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I would like to hear your thoughts on how cross grain stiffness affects the tone of a guitar. What does loose cross grain stiffness sound like compared to strong cross grained stiffness? Do guitars built for certain music styles maybe have a particular cross grain stiffness that produces a certain tone for that music style? (for example, possibly loose CG stiffness for bluegrass and maybe stiffer CG stiffness for jazz......I'm making the example up but you get the point)

Do you shoot for loose crooss grain stiffness, strong cross grain stiffness, or a particular balance of cross grain stiffness vs stiffness along the grain? The ratio of cross grain stiffness vs stiffness along the grain would certainly affect the vibration modes........so could choosing the cross grain stiffness carefully help achieve a stronger monopole mode by rounding the pattern and maximizing air movement?

Appreciate your thoughts.

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Formerly known as Adaboy.......

Hey Darryl,

I have been getting more picky about this lately. I try to find the best cross grain stiffness (and most consistent) I can for a few reasons. I do feel that the plate having a closer ratio of long to cross stiffness is beneficial for tone (although I' not sure I could explain exactly how it influences it). One thing I really look for though is uniform stiffness across the grain. If the top goes off quarter at a certain spot it is really obvious when you are flexing the plate, you can feel that spot. To me this is a bad thing as there is a major change in one small area of the top. You could argue that you do this anyway with the braces but I feel that the wood you start out with is the most important part of the top system. I want to be able to control the cross stiffness in smaller gradations through thinning the top where I want to rather than working around where the top goes off quarter.

In practice, tops that are fully quartered usually have good consistent cross stiffness. I read somewhere that the cells in the spruce are somewhat square shaped, so when you line them all up they reinforce each other. If they are at slight angles to each other though they begin to slip. I am not sure how true this really is but that visualization helps me "see" in my head what I certainly feel in practice. Also, wood that is off quarter for the whole width may have consistent cross grain too, it would just be less stiff than wood that was on the quarter.

This is what I am looking for now but I also have to admit that so many great guitars have been built with wood that goes from vertical grain to slightly off on the edges. In theory by thinning the edges we are more or less copying this anyway. I just want as much control as I can get over exactly where that happens.

In terms of Chladni modes, I am very influenced by Mark Blanchard's ideas about certain tops having the correct ratio of long-cross grain for a certain size top (this is assuming a clean ring+ mode is what you are after). I don't have enough data to have any meaningful guidelines for my shapes but part of compiling all of the data is to someday be able to.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

Burton, thanks for that information. Very interesting that you are looking for consistent cross grain stiffness across the entire width of the booard plus you like the ratio of cross-grain to long-grain stiffness to be in a specific range. Contrast this with another current thread where they mention a well known luthier that apparently likes the grain going a little off quarter near the soundboard edge. I think most would agree that good guitars could be built with either luthiers ideal soundboard........it sure would be nice to know how either example influences tone.

I'm just speculating here but it would be interesting to see if a particular cross-grain stiffness or ratio of croos-grain to long-grain stiffness could be selected for a more powerful monopole mode.......or to strengthen another vibrating mode of the guitar. It would guess that a carfully chosen ratio would round the monopole shape possibly increasing the moving surface area which might increase the volume or power of that vibrating mode. That would surely change the tone (good or bad).

I spoke with a gentlemen in our area who has built guitars for several big bluegrass names and he likes the floppy cross grain stiffness.

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Formerly known as Adaboy.......

Forgot to mention this, but a lot of folks shave their finger braces down considerable stating they like the tone it creates. So how would this differ from choosing a top with less cross-grain stiffness?

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Formerly known as Adaboy.......

The finger braces reinforce a crucial area on the outer wings of the bridge. Mine are almost nothing, .200" triangular feathering to the x-braces and under the linings. But I use 4 of them, even on small guitars. I feel it gives more control in voicing the top.
I tend to use the tops that are least stiff accross the grain on small guitars, like 0 or 00s, regardless of what the plates size allow.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Wood stiffness in bending depends on the stretching and compression of the cells, particularly near the surface of the piece, and correlates well with the orientation of the cells. Most of the cells run along the axis of the tree, assuming it grew without too much twist, so that's the grain direction that has the greatest stiffness. The small bundles of medullary ray cells that run in an out along the raduis of the tree serve to increase the stiffness along their axis, so the cross grain stiffness is usually at least somewhat higher in radially cut wood (vertical grain) than it is in wood cut along the tangent (dead flat cut).

The low cross grain stiffness of 45 degree skew cut wood is probably related to cell distortion, as Burton posits, but working in a different way. Imagine a bunch of square tubes that have been glued together to form a plate of some sort. If you try to bend it across the grain you've got to stretch or compress the flat surfaces that comprise the top and bottom surfaces of the cells. If you glue a similar bunch of square tubes together, but on the bias, then bending the plate in the crosswise direction just calls for changing the angles of the corners of the squares, and that would probably call for less force. The cells act like accordion pleats.

According to Daniel Haines, who did a lot of measurements of wood properties, skew cut wood has a much higher shear modulus than either vertical grain or flat cut wood. FWIW

Al, is silking in a top a sign of heavy medullary rays and does a silky top typically have higher cross grain stiffness?

I would still love to hear from anyone that has built similarly braced guitars with a wide range of cross-grain stiffness and have them describe the tone differences. Likely most change the bracing based on the cross grain stiffness so no one may have done this. A lot of folks are careful (or picky) about the cross grain stiffness they use......so I'm a little surprised that its not common knowledge how "loose" or "stiff" cross grain stiffness affects tone. Maybe the tone difference is subtle and difficult to describe?

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Formerly known as Adaboy.......

Yes
No. It does indicates however a perfectly quartered piece.
For what, in what context, with what style of bracing, with what range of thickness for the top, with what bridge/bridgeplate system, do you thin the perimeter of the guitar, if yes how much compared to the original thickness?
How old is a tree?
I indicated that I tend to use tops with the least cross-grain stiffness on small(er) guitars. I would surmise I am not the only one. IME it's easier to coax a little more bass response this way, logically. I prefer to have more control on larger bodies and arrive at a similar result by thinning the perimeter of the lower bout until I am satisfied by the bounciness of the top. Almost all roads lead to Rome.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Within one log though, the tops with better quartering will be noticeably stiffer crossgrain than the others.

I built one classical with a very floppy crossgrain spruce top, while lengthwise it was very nice. I made sure all crossgrain braces were not overly scalloped, and that the bridge was stiff; still nothing major, you wouldn't be able to look at it and think "this guy compensated something here" and it worked too well even, ended up plenty stiff. My conclusion is that at least for a small guitar it is less bad than it looks (looks = feels, when you flex these they are like rubber)

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Build log

The meduullary rays are there, no matter how you cut the top. Some species of wood seem to have heavier meds than others, but in whatever species they will be most visible in perfectly quartered tops. And, yes, compared with other less well quartered tops from the same tree, the ones with more visible silking will generally have higher cross grain stiffness.

I think the reason there's no concensus is that it's not something you can do controlled experiments on easily. With all the variables inherent in building any two gutiars, how do you attribute some difference in sound simply to cross grain stiffness?