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I am building 3 OM's at the moment and have made some observations as I move forward on my luthiers journey. This is guitar numbers 69, 70 and 71.

I have always used the 'sheet metal test' to determine top thickness and at some point some years back built my own deflection testing jig then started using the Gore/Gillet method in conjunction with all 3 methods to determine top thickness. I have found that the Gore/Gillet method almost identically matches my reflection method and the sheet metal test gets close.

The 3 OM's I am building now are two with torrified Red Spruce tops and one with a regular Euro spruce top all top quality master grade stuff. When thinning out the tops and measuring the deflection I found that the Euro spruce top is dramatically weaker in it's cross grain stiffness then the two T-tops. In fact the T-tops have almost the exact same properties which makes me wonder if the T-process basically makes 'blanks' almost like carbon fiber but I digress...

I have found from previous OM builds that with my deflection testing set up a deflection of .3in makes a pretty nice sounding guitar. So I aim to get that on all 3 tops. The two T-top guitars are .1 and .107 inches thick and deflect at exactly .275 while the Euro spruce top is .110 thick and deflects at .240. I have stopped short of my .3in goal on the Euro spruce top because it is incredibly floppy across the grain.

Which brings me back to the main topic... What does this mean?

I can imagine that the X-brace and the two angled lower cross braces will sort of 'lock in' the cross grain properties of the top anyway. It would for example probably have more influence to tone on a fan braced guitar. But it's got to mean something right? We always seem to care more about the long grain stiffness of the top which of course makes sense but with these three tops I am working with the difference in cross grain is pretty dramatic especially given that at this point the Euro spruce top is actually a bit thicker but deflects quite a bit less.

Is it known that T-tops have stiffer cross grain?

I gave up on deflection testing on raw tops to me braced tops are what matter.
I have found that tuning a top to a C gave to me the better sounding guitars. I know Wayne uses this method as well.

Still you have to find what works for you as there are many ways to make a good guitar.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Cross grain stiffness is primarily determined by the cut. If you've never handled a top with 45 degree grain across the whole width, it's an eye opener. Incredibly flexible.

And for long grain stiffness, you need density measurements to see if there's any real difference between the tops.

I do deflection after bracing too. When you tune to a C is that with the braced guitar top? And how do you go about measuring that? Tap tones?



I have indeed but that is also why I wanted to be clear that these are high quality tops, perfectly QS across the whole top.

I have noticed that torrefaction increases cross-grain stiffness. It also reduces density.

Sent from my SM-G950U1 using Tapatalk

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John

Ok cool, yeah it really seems significant too.

Question is, what do you think it has to do with tone on a steel string guitar?

yes I tune to a C after bracing the top and as John A points out Torrification does change the top characteristic . As for the sound it is different that the raw spruce and to my ear it does sound older for lack of a better term. I made about 20 guitars with Torrified spruce both red and sitka.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

This will only be my 7th with T-wood, all Red Spruce. It is a different animal that's for sure.

Mark Blanchard and I both use 'Chladni tuning'; the 'tech' version of tap tones. He looked into tuning the un-braced top modes by graduating thickness some time back. He said he wished he had back the time he wasted on that project. What he did find out that was useful was that a ratio of long- to cross-stiffness that produced certain mode shapes on the unbraced top blank tended to produce the 'best' guitars, in his opinion. These mode shapes carried over into the 'free' braced tops. Altering the bracing (within reason) would not produce the 'right' mode shapes on a top that didn't have them in the unbraced condition. He drew the conclusion that 'the sound is in the top': a bad bracing job can ruin it, but a good bracing job won't help much if the stiffness ratio is out of range.

It's all about the aspect ratio; the width of the top as compared with the length. Unsurprisingly, a wide top, like a Jumbo, works best with really high cross stiffness wood, while on something narrower, a size 1, you don't need it.

Mark now tests all of his top blanks (so far as I know) by joining them and taking them to a fairly large uniform thickness. He cuts the outline of his largest guitar, and looks at the shapes and frequencies of some 'characteristic' modes that speak to the relative stiffness along and across the grain. If they're not 'right' he cuts them to the next smaller size (which is relatively narrower) and checks again. When he find the shape that works best for that top, that's what he uses it on. You can, of course, measure the actual long- and cross-grain stiffness or Young's modulus, by whatever method, and work out how that correlates with Blanchard's modes. You may or may not agree with the logic, but Mark makes very consistent guitars that sell for top dollar...

You don't quote any "transferable" numbers (like Young's modulus) so it's hard for me to compare what you have.

However, check out Section 4.5.4 of the Design book and also Table 4.5-3. All the wood in that table (apart from the Port Orford cedar, which I didn't use) made fine guitars.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

you can add stiffness.
That's what bracing does.
what note the top is before bracing will change after bracing.

just plan your bracing right- stiff in the center!
Taper down to the sides.
On classicals the # 1 & 2 braces (above & below the s.h. -lock into the linings.

mike

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Mike Collins

Mike Collins wrote:
"...just plan your bracing right- stiff in the center!
Taper down to the sides."

It all depends on the sound you're after: that's not the preferred profile for some styles of music.

As Mark found, bracing can't go all that far to make up for the 'wrong' cross grain stiffness. Guitars seem to sound best when the tone 'just happens'; the harder you have to work to get what you want the less likely you are to be happy with the result. It all starts with choosing the 'right' set of wood for what you're trying to do.

Thanks for the discussion.

Yeah I do not have any hard numbers and since I already cut the sound holes out that's not possible now. But just by feel it was like handling cardboard then a piece of paper. I will check out the sections of the book too.

These are OM's that will have scalloped braces to loosen up the center of the guitar. I built one straight braced OM last year and it sounded good but not what I am after on these.

I also always thin out the perimeter of the top.

I think I will brace these all up and make a decision. Worse come to worse I can use the floppy one for an 00.

You can stiffen up the floppy top by adjusting the angle of the tone bars below the bridge. Making them a bit more perpendicular to the centerline should increase the cross grain stiffness. You may also not need to do much perimeter sanding to loosen up an already loose top. I haven't worked with Euro spruce, but I'm assuming it works like the other spruces.

It works for me! I trust my instincts & hearing It's my sound.& Players buy my guitars .

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Mike Collins

Mike I have built many guitars with tapered bracing. In fact I think it is my preferred style personally. It gives a nice well balanced tone. But for these Martin-esque replica's I'm going for that bass balanced scalloped bracing.