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Old 03-23-2013, 09:42 PM
jcbmv11 jcbmv11 is offline
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Default Bracing Question

This may be a question for a luthier. I have always been interested in the placement of bracing. I know the main purpose is to reinforce the top. It's obvious that the x brace takes the brunt of the force, but there are other strange braces which I'm curious about. More specifically,

What are tone bars? Why are they placed how they are? Why are there so many? What effect do they have?

What's the purpose of the soundhole strips?

The tone bars are slanted, and kind of in an odd position. I am an engineer, so the reasons for this design fascinate me.

http://martinrep.com/library/topstdscalp.jpg
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Old 03-23-2013, 10:38 PM
gitnoob gitnoob is offline
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I'm looking forward to the responses, but allow me to make a noob attempt at it.

Let's start from the top and work our way down:



The brace labeled "top plate" is also known as the "popsicle brace." It's literally a band-aid. I think Martin added it after they noticed cracks forming in that area under the fingerboard extension. The fingerboard wood expands/contracts at a different rate than the spruce, so that band-aid tries to hold the spruce together cross-grain-wise.

The brace labeled "number 1" is also known as the upper transverse brace, and it's probably the most important structural brace. It takes the load from the neck as it tries to rotate into the sound hole from string tension.

The "soundhole strips" are poorly designed IMO. Martin probably added them to support the exposed end-grain from the sound hole, but that hole in the top also makes it very weak in that area. Taller braces would be much better structurally.

The X does provide structural support, but it also enables several vibrational modes of the top and gives us the tone we all love. You'll notice that the legs of the X cross the wings of the bridge -- the strings set the bridge in motion, and the bridge sets the X in motion.

The side tone bars are sort of optional IMO. They don't provide much structural support, but they add stiffness to an otherwise large open area. Some designs only use a single tone bar on each side, but basically they help with higher-frequency response by adding cross-grain stiffness.

The bridge plate primarily keeps the spruce from getting chewed up by string ball-ends, but it can also help resist bridge rotation.

And finally, the mysterious bass bars or lower tone bars or lower face braces....

You'll notice that the slant gives you a more open area on the bass side. Somebody thought that this would enhance the bass response from the bass strings, but that's not how it really works. So really the slant is a historical curiosity, and plenty of builders do something different.

They also add some resistance to bridge rotation in the lower bout as well as cross-grain stiffness (which actually reduces bass response).

Computer models have shown that the mass effect of those bass bars is pretty significant, so they probably reduce responsiveness more than anything else.
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Old 03-23-2013, 11:04 PM
charles Tauber charles Tauber is offline
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I'm an engineer as well. A fluid mechanics teacher I once had liked to say, "An art is a science with too many variables." Guitar design is an art, in this sense. It is difficult to control and/or isolate each of the variables involved and definitively say do this and you'll obtain that exact result: there are too many inter-related variables.

The object of top bracing is to achieve a compromise between response and structure. The response would have the top and its bracing low in mass and low in stiffness. The structure requires stiffness.

The bracing, in conjunction with "sound hole" placement and top thickness (and to a lesser extent many other variables) is aimed at providing this compromise. There are many ways that people have tried to best achieve this compromise. Sometimes it is empirically determined - trial and error - other times theoretically based. However, what is the "best" response is subjective: ask different people, you'll get different answers on what sounds best. Hence, there are many different variations on how to best achieve the common goal.

Regarding the common X brace, practice suggests that one can alter the response of the guitar by changing the angle of the arms of the X brace. The placement and angle of the "tone bars" can also be changed to add greater or less stiffness along the length of the top or across the width of the top. What size to make the braces, how many braces to have, the placement and angles of the braces all depend upon the maker's "vision" of how they function and what he or she believes is needed to optimize the compromise between response and structure. In short, there is no single answer and the bracing can't be considered in isolation from the other variables that enter into response and structure.

Here are just a few examples of other "visions" of how to best achieve that compromise:










Last edited by charles Tauber; 03-23-2013 at 11:10 PM.
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Old 03-24-2013, 12:15 PM
Alan Carruth Alan Carruth is offline
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One of the interesting things about bracing is how little it does to the sound.

The actual main function of bracing is to add stiffness to the top to allow it to withstand the torque of the bridge, without adding too much weight. It's interesting to note that an unbraced top off the guitar will vibrate in much the same way as a braced one. If you simply left it thick enough to have the requisite stiffness, it would have much the same timbre as a properly braced top, but the mass of the thick top would cut down on power a lot. In this respect, a good bracing design is one that doesn't do too much: that 'keeps out of the way' of the top, so to speak.

IMO, it's more important to get the bracing working well with the top than it is to use a particular brace pattern. Good guitars can be made with a wide range of bracing patterns if they're done well, and anything done badly makes a bad guitar.

That said, different bracing schemes do sound different. Several years ago I made a 'matched pair' of maple Small Jumbos with different top bracing, and took them to a luthier convention. There, I had as many people as possible try them out without telling them what the difference was, and asked them which they liked better. In two cases out of three these experienced builders preferred the one with 'double-X' bracing over the 'standard' top by a slight margin. It was really interesting to hear the guesses people gave as to what they thought the difference was. When I told one maker that the guitar she preferred had double-X bracing, she exclaimed: "But that doesn't WORK!" I shrugged.

In most traditional designs, both the top and the braces are taking at least some of the load, but this is not the case so much with some of the modern designs. The Smallman 'lattice', which is more or less like Charles' next to the last photo, takes almost all of the load on the carbon/balsa bracing, with the veneer thickness top being there mostly to fill in the gaps and push air. I think of this as 'distributed bracing'. The use of high tech materials cuts down on the overall weight of the top. Some 'sandwich' tops have been made that also delete the bracing entirely, or nearly so.

To the extent that a 'different' braving system actually works differently, it will alter the tone of the guitar from the expected one. As often as not this is a disadvantage in a world where there is a lot of existing music that people will want to play on the new instrument. On the other hand, a well designed and executed 'different' bracing scheme can make a very nice instrument, and be a decided marketing advantage.

Finally, to the OPs question:
You have to remember that most bracing schemes were not 'designed' in any rational way. They are mostly the outcome of seat-of-the-pants engineering and a lot of cut and try. Given enough time this sort of evolutionary process can get you a pretty well optimized system, but there will be a lot of failed experiments along the way. We mostly don't see those, of course; the luthiers bury them.

It's also really hard to say in many cases what a particular brace 'does' that's different from all the others. In a general sense, as I tell my students, the edge of the top at the upper block is primarily about structure, and the lower edge, at the tail block, is about tone. It usually does not make a lot of difference to the sound if you remove wood from the bracing above the bridge, and lightening the bracing below the bridge does not affect the strength as much.

Every scheme is a compromise: we want low weight and high stiffness: mobility to produce sound and stability over time for playability. You can't have everything you want, and each choice you make comes with a cost. In the end, there's at least as much 'art' in this as 'science'. A good top is not so much the prose of a technical paper, where each brace has a well-defined purpose, as poetry, where it all works together to produce meaning. A successful maker knows how to work with both aspects.
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Old 03-24-2013, 12:29 PM
gitnoob gitnoob is offline
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Somebody recently converted an old ladder-braced Harmony to X bracing.

He also kept another nearly identical Harmony ladder braced.

And then he made a video comparing the two.

http://youtu.be/iYkt4da6kwc
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Old 03-24-2013, 01:53 PM
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Quote:
Originally Posted by Alan Carruth View Post
One of the interesting things about bracing is how little it does to the sound.

The actual main function of bracing is to add stiffness to the top to allow it to withstand the torque of the bridge, without adding too much weight. It's interesting to note that an unbraced top off the guitar will vibrate in much the same way as a braced one. If you simply left it thick enough to have the requisite stiffness, it would have much the same timbre as a properly braced top, but the mass of the thick top would cut down on power a lot. In this respect, a good bracing design is one that doesn't do too much: that 'keeps out of the way' of the top, so to speak.

IMO, it's more important to get the bracing working well with the top than it is to use a particular brace pattern. Good guitars can be made with a wide range of bracing patterns if they're done well, and anything done badly makes a bad guitar.

That said, different bracing schemes do sound different. Several years ago I made a 'matched pair' of maple Small Jumbos with different top bracing, and took them to a luthier convention. There, I had as many people as possible try them out without telling them what the difference was, and asked them which they liked better. In two cases out of three these experienced builders preferred the one with 'double-X' bracing over the 'standard' top by a slight margin. It was really interesting to hear the guesses people gave as to what they thought the difference was. When I told one maker that the guitar she preferred had double-X bracing, she exclaimed: "But that doesn't WORK!" I shrugged.

In most traditional designs, both the top and the braces are taking at least some of the load, but this is not the case so much with some of the modern designs. The Smallman 'lattice', which is more or less like Charles' next to the last photo, takes almost all of the load on the carbon/balsa bracing, with the veneer thickness top being there mostly to fill in the gaps and push air. I think of this as 'distributed bracing'. The use of high tech materials cuts down on the overall weight of the top. Some 'sandwich' tops have been made that also delete the bracing entirely, or nearly so.

To the extent that a 'different' braving system actually works differently, it will alter the tone of the guitar from the expected one. As often as not this is a disadvantage in a world where there is a lot of existing music that people will want to play on the new instrument. On the other hand, a well designed and executed 'different' bracing scheme can make a very nice instrument, and be a decided marketing advantage.

Finally, to the OPs question:
You have to remember that most bracing schemes were not 'designed' in any rational way. They are mostly the outcome of seat-of-the-pants engineering and a lot of cut and try. Given enough time this sort of evolutionary process can get you a pretty well optimized system, but there will be a lot of failed experiments along the way. We mostly don't see those, of course; the luthiers bury them.

It's also really hard to say in many cases what a particular brace 'does' that's different from all the others. In a general sense, as I tell my students, the edge of the top at the upper block is primarily about structure, and the lower edge, at the tail block, is about tone. It usually does not make a lot of difference to the sound if you remove wood from the bracing above the bridge, and lightening the bracing below the bridge does not affect the strength as much.

Every scheme is a compromise: we want low weight and high stiffness: mobility to produce sound and stability over time for playability. You can't have everything you want, and each choice you make comes with a cost. In the end, there's at least as much 'art' in this as 'science'. A good top is not so much the prose of a technical paper, where each brace has a well-defined purpose, as poetry, where it all works together to produce meaning. A successful maker knows how to work with both aspects.
Well stated Alan. Thank you.

Kevin.
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Old 03-24-2013, 02:04 PM
Tom West Tom West is offline
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Alan is one of the best teachers on the various guitar forums. He puts a lot of thought and work into his answers and I for one seem to get something new every time he appears. Thanks for the enthusiasm Alan.
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Old 03-25-2013, 08:41 AM
redir redir is offline
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I don't have anything to add to the information already provided but I will say that I tried several experimental bracing patterns and none of them sounded as good as the traditional designs. So it was really just a waste of my time and I build with X-Brace for steel and Torre's fan bracing for classical. I did however build with a double-x and it was a good guitar, I'd probably do that again.

I'm not an engineer, nor do I play one on TV, I do have a degree in Geology however and structural geology is about as much engineering as I can stand. So, having so boldly stated my qualifications, I've never really 'got' scalloped bracing. Seems to me you want the braces to get weaker as they approach the stiff rim. So I just taper my braces to the edge. I know that it looks like a pretty little Brooklyn bridge's under there but is that structurally practical in guitar building? It's essentially designed to weaken the top in what seems to me to be an area that needs strength, it also leaves a big heavy blob on the top at the location of the peak. So like I said, I don't get it but apparently it makes a good sounding guitar

Talking about bracing is like talking about the best way to make chili. Everyone has their own recipe and most of them are pretty darn good.
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Old 03-25-2013, 02:38 PM
Alan Carruth Alan Carruth is offline
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redir wrote:
" I've never really 'got' scalloped bracing. "

It's all about the sound you're trying for.

The top of the guitar vibrates in several different 'modes' at different frequencies. the power and timbre of the guitar have a lot to do with the amplitudes and relative frequencies of these modes.

The one that produces the most actual sound is the 'main top' resonance, where it's moving in and out like the cone of a loudspeaker. Most speakers have a (more or less) rigid cone and a 'soft' rim, but you can't go too far in that direction on a guitar top, simply because it has to withstand the bridge torque over the long term. How well the top moves, and how much air it can push in the process, is thus a function of how stiff it is in different places.

A lot of makers thin the edges of the top to get it to move more. This works, but,again, you can only go so far. Modeling that has been done, by Evan Davis, and maybe others, has shown that it produces more sound to make the center of the top flexible, rather than the edge. It's a nice trick to make the center as flexible as you can, and keep it as stiff as you need, but it can be done.

What happens in that case is that the bridge moves a lot, extracts the energy from the strings in a hurry, and turns it into sound. You get a strong attack, with a lot of fundamental, and plenty of power.

The peaks near the outside ends of the braces serve to stiffen up areas of the top that move more in higher order modes. These are not as efficient at producing sound as the main 'loudspeaker' mode, and suppressing them and moving them up in pitch helps emphasize the fundamental.

'Tapered' bracing, that's higher in the middle and low at the edges, works the opposite way; bringing up more of the higher partials, and giving more sustain.

Nether of these is intrinsically 'better' than the other, they're just different. Scalloped bracing tends to work well for some styles of flat picking, and for accompaniment, where you want the 'punch' to come through and don't want a lot of sustain to muddy things up. Fingerstyle players tend to like the even and sustained sound of tapered bracing. 'Straight' bracing fall somewhere in between, and can either bridge the gap or leave everybody cold, depending on how well it's done.
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Old 03-26-2013, 07:29 AM
redir redir is offline
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That's interesting Allan. I have always gone for guitars with more sustain and perhaps that's why I have gravitated in that direction (without even knowing). That's the problem with taking the shotgun approach I suppose. I've got the Gore and Somogyi books on my shelf but just have to sit down one week/month and read them
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Old 03-26-2013, 02:57 PM
Alan Carruth Alan Carruth is offline
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I'd concentrate on the Gore/Gilet books.
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Old 03-27-2013, 07:32 AM
charles Tauber charles Tauber is offline
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Quote:
Originally Posted by Alan Carruth View Post
I'd concentrate on the Gore/Gilet books.

That's very funny - a belly laugh. For all of the unstated reasons.

I'm not going to review the books here, but I'm currently slugging through the Gore/Gilet books. In my opinion, they are a must-read for anyone serious about guitar making. I think they are the best books ever written in the English language on the subject. I was stunned by the thoroughness of the second book dedicated to physical construction. (The first book is not lacking either.) About the only thing not covered was the use of CNC machinery. Thanks, Alan, for your contribution to the books.

Its fascinating to me that two different builders published two different sets of books in the same basic format: an expensive, glossy, hardcover two-book set with the first book dedicated to theory, the second to practical how-to guitar making. In my opinion, as a generalization, Somogyi's books are the "warm and fuzzy" intuitive approach, while Gore/Gilet's is the hard-core scientific approach. I think that the discussion and topics that Somogyi brings up are valuable contributions, but less to the point of improving one's ability to understand guitars and improve the guitars one makes.
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Old 03-31-2013, 11:55 AM
pickinbob pickinbob is offline
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I am the "new guy on the block" posting in this forum. I have been reading diligently all the information I can about various aspects of building an acoustic guitar. I am currently in the process of building jigs and forms. Relative to this thread, I have a couple of questions. (1). Does it make any difference what kind of wood one uses to make the bracing? (2). I have noticed various plans have various sizes with regard to thickness, length and with of the bracing. How crucial is this issue?

You all seem to be very knowledgeable about this so any help will be appreciated.
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Old 03-31-2013, 12:31 PM
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Depends on what you are building, but most plans I have seen are design built for average results, and are somewhat over built with regard to bracing. You really need to find out from someone who has used the plan. Yes it does matter what you use for bracing. There are lots of opinions out there, and you'll have to decide for yourself what works for you. Again, it depends on what you are building. I build classical guitars, and I now use Lutz Spruce exclusively for all my top bracing, even on Cedar and Redwood tops. Why? Because it works for me and I have a very reliable source for my brace wood.
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Old 03-31-2013, 12:44 PM
pickinbob pickinbob is offline
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Quote:
Originally Posted by WaddyT View Post
Depends on what you are building, but most plans I have seen are design built for average results, and are somewhat over built with regard to bracing. You really need to find out from someone who has used the plan. Yes it does matter what you use for bracing. There are lots of opinions out there, and you'll have to decide for yourself what works for you. Again, it depends on what you are building. I build classical guitars, and I now use Lutz Spruce exclusively for all my top bracing, even on Cedar and Redwood tops. Why? Because it works for me and I have a very reliable source for my brace wood.
OK, perhaps I was not specific enough. I am going to build a "Martin" type dreadnought guitar. The sides and back will be walnut and the top will be sitka spruce. I have read every thing from mahogany, spruce, pine, cedar or basswood used for bracing. I have no idea what I am shooting for with regard to sound except that I like the deep loud bass sound of acoustic guitars. I do plan to make it an acoustic/electric guitar and will install an under saddle pick up. by the way, I am a recreational player, mostly for my own entertainment.

Last edited by pickinbob; 03-31-2013 at 02:00 PM.
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