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  #1  
Old 09-22-2011, 02:04 PM
tjp tjp is offline
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Default Tapered X-braces, scalloped tone bars?

I'm in the process of gluing the braces on my 00-12, and my instincts want me to taper the X-Braces, mostly because it seems to make better structural sense than scallops. They will be relatively thin, 1/4 in or so, and the cross-section will be decidedly tapered from bottom to top - try to minimize mass while keeping some height. The tone bars, on the other hand, don't seem to do much structurally, and I have some pre-scalloped pieces sitting around, so I've scalloped them a bit more and fashioned some pretty minimal tone bars. Idea is that the minimal tone bars should increase bass response a bit. They will also be trimmed from bottom to top to reduce mass.

Second thought. On a small guitar such as mine, it seems reasonable to me to go with one finger brace, the one that connects near the outside of the bridge. What effect, if any, is this likely to have on sound?

Anything completely whacky about my thinking? I won't be able to submit pictures until they are glued up, so I'm just seeking input at this point.

I've learned a tremendous amount from you all by lurking. Hoping to up the knowledge a bit. Thank you!
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Old 09-22-2011, 02:51 PM
gitnoob gitnoob is offline
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I still struggle with the function of the finger braces and tone bars. I see them as the means to partition the top into smaller areas. So I kept the partitions relatively even in terms of area.

Mine is also a 00. Since I used a large bridge plate, I went light on the finger braces and tone bars.



I also tapered my X and avoided scallops -- just because they don't make a lot of sense to me.

FWIW, I like the tap tone so far.
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Old 09-22-2011, 03:22 PM
arie arie is offline
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here's what i'm doing on a 3/4 mini slope shoulder:


mini slope rough top by arie08, on Flickr


one finger brace per side, one tone bar. don't know how it will sound -good I hope. so far w/o the utg on yet, it taps clear and even across all areas with a nice marimba like tone.

as far as scalloping goes i need to research a little bit more. i feel that they could provide "spikes" or some other localized tonal change in the top, or can be used as an enhancement. don't know yet. or maybe some guy in the past made straight braces and just cut away sections that induce top movement and lower pitch.

imo, the point of finger braces and tone bars is to allow vibrations from the bridge area to spread out along a greater area of the top in order to get more of it in motion.

gitnoob, i like how you have the top divided up into equal sections -very good! imo

Last edited by arie; 09-22-2011 at 06:00 PM.
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Old 09-22-2011, 03:29 PM
gitnoob gitnoob is offline
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Quote:
Originally Posted by arie View Post
imo, the point of finger braces and tone bars is to allow vibrations from the bridge area to spread out along a greater area of the top in order to get more of it in motion.
If the braces were tightly coupled to the bridge plate, I guess I can see how that could work, but they are usually loosely coupled, so they seem to just add directional stiffness.

I may try a more traditional diagonal tone bar next time just to see how it compares. The orientation seems a bit random to me, but I guess it makes some sense to add cross-grain stiffness to the lower bout.
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Old 09-22-2011, 03:35 PM
tjp tjp is offline
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"The orientation seems a bit random to me"

Amen to that. I really wonder how much guitar tradition started out with a "hey, let's try this...", and turned into gospel. And the popsicle brace reminds me of something my hillbilly dad (bless his heart) would have used to "fix" a crack....

I LIKE the fact that we know so little about what makes guitars sound good, and why.
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Old 09-22-2011, 03:41 PM
gitnoob gitnoob is offline
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Take a look at Martin's A-frame bracing. It was apparently the result of Martin rethinking their traditional designs. Supposedly, the designers were so happy with it that they wanted to move all of their guitars to the new style, but they decided that "tradition" was a selling point for their guitars, and they didn't want to lose that market.



Besides improving the strength of the upper bout with the A-frame, they relocated the lower tone bar to create a "box" around the bridge for increased resistance to bridge rotation. They also went to single flat finger braces.
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Old 09-22-2011, 03:51 PM
arie arie is offline
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for me i see martin's a frame style an exercise in "warranty management" and "cost reduction". less labor and preventing bridge roll up while leaving so much empty real estate behind the bridge undeveloped -jmo.
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Old 09-22-2011, 03:55 PM
gitnoob gitnoob is offline
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Quote:
Originally Posted by arie View Post
for me i see martin's a frame style an exercise in "warranty management" and "cost reduction". less labor and preventing bridge roll up while leaving so much empty real estate behind the bridge undeveloped -jmo.
Perhaps, but they also offer the more traditional "hybrid scalloped" bracing in the 16-series. I've compared similar guitars using those two patterns, and I much preferred the 15 style. People seem to universally consider the 15's to be responsive. To me, their only weakness is the treble response, which could be due in part to that large unbraced area of the lower bout.

Edit: I should add that Martin uses pretty thick tops on the 15's, and that obviously affects both tone and structure.
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Last edited by gitnoob; 09-22-2011 at 04:05 PM.
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  #9  
Old 09-23-2011, 01:12 PM
Alan Carruth Alan Carruth is offline
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'Scalloped' vs 'Tapered' bracing has to do with the relative pitches of the various low-order resonant modes of the top, and the 'impedance' at the bridge. These are things that color the tone, and alter the overall output of the guitar.

Most of the power of the guitar, and the low end 'whomp', is produced by the 'main top' resonant mode; the one that has the whole lower bout moving like a loudspeaker. You can think of this as a piston, pumping air in and out of the hole, and also making sound directly off the top. A lot of folks think that lossening up the edges is the way to make this work better, but, in fact, it's just the opposite. The trick is that when you make the edges loose the middle does move more, but not a whole lot more, and youve got to move the whole mass of the top to do it. Loosening it up in the center, by scalloping the braces, actually makes the 'main top' mode more active, and moves more air to produce more sound.

Since scalloped bracing is less stiff, and less massive. in the middle, it also has lower impedance; that is, it's easier to move at all frequencies. This gets pretty complex, but the upshot is that you can end up with increased output all the way up the frequency range, even though the main observable effect is that the 'main top' resonant mode pitch drops.

In most cases, scalloped bracing has peaks in the outer parts of the lower bout. These add a lot of stiffness in areas where ther 'cross dipole' and 'long dipole' modes are bending a lot, so those modes can end up higher in pitch. The 'cross dipole' is not a very efficient sound producer, and, in fact, seems to me to act more as a 'cutoff' for the 'main top' mode. Raising the cross dipole pitch tends to yeild a broader and taller 'main top peak in the output spectrum, which usually comes across as a more 'solid' or 'fuller' tone.

The 'long dipole' is, in isolation, also not a very effective sound producer. However, it can 'couple' strongly with one of the inside air resonant modes; the 'A-1' mode. The A-1 mode is, in itself, not usually a strong sound producer either, but when it couples with the top mode you can end up with a pair of air modes, one of which can make a lot of sound at the hole. I have not done enough work with scaloped bracing to know whether the scalloped tone bars make the long dipole stiffer or looser.

All of that activity of the top at the bridge comes at a price; the same one the 'cellist pays: wolf notes. It's easy to lose the mid-range if you're not careful, and end up with strong and weak notes.

Anyway; tapered bracing is pretty much the opposite of scalloped in the way it works. The higher impedance at the bridge tends to give a more even, if less powerful, voice, with more sustain and 'headroom'. The relatively higher 'main top' pitch gives a tone that is, perhaps, a bit 'clearer', with less 'attack'. You also tend to end up with a more stable structure, and there's not that angst about whether you took off one shaving too many, and won't find out for three years.

It will be interesting to hear how your hybrid experiment works out.
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  #10  
Old 09-23-2011, 01:38 PM
tjp tjp is offline
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Default Thanks Alan

I was hoping you would chip in here! I'll need to do a bit of research to totally understand your response, though I'm pretty sure I get the gist of it.

I'm curious how the hybrid will turn out too. It's going to be kind of a Martin/Taylor/Goodall monstrosity of a 00. Deep (4.5 in) body, A-frame, dovetail converted to a Goodall/early Taylor style bolt-on (visited the Goodall shop a few weeks ago, and was impressed but a lot of things, not the least of which was the simplicity of neck attachment), reversed kerfing...just following my instincts as I go.

Knowing it's a short scale, I'm leaning toward tapering the braces pretty severely while tapping a lot to make sure I don't go too far over the edge. This is going to be fun...
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