it matched my deflection almost to a tee which made me feel better about it. The top is torrified REd Spruce.

Be interested to hear about the correlation between your deflection testing and the Gore method...

I’ve got a stack of very stiff Sitka tops that consistently end up at .095” based on my target deflection numbers. Thanks to John Parchem I’ve got the Gore plate analysis set on my laptop. I did a quick test on 2 of those tops and low and behold the test gave up a suggested thickness of like .093” and .094”. My intention is to start using the Gore method on every new build once the current pile of instruments are complete. Good stuff!

M

Or course the Gore plate analysis target figures suggested in the book pertain to his falcate builds.
He is effectively matching the top to the bracing.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Frankly I have only read the first half of the design book so I didn’t know that the testing was aimed at falcate bracing (my brain was gonna melt if I kept reading). I’d still think the Gore method would have merit regardless of the bracing pattern.

Understand completely, my brain hurt for a month..
And of course it's useful regardless of bracing pattern, just his target thickness may not be your "normal" for your bracing was all I was trying to point out.
But as you implied, you would be doing your own deflection testing and using the Gore method in parallel, so it won't be a problem.
I've used it as well as doing my own deflection testing and not using falcate bracing.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I've not been using the Gore method, only deflection testing. But I have a feeling if one got quite a bit more analytical with their brace stock, you could apply it in a similar fashion. TBH it's been a long time since I've had my nose in the books, I've just been using the answers I needed.

I surmise that with the carbon fibre caps in the falcate style, the CF will totally dominate the stiffness characteristics of the spruce, making the actual properties of the brace stock less significant. I think if you sorted your brace stock by known properties and used it consistently/intentionally, the tap method would be applicable.

Back when I had an apprentice, I had the luxury of tasking him to do deflection testing on the brace stock, and would sort ranges for body size. That was part of how I got my initial target deflection numbers. I should get back to doing that come to think...

I wish I could give credit to the luthier I learned this trick from on a Youtube video that he was kind enough to make but I forgot his name. But years ago I came across this video where he thinns the top and after every run through the sander hhe holds the top on each of it's long sides and shakes it back and forth. When it starts to have the whopm whopm sound you get like when you shake a piece of sheet metal you are done.

I've been using that trick for years when I adopted Brian Howard's method of deflection testing. So I found a dedicated brick and built a consistant method of deflection. I would thickness the tops till I got the sheet metal sound, run a deflection test and then record the data.

So far I am only about 15 guitars in doing deflectinon so it's not a lot of data.

Then I started doing the Gore acoustic testing and what I found was that it almost matches perfectly to the "sheet metal" test. I will find the target thickness doing the Gore method and then start to thin the top out checking constantly for the sheet metal sound and when I get there I am usually very close to the Gore thickness.

It's just a stroke of luck or the luthier that I learned that method was just old school brilliant or both.

I think I remember someone telling me that Robbie O/Brain does something like this. Could that be who you are thinking of? I'd love if you could find the video; I would like to hear an example of the sound. That is one of the tactile tests I do to tops when I pretend like I know what I am doing. . .

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I saw it in a Robbie O/Brain video. He may have gotten the idea somewhere else. I used it but I'm not even close to finishing #1 so...

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

I don't recall that it was Robbie, pretty sure it was someone else. IT was probably ten years ago too. Can't seem to find it on the Googles.

Maybe Chris Paulick?

This one?

https://www.youtube.com/watch?v=xDDJVAaD9n8

That's about the sound I listen for but again, I'm no expert. I flex and tap too hoping someday it will all come together in my mind.

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Kent Everett does the sheet metal thing.

Kent Everette is my guess.

Thankyou Kent rings a bell I believe that is who it was.

Yes Bryan, that's the video I remember. In it Chris refers to Kent Everette as the source of the "wobble" sound technique.

Kent (Carlos) taught that in his voicing class that I took quite a few years ago. Works good for me but be careful with torrefied tops, they can split easy (yes, I did). After I learned the womf womf thing I stopped deflection testing.

_________________
Steve Smith
"Music is what feelings sound like"

Here's what I don't get about the 'sheet metal' deal. Is that not only an indication of cross grain stiffness, when it's long grain stiffness we're concerned with?

I've talked with others who have reported correlation to the floppy test with the Gore method, but I don't trust it...

Indeed. Unless you always use perfectly quartersawn wood, it should give inconsistent results.

I suspect it's a contributing factor in the accumulated "luthier wisdom" that quartersawn wood sounds better. If your teacher tells you to do that, and you don't know anything else, then perfectly quartered tops will end up having the lowest long grain stiffness, and thus probably sound better. So then you teach your students how to do the wobble test, and tell them that quartersawn wood sounds better. Then someday they might decide to try an off-quarter top, and it will end up being too stiff along the grain and thus sound bad, confirming the belief which will then be passed on again

But if you measure long grain stiffness directly, then off-quarter wood sounds just as good.

This could actually be used to calibrate a deflection system. Thin a perfectly quartered top until it makes the sheet metal sound, and then use it as a reference for how much long grain deflection you should aim for on the off-quarter ones

That was exactly what my goal was when I started doing the deflection based on the wobble sound. Deflection measures actual numbers, Youngs Modulus in particular, regardless of the orientation of the grain. If X piece of wood deflects X distance then you have Y piece of wood.

---

IDK how the wobble method works in relation to deflection and the Gore tests but it does. The acoustic sound of the wobble may not be strictly associated with cross grain stiffness.

Also there is a range of wobble too, from when you first detect it through when it is quenched. I used to play within that range when targeting a specific sound dynamic for a guitar. When it first starts is good for a Bluegrass Dread, when it's quenched it's probably good for nothing but I've never gone there.