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Parabolas are used in many applications because they have the best strength to weight ratio. However, in guitar bracing, they must also be matched to the forces acting on them in order not to dampen the surface to which they are glued, both top and back. There is nothing magic about using parabolic braces that are too strong. And since tops, especially, have different stiffness, I feel that its braces must be fine tuned after the guitar is together to make sure there are no areas over the braces that cannot vibrate due to too much meat.
As Brownie McGhee used to sing:
"I got an order from across the street,
I delivered the goods, she said that's too much meat..."
Hesh's photos show braces that are WAY too big. In an email, he wrote that this guitar doesn't have much bass, and I can see why. He is also using what I call a skewed parabola on most of the top braces. I developed the idea of doing this in my brace shaping to deal with braces that had more force acting on one end than the other. However, this does not apply to all braces on the top, only the ones that meet this criterion.
When re-voicing a scalloped guitar, in order to make the belly braces parabolic, the low height of the center of a scalloped belly brace becomes the highest point of a parabolic brace. Depending on the depth of the scallop, shaping especially the upper belly brace parabolically would not leave much of a brace. But the center of the brace is usually not over the centerline of the top, and has grown taller by the time it is. A skewed parabola can be shaped from a scalloped brace, whose tallest point is moved over to the center of the top, leaving more strength under the bridge.
Last spring, I re-voiced a 1989 HD-28 that had been so radically scalloped at the factory that the top was close to failure. The bridge belly had some loose from the flexing/bellying. I read somewhere that the guy at Martin who did the scalloping then spent 45 seconds (!) on each top. This one showed it. I fashioned a supplemental piece of spruce to fill in the deep (and irregular) scallop in the upper belly brace, reshaped it into a skewed parabola, and saved this guitar.
However, by the time the string tension gets to the lower belly brace, it is spread out enough so that the lower brace can be fashioned into a true parabola. And being farther away from the bridge, the lower belly brace does not need to be as strong as the upper one. When building, I make them narrower, too. In fact, a skewed parabolic lower belly brace will only interfere with the top's vibrating--believe me, I've tried.
If I were re-voicing Hesh's OM, I would cut the upper belly brace down to less than half its current height, and leave it skewed. The lower belly brace could come down to about 5/16" high in the center, and I would make it a true parabola.
I have a picture on my web site of a factory Gibson top which I reshaped in 1997 before it was a guitar. Looking carefully, one can see how the belly braces are as I described above.
As to the side braces, I have found in re-voicing and building my own guitars that the upper side brace needs to be skewed, with more height towards the X brace, since the string pressure wants to push the top down in this area. But I make mine no taller than 5/16" to 3/8" by 1/4" wide.
The lower side brace, instead, is off to the side of the bridge, where the forces are in transition from pushing down to pulling up. And so, this brace does not need to be skewed, and when I built one with skewed lower side braces, I reshaped them into true parabolas, about 1/4" high, because they were dampening the B string.
I have been building guitars with parabolic X braces, and have been frustrated that I can make a scalloped guitar sound better than mine. Sometimes sticking to one's principles is limiting. I tried lowering the height of the X's cross point for more life with limited success. I have been using X braces that are 5/16" wide, and was down to under 1/2" and still not happy.
This spring, I put theory aside and started reshaping the legs of the X into a compromise with scalloped, after working on an OM whose X legs started the concave curve right at the cross point, instead of the usual start, a few inches below the cross point. However the peaks left lower down by scalloping are always dead spots, so I made just a gradual curve levelling off towards the bottom, and am closer to happy. Maybe a bit more on one of mine.
Another thing I've discovered is that the cross sectional curve of the X brace's legs needs to be continuous to the end, not squaring off as most builders shape them. These corners keep the bass from being released into the top and sides.
Moving to the photo of Hesh's back, so to speak, I would advise making the lower two braces much lower in the center. The lower bout of the back reinforces the bass end. I would make the lower two braces no taller than 3/8" in the center, given the width shown.
The upper bout reinforces the mid range. If you take the model I've developed, which says that any given note finds resonance in a ring, depending on the frequency, you can analyze whether the back braces are too big by plucking the strings, finding the rings on the back with which they resonate, and follow the rings with your finger tips over the braces. If there is no vibration over a brace, it means there is too much meat underneath. This goes for the top, too. When the strength of braces is matched to the forces acting on them at any given point, the braces no longer dampen the surface to which they are glued. Ergo, more volume. The shape must also be perfect in order for the energy to flow through them and be released where it finds resonance.
On the back's lower bout, you will find the low E ring near the outside perimeter. The A and D rings are further in towards the center. On the upper bout, you will find the D ring around the perimeter (no A and E reinforcement here, due to the smaller diameter) and the G ring inside of the D's.
Another aspect of parabolic bracing is that the shape must be absolutely perfect. Any irregulaties will prevent the brace from flowing with the energy instead of fighting it. Not to be critical of Hesh's work, which overall is very cool and carefully done, but look at the third back brace down. Notice that to the left of the center line, there is a slight hump, i.e., the tallest part of the brace is not in its center. Ditto for the very bottom back brace, but not as much. Whereas the top two braces appear to be true parabolas.
I went over all of this and much more in my workshop at the recent GAL convention in Tacoma. The folks at ASIA are not interested, but if any of you are, I can give individual workshops here in southwest New Mexico to show how to find the rings, and, tapping and feeling, reshape braces until they work. I can also re-voice a guitar via UPS, if anyone has one which is disappointing. Cost of re-voicing is $1200, but I will give the same 25% discount to builders that I give to musicians and dealers.
Overbraced parabolic guitars will not only be disappointing, they will give parabolic bracing a bad name.
Don't give up on parabolas.
Scott
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FAQ
I generally build very light, with light thin bracing and but I tune the top in just the same way as normal, the wood will tell you how thin it wants to go. My idea of balance is where no part of the sound spectrum dominates any other, but that said I like the mids and uppers, where the melody tends to be, slightly forward. Final tuning of the top for me is when it's on the box and I work the outer edge of the top. and get tapping at the bridge area.
