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at the bridge location, which is more important: longitudinal or cross grain stiffness?

how about the area between the neck joint and the sound hole?

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sweat the small stuff.

Purely structurally speaking in the bridge area you are braced by the X brace and the bridge plate is laminated perpendicular to the top plate so that area in of its self is very strong in regards to the loading it sees. So in terms of the tops structural integrity neither play a more critical role.

At the neck joint it is really the neck block, and UT brace seeing the majority of loading there. If there is no flange on the neck block supporting the fretboard extension or other bracing supporting the downward loading of the fretboard extension then there is a high risk of in line grain splitting occurring at the edges of the fretboard extension. So I guess you could say that cross grain stillness in important here. But that is really miss leading because a top plate with very stiff cross grain will still likely split along the grain at the fretboard extension if not enough bracing is provided. It is really the bracing giving the structural stability to the top not the stiffness of the top. The stiffness of the top has a greater roll in its ability to perform well as a guitar top tonally than it does in creating a stable structure. Theoretically I could make a well braced top out of cardboard and make the structure stable. It not likely to sound very good but the box could be structurally sound.

i guess i should mention that what got all of this started for me is the picture of the classical guitar top that alexandru showed in another thread and some of the comments he had about steel string guitars compared to classical. you can see that picture here: download/file.php?id=13308&mode=view

i dont think there is anything too unusual about that top, but i just usually dont pay too much attention to classical guitar construction. i realize the string tension on a classical is much greater than a steel string, but as far as i can tell the kind of load is the same.

what i noticed is the two great cross grain braces running across the upper bout and a myriad of longitudinal braces in the lower bout. it makes sense to me, but the majority of the braces in a steel string run diagonal across the grain. go figure. it just got me thinking and that is a good thing.

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sweat the small stuff.

You mean, the load is far less strong on a classical. It is just about 100 pounds.

One reason for adding a lot of crossgrain stiffness in the upper area is to fight against cracks along the fingerboard, which seem to be quite common on old guitars. One slightly different approach is the use of the A-frame. Two braces running at lets say 45 degrees start from each side of the soundhole, cross under the UTB and are tucked in the endblock. This is a smart trick as the long-grain of these braces offers extra compression resistance too.

Another reason for which I wouldn't want a weak UTB is that the neck of a classical is not removable without major reconstruction work. In addition I make the foot as long as possible. All this to prevent any rotation of the neckblock area.

As for what happens in the lower bout, I think that the fan braces do what the X does, only that in a spread-out way. Additional cross-grain stiffnes is gained from the center transverse, in most cases solid with no arches, and from closing crossgrain braces near the tail, or a crossgrain strut/patch under the bridge, or a combination. Or a lattice.....

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Build log

And this is a more traditional thing. You can see both upper transverses, the main one and the smaller one near the block. This small one was even smaller as designed by Torres, just 1/4 x 1/8. It seems it was not enough. I'm making it double in height. The patch in between (1/8 thick) is probably Hauser's addition and I've picked it up from Romanillos. Some people make that second transverse just as big as the main one! So it can be even more "done". Obviously those guys don't think that area has any major tonal contribution.

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Build log

yeah, nice catch. i even proof read it like 5 times.

im staring at the classical bracing and it just seems so much more intuitive to me, regardless of string tension. dont get me wrong, i love the sound of the x braced steel string. the classical bracing just looks like it will handle load and transmit vibration better. i only have a single build under my belt (steel string), so i know little to nothing about what makes a guitar tick, so to speak. im just kinda thinking out loud i guess.

anyway, i guess my original question was about the structure of the guitar and how best to handle the pull of the strings. how that affects the vibration of the plate is a different story.

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sweat the small stuff.

There are a few folks out there making fan braced steel strings. Bear Acker comes to mind. He claims that the fan braced steel string is much better on strings, and that he rarely has a string break, because the fans allow for more movement at the bridge than the X braced system. I don't know if this is true or not, and am not trying to start any arguments. I'm just saying there are some who build that way. My understanding is that the X is designed to increase the rotational force at the bridge that is produced much more intensely in a steel string than in a classical.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

The X brace adds a lot of strength between the soundhole and the bridge , the place where many old guitars fold up on themselves. Originally this bracing pattern was created for gut stringing.

i read this some time ago and stumbled upon it again today. some interesting stuff in there about why the x brace works.

http://www.esomogyi.com/ssg2.html

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sweat the small stuff.

The main _structural_ problem is the same for both steel string and classical guitars: making a top that will hold up over the long term (however you define that) under the torque of the bridge. The issue is that wood 'cold creeps', deforming progressively under any load, even if it's well below the nominal 'yield' point. This is why we put truss rods in necks: they allow you to apply a countervailing force against the upward pull of the strings to keep the fretboard as flat as you like and prevent the action from getting higher over time.

We could, of course, simply make the top thick enough to withstand the bridge torque, but that would make it too heavy to produce much sound. Bracing adds a lot of stiffness without a lot of weight. Structurally either fan bracing or X bracing will do the job. I'll note that both of them tend to put more wood between the soundhole and the bridge, which seems to be the critical area.

X bracing was first used on gut-strung guitars, but really came into it's own when steel strings came in. There are a couple of reasons for this. One is that X bracing is a little more efficient structurally than fan bracing. This may be so simply because the main X braces are so much taller than fans can be. So the real question becomes why more nylon strung guitars aren't built with X bracing.

The answer, I think, has to do with acoustics. Basically, the X does tend to add a lot more cross grain stiffness to the soundboard than the usual fan. The overall stiffness to weight ratio of the two does seem to be similar. If you look at the resonant modes of steel string and classical guitars they tend to be at much the same frequencies, except for one: the 'cross dipole'.

All guitars have a 'main top' resonant mode, where the whole lower bout is acting like a loudspeaker, at around the pitch of the open G string. Above that pitch, the modes are comprised of two or more areas that are out of phase with each other. For example, the 'long dipole' mode has the area of the top behind the bridge rising, while the area in front of the bridge falls, and vice-versa, as the bridge rocks. This mode is often around F on the high E string on both steel string and classical guitars.

The 'cross dipole' mode has the treble side of the top moving 'down' while the bass side moves 'up', with a staionary 'node' line up the center (more or less) of the top. On classical guitars this is often at around 250 Hz: the pitch of the open B string, or a bit below. On steel strings it's usually higher, say, around the pitch of the open E string or a bit below that. This has an effect on the timbre of the guitar.

The 'main top' mode motion is by far the most effective sound producer on the guitar. With the entire active area of the top moving air in the same sense, it can really produce some sound. Once you divide the top into 'mulitpole' modes the losses go up. The 'cross dipole' mode is trying to produce a sound with a wave length of about 3-4 feet, but the centers of the moving areas are only about 8" apart, so it's easier for the air to 'slosh' from one side of the op to the other than it is for it to make a sound. When the cross diplole mode is low in pitch it tends to 'cut off' the power of the 'main top' mode, producing a tall, but relatively narrow peak in the output spectrum.

One researcher found years ago that this type of spectrum is associated with a 'sharp' or 'forward' timbre. A broad 'main top' peak tends to sound more 'full'. On a classical guitar, where there is not much high frequency energy in the strings, that added 'edge' to the tone can be helpful. I note that Flamenco guitars often have very low pitched 'cross dipole' modes, and that probably helps with the 'cut' of the sound.

On a steel string you've got more than enough high frequency in the strings: the problem is to get enough bass to balance off all the treble you've got. Shifting the cross dipole upward seems to help with that: lending the sound a more 'solid' or 'full' timbre.

None of which is to say that you can't make a good steel string with fan bracing, or a good nylon with X bracing. I've made a couple of very nice sounding X-braced classicals, and I've heard of good fan braced steels. Still, it does make sense to just 'go with the flow', considereing all the effort that's gone into developing these systems over the years.

alan, that was a lot to take in. ive read your post numerous times and i think im mostly understanding what you have to say. i really appreciate your input. at the end of the day i think i lean towards your last statement: "Still, it does make sense to just 'go with the flow', considering all the effort that's gone into developing these systems over the years." thanks again. youve given me plenty to think about.

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sweat the small stuff.