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| The Purpose of Bracing http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=10595 |
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| Author: | LarryH [ Sun Jan 28, 2007 11:26 am ] |
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My sincerest apologies for the topic title as I know that the subject of braces comes up at least a few times a day but I have a question that arose in my mind as I was finish trimming the braces on my top. I was trying to decide if I should let the braces into the kerfing or not and was wondering what those braces were actually for. The first thought I had was that they were for bracing (sorry, really been a long day). So then I thought that there is certainly no need to have a brace within 1" - 1 1/4" of the kerfing. That small area of sound board between the end of the brace and the kerfing should be perfectly fine - no real need for reinforcement there. Or is there? Then I thought about coupling the braces to the kerfing and hence the sides which might involve the sides a little more in the sound production suggesting that another purpose of the braces is sound production and it seems to be agreed that a sound board without braces would not sound very good so obviously another purpose is for sound production. I ended up shaving the lower X, both finger, and in this case both symmetrical tone bars, short of the kerfing and letting in the upper X, transverse, and upper transverse brace into the kerfing. Don't know why but it seemed like there was no need to let in the kerfing on the lower bout and for some odd reason it made sense on the upper bout. Any thoughts? |
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| Author: | CarltonM [ Sun Jan 28, 2007 12:23 pm ] |
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More experienced people will chime in, I'm sure, but here's what I know. Either way is probably okay. Some seasoned luthiers inlet the "X" and some don't. However, if you stop short of the linings, you'll want to taper the brace down to virtually nothing as it gets near the lining. If you don't, you're askin' for a crack in your top between the brace and the lining. Why...because if the brace stops abruptly, the top won't be able to flex if it sustains a blow (or even with seasonal movement) and is more likely to break. Also, the same conditions raise the possibility of your brace coming loose at its end. I think virtually everyone does inlet the upper braces, except for the one refered to as the "popsicle" brace, so good call there. There are also tone considerations, but I'm not experienced enough to speak to that. |
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| Author: | LarryH [ Sun Jan 28, 2007 12:27 pm ] |
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[QUOTE=CarltonM] More experienced people will chime in, I'm sure, but here's what I know. Either way is probably okay. Some seasoned luthiers inlet the "X" and some don't. However, if you stop short of the linings, you'll want to taper the brace down to virtually nothing as it gets near the lining. If you don't, you're askin' for a crack in your top between the brace and the lining. Why...because if the brace stops abruptly, the top won't be able to flex if it sustains a blow (or even with seasonal movement) and is more likely to break. Also, the same conditions raise the possibility of your brace coming loose at its end. I think virtually everyone does inlet the upper braces, except for the one refered to as the "popsicle" brace, so good call there. There are also tone considerations, but I'm not experienced enough to speak to that.[/QUOTE] Yeah Carlton thinning the brace to nothig seemed the proper thing to do. Do you inlet your braces? Or just a few? |
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| Author: | Serge Poirier [ Sun Jan 28, 2007 2:13 pm ] |
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Just the X brace here, the others thinned to nothing at the kerfed lining, i want my tops to move as freely as possible like Paul Woolson does and others i believe. |
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| Author: | j.Brown [ Sun Jan 28, 2007 3:12 pm ] |
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I put the X's and the upper transverse all the way through the kerfing. Right about 1/8" tall at that point. All others are tapered to Zero. I wouldn't exactly call myself "experienced", but my mentor builds his this way, its how I learned, and I'll probably continue to do so for awhile. -j. |
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| Author: | Alan Carruth [ Sun Jan 28, 2007 4:03 pm ] |
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Braces are there to brace the top, so we call that stuff 'bracing'...... :) Aside from that, there are probably more opinions on this than there are luthiers. I tell my students that the rule of thumb is that the area of the top at the upper block is 'all' about strength, and the area at the lower block is 'only' about tone. you can leave the braces at the upper end as heavy as you like without altering the tone a lot, with the _possible_ excepetion of the 'popsicle stick', if you use one. You can shave the braces at the lower end to practically nothing, without too much risk, so long as your top is not too thin. In terms of top motion, reducing the brace height around the edges is probably less effective than reducing them in the center. Of course, there's less structural risk to thinning the edges. The tone will be different in each case, too. Any brace that is more than aboout 1/3 as tall as the top is thick at the brace end should be inletted, to avoid stress concentrations that will promote cracks and peeling braces, as has been said. Since there is a download on the top in front of the bridge, it's probably more necessary to inlet in front of the bridge than behind. As far as I've been able to tell, most of the sound that gets from the top to the back does so through the air in the body, with little, if any, transmission of force through the sides. It stands to reason if you think about how hard it is to 'flex' the sides in a vertical direction. Coupling the braces to the liners probably has no real acoustic function in that sense, although, no doubt, there's a difference in tone between braces that are tall and low at that point. |
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| Author: | LarryH [ Sun Jan 28, 2007 4:17 pm ] |
| Post subject: | |
[QUOTE=Alan Carruth] Braces are there to brace the top, so we call that stuff 'bracing'...... :) Aside from that, there are probably more opinions on this than there are luthiers. I tell my students that the rule of thumb is that the area of the top at the upper block is 'all' about strength, and the area at the lower block is 'only' about tone. you can leave the braces at the upper end as heavy as you like without altering the tone a lot, with the _possible_ excepetion of the 'popsicle stick', if you use one. You can shave the braces at the lower end to practically nothing, without too much risk, so long as your top is not too thin. In terms of top motion, reducing the brace height around the edges is probably less effective than reducing them in the center. Of course, there's less structural risk to thinning the edges. The tone will be different in each case, too. Any brace that is more than aboout 1/3 as tall as the top is thick at the brace end should be inletted, to avoid stress concentrations that will promote cracks and peeling braces, as has been said. Since there is a download on the top in front of the bridge, it's probably more necessary to inlet in front of the bridge than behind. As far as I've been able to tell, most of the sound that gets from the top to the back does so through the air in the body, with little, if any, transmission of force through the sides. It stands to reason if you think about how hard it is to 'flex' the sides in a vertical direction. Coupling the braces to the liners probably has no real acoustic function in that sense, although, no doubt, there's a difference in tone between braces that are tall and low at that point. [/QUOTE] Thanks Alan for your usual clear and concise explanation. I am inexperienced enough to have to rely on my intuition in a lot of respects but seemed to have followed a perhaps well worn path. The shame as always is never knowing how the tone will have been affected if the braces took on a different shape/height or were stopped at a different location. This is a redwood top as well so the tone will have to include some sort of redwood magic as I'm sure it will. |
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