Official Luthiers Forum!

I've spent at least four hours searching and reading the net for information on bracing theory, and I can't seem to really find anything.

After building my first guitar, many questions popped into my head. For example, why the "harmonic" or "tone" bars above and below the sound hole? Why a fan shape? Why seven fans, nine, three? What's the point of a bridge plate? If the upper bout is tonally "dead", could it be because there is so much bracing in that area that it can be nothing but dead, and if not, why have the upper bout at all beside being a nice resting area for your leg? The bridge is really like an external brace, so why not use it as one, like a spiderweb or "fingered" kind of bridge?

And you know what is really cool, after all this looking around, the best thing I found about all this was right here on the forum. It happened to be the last thing I read too, and it's not just because I found it so I stopped looking. It was the last of 30 screens I had opened and read.

It was this one here.

These two were also good as well: stiff back and ebony

Of course I still really don't have many answers and I thought it was odd that there is not a book about this. Is the only research being done trial and error by luthiers? Simpling comparing guitars and guessing what makes them different? I know Kasha and Schneider did alot, and people like this guy and this guy as well are using it.

But really, no book or anything that says essentially "purpose of bridge plate is..." "fan bracing is believed to ----- while lattice bracing does -----"?

And of course I have a million more questions, like if the tension from strings is the cause for all the bracing, why not use a tail piece, and maybe use a sound post like violin. And why stop there, why not an elevated fingerboard, but not like Humphrey, but like a violin yet again, raised and detached from the soundboard so the whole of the soundboard can now vibrate more freely. Heck, and either move the sound hole to the side where people put a sound port, or elsewhere on the face of the guitar.

Well, I don't know what I'm really getting at in this post. My head is swimming with info, (heck, I just learned of all these luthiers I mention about in the last few hours. I'm serious greenhorn when it comes to things like who's who and does what.), ideas, and questions. And I probably forgot about 90% of the stuff I just read... So I better just stop. Whew!

I think I know why no one has written a book about it...most full time builders who could actually gather that kind of data are not experimental in their builds because they are trying to make a living selling well made good sounding guitars. Experiments fail most of the time, and they cannot sell these. Also experimenting takes away from profit. I think the hobby builder who has a strong engineering background has a better chance of getting this data, but the problem is that they do not make enough guitars to have the emperical data they need. See the problem?
Tracy

Yes, that does make sense, especially with the radical changes I was suggesting. But there have been scientific experiments done, or so I believe. I would think those findings would be available (Kasha's findings and others). And there must be SOME sort of, at the very minimum, conjecture behind the theory of the basic bracing structure that has been around since Mr. Torres that could be written about. Yes/no?bbeardb38716.7237268519

Drop a note to Al Carruth. He should be able to point you to a published reference or share some of his in-depth knowledge to help answer your question.

_________________
Peter

There has actually been rather a lot of work done on top bracing theory. Most of it has been published in PhD theses, done by engineering students and physic majors who want to learn about vibration in a system that's not as complicated and expensive as a satelite. Richardson, at the Universtity of Wales in Cardiff has directed several good projects. Meyers has also published papers, as has Jansson. There are others, too, but the journals can be hard to get hold of.

Ultimately you have to understand that there was no 'theory' involved in devising most of the standard bracing systems, if, by theory, you mean applied mechanics or physics. Most makers have a 'model' in their heads of how they think the guitar works, and where the sound comes from, and they work out bracing schemes based on these models. It's entirely possible to do all the right things for all the wrong reasons: some of the models I've heard about are pretty far from real physics.

Almost anything you build that looks like a guitar will sound more or less like one, so it's a pretty robust system: look at Ovation! At the same time, the small objective differences between very good and bad guitars are really important. The 'standard' designs have all been worked out over generations to yield good to excellent results when made with reasonable care and good materials. This is far from saying they are the ultmates, and can't be improved on; just that it's going to be difficult to do so.

Finally, the big manufacturers opperate under the theory that there's a buyer for every guitar. Tastes vary so widely that something you think of as a tin can is likely to be somebody else's Holy Grail. All they have to do is find that guy. Any bracing scheme you come up with will probably please some folks, and if you can get the word out widely enough you're in business.    

[QUOTE=Alan Carruth]

Ultimately you have to understand that there was no 'theory' involved in devising most of the standard bracing systems, if, by theory, you mean applied mechanics or physics. Most makers have a 'model' in their heads of how they think the guitar works, and where the sound comes from, and they work out bracing schemes based on these models. It's entirely possible to do all the right things for all the wrong reasons: some of the models I've heard about are pretty far from real physics.
[/QUOTE]

Darn it. Well let's bring it down to more a more simple basis on the thoughts behind certain things. The commonly accepted or known reasons for doing it just so.   I know not everybody does what I list below, but from what I have seen, the following seem rather common. For example:

- Why the closing bars at the tail of the guitar behind the fan struts?

- Why the excessive bracing in the upper bout, ie the "harmonic" bars?

- Why a fan shape at all and not straight with the grain?

- Why use a bridge plate or lateral bar beneath the bridge?

- Why use a bridge as opposed to a tail piece that would reduce the stress on the top of the guitar (related to bracing as that seems to be what a lot of bracing is used for)?

- I feel like I'm being annoying, am I?

Well I'm only on my second guitar, but I'll give you my thoughts on this.

[Quote] Why the closing bars at the tail of the guitar behind the fan struts? [/QUOTE]
I think these are to add some extra stiffness & strength to the lower bout area. Plus it looks cool.
[Quote]- Why the excessive bracing in the upper bout, ie the "harmonic" bars? [/QUOTE]
I'm not to sure about this one. Maybe the guitar just needs the extra strength.
[Quote]- Why a fan shape at all and not straight with the grain? [/QUOTE]
Maybe to cover more space with out adding to many braces? Plus it looks cool.
[Quote]- Why use a bridge plate or lateral bar beneath the bridge? [/QUOTE]
This is one I've been thinking about since I just finished bracing my sound board and I think it's mainly for strength since all of the string tension is focussed there.
[Quote]- Why use a bridge as opposed to a tail piece that would reduce the stress on the top of the guitar (related to bracing as that seems to be what a lot of bracing is used for)? [/QUOTE]
I think that would probably change the sound of the guitar. That isn't nessisarily bad, but it might not sound like you expect it to.
[Quote]- I feel like I'm being annoying, am I?
[/QUOTE]
I hope you aren't being annoying because I want to hear the more experienced members opinions on this also.

Thanks!

[QUOTE=bbeardb]
- Why the closing bars at the tail of the guitar behind the fan struts?

- Why the excessive bracing in the upper bout, ie the "harmonic" bars?

- Why a fan shape at all and not straight with the grain?

- Why use a bridge plate or lateral bar beneath the bridge?

- Why use a bridge as opposed to a tail piece that would reduce the stress on the top of the guitar (related to bracing as that seems to be what a lot of bracing is used for)?

- I feel like I'm being annoying, am I?[/QUOTE] First of all, understand that the wood is much stiffer along the grain rather than across it. Bend it in both directions and see. Then take a braced up classical top and bend it in both directions. Notice the stiffness is much more equal in both directions now. Cross braces add to the stiffness.

As to why the cross bracing in the upper bout, you have to do several things. Add support for the fingerboard, keep the wood from spliting (the ebony fingerboard moves a lot, and can easily cause splitting of the top), and, there is only limited energy in the classical string. Torres cut arches under many of his harmonic bars to get the upper bout more involved in sound production. However, the more energy that you use moving the upper bout, the less you have to move the lower bout. Tradeoffs.

If you have a chance, listen to a nylon string guitar with a tailpiece. They sound terrible, generally. No, it's not 'theory', but it sure is a useful piece of emperical information. :)

If you read GAL or other sources, you will see some makers claim the bridge plate is there for structural support, others think it helps "tie together" the sound (whatever that means), others have decided it does nothing and leave it out.

The best written source I know of is Romanillos' book "Antonio De Torres: Guitar Maker-His Life and Work". While heaver on emperical data than theory, it is the best description I have read of a maker's thought on soundboard construction. It's spendy, but I picked a copy up at the Strand this week, and have been kicking myself since then for not buying it earlier.

I've read a lot on the topic, and realize that when you read what different people write they often contradict each other. Nobody really knows the answer to many of the questions, besides the obvious points that cross bracing increases side-to-side stiffness, etc. Build a hundred guitars and you find a system that works for you - and this may be an entirely different set of choices made by another, equally successful builder.

Sometimes it's distressing to fumble around in the dark, but remember, as soon as it is easily codified every builder is going to go out of business except for the 3-4 who corner the "art guitar" market. The factories will have the rest.

p.s. Not trying steer traffic away from this forum, or stop discussion in this thread, but Alan Carruth is one of the well known experimenters on guitar construction, and the library at MIMF is filled with posts by Alan answering all of your questions in excruiating (sorry Al! :)) detail. It's not well organized in a book form, but reading around will help to give you a handle on the issues.rlabbe38717.4716782407

[QUOTE=bbeardb]

- Why the closing bars at the tail of the guitar behind the fan struts?

- Why the excessive bracing in the upper bout, ie the "harmonic" bars?

- Why a fan shape at all and not straight with the grain?

- Why use a bridge plate or lateral bar beneath the bridge?

- Why use a bridge as opposed to a tail piece that would reduce the stress on the top of the guitar (related to bracing as that seems to be what a lot of bracing is used for)?

- I feel like I'm being annoying, am I?[/QUOTE]

Okay, here's what I figure:

#1 - The "closing bars" aka 'bottom angle braces' definitely add a bit more structural integretity to the bottom of the soundboard, and if arched, help preserve the soundboard arch. They may well play a role in preventing cracking of the soundboard in that area as well.

#2 - I figure the primary reason for the harmonic bars is to brace the soundboard around the soundhole, which vastly weakens the top's structural integrity.

#3 - The fan shape was first employed by Panormo, I believe it was. The modern fan shape was developed by Torres. I believe the "why" is a matter of tradition now. As for why Panormo began building with a fan pattern, it might have simply been a "what if?" idea that payed off. Torres then just "improved" on Panormo's pattern.

#4 - The bridge pad was most likely used to resist "bridge roll". Torres didn't use bridge pads. They came later. This might actually be an influence that came into practice in classical building which was borrowed from steel string building. The cross brace beneath the bridge, as opposed to a bridge pad, however, reinforces the top arch, and it is my belief based on my own builds, that it improves sustain.

#5 - Well, if you look at guitars that use tailpieces, like archtops, you'll note that they have bracing to resist forces too, just different. The arched tops themselves supply considerable stiffness to counteract string forces. This is an area where I defer to folks like Al Carruth, but based on my readings, I'll just say this -- guitars with bridges that are fixed to the soundboard are mostly dependent upon their volume production due to the bridge rocking fore and aft when a string is plucked. Guitars with tailpieces and violin-style bridges are more dependent upon the up-and-down movement of the soundboard to produce volume. I suspect that having the strings terminate somewhere around the middle of the soundboard's greatest sound production area is a more efficient means of volume production that that method used in archtops. (I've played a few archtops, and I would not describe them as being loud.) Of course, I'm also thinking about mandolins and banjos as I write this, so take it with a grain of salt . . .

#6 - Nope. I, for one, love this stuff.

Best,

Michael
Michael McBroom38717.4734027778

_________________
Live to Play, Play to Live

Don't forget there were stringed instuments before the guitars your talking about. There were fan braces below the bridge in lutes a hundred years before Panormo was working in Bloomsbury. I repaired a 1764 13 course lute that had fan braces about 6 ins long below the bridge. It is thought that they were employed to counter the increased tensions on the baroque lute. There are very few quantum leaps in bracing as far as I can see. Most changes are incremental: X bracing being a development of the diagonal braces on early ladder braced guitars, fan bracing an extension from lute fans, harmonic bars from ladder braces etc. There is almost nothing new that hasn't evolved from earlier models.

So, we remember the man who made the biggest 'commercial' success of it, Torres for the fans, Martin for the X brace. But, what did Newton say of his work "Standing on the shoulders of giants".

Colin

By the way I lived in Newton's rooms at Trinity College, Cambridge for three years!

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Yeah, so far as I know fan bracing was being used on Spanish guitars around 1775. Panormo advertised himself as building 'in the Spanish style', so he was just borrowing the idea. Torres didn't really _invent_ anything, he just got it to work better than anybody before him. It's like Stradivarius, who basically took a system that Nick Amati and his family had worked out, and refined it that last little bit.

One way to think about the function of the bracing is to say that the upper edge of the soundboard, where the neck attaches, is _all_ structural, with nothing to contribute to tone, and the lower edge, at the tailblock, is _all_ tone, with no real structural function. That is NOT STRICTLY TRUE, but it's a helpful way to think about it.

All that bracing under the upper bout is thus structural, and has little effect on the tone. Why have an upper bout then? For the air mass, of course! Those little 'cuttoff bars' at the lower end of the top do have an effect on the tone, and, no, I can't say just what it is.

With a tailpiece you have the problem of getting enough breakover angle on the bridge saddle to stop the strings effectively. You need around 12-15 degrees, probably not more, but you do _need_ it. It's hard to get that much angle on a flt top, without 'cranking' it, like a Neapolitan mandolin, and that changes the sound. That much break angle puts a fair amount of down pressure on the top, which needs to be either beefed up or arched, or, more likely, both, to stand up, and that effects the sound too.

I have seen guitars with glued-on 'normal' bridges where the strings were just run out through the tieblock holes and tied off to a tailpiece. The down angle is set by the tieblock, and that also gives you the 'normal' torque on the bridge more or less, minus the tension load from the strings, of course. It works.

Torres didn't use bridge patches, and I don't find them much use myself, but that's just my opinion. They do add mass and stiffness at the bridge, usually about in proportion: that is, they don't change the resonant frequencies much, if at all. What a patch, or, especially, a bar, as in a 'Bouchet brace', does, is to raise the impedance of the bridge a bit. It takes a bit more 'push' to get it going, but it also can work better with higher power levels, or so it seems. One person I know uses a version of the Bouchet brace, and I note that his guitars tend not to sound as good at low levels; they require a certain amount of energy to get them going. However, they don't 'top out' as soon as some guitars, either, so if you're a strong player or need the power, it's there.

I've long felt that the standard guitar designs do a nice job of balancing the structural and acoustic needs. You need some bracing to get the top to work right, and if you do it correctly, that's just what you need for strength as well.   

To the best of my knowledge, the first known use of fan bracing in a guitar (or vihuela) is in a 1759 Fancisco Sanguino.

_________________
Instagram: @jfrenchluthier
Web: https://www.jfrenchguitars.com/

[QUOTE=Alan Carruth]What a patch, or, especially, a bar, as in a 'Bouchet brace', does, is to raise the impedance of the bridge a bit. It takes a bit more 'push' to get it going, but it also can work better with higher power levels, or so it seems. One person I know uses a version of the Bouchet brace, and I note that his guitars tend not to sound as good at low levels; they require a certain amount of energy to get them going. However, they don't 'top out' as soon as some guitars, either, so if you're a strong player or need the power, it's there.[/QUOTE]

Wow. So that's it. One of the things I discovered, quite by accident, with the first guitar I built using a Bouchet-style cross brace, was that the guitar didn't "flab out" when I struck the strings really hard. Hehe. The wife said something to me that kinda got me PO'd while I was playing and I just really bore into it hard -- much harder than I normally do, even though I consider myself to be a fairly loud player. I was stunned. The guitar didn't "flab out" -- it just got louder. I completely forgot about my wife's remark or even being angry about it, and just stared down at the guitar in amazement.

One thing I did to that guitar that might improve the sound at lower levels, is I ported it. I drilled a 1.5" diameter port into the top side of the upper bout. Kinda like a personal stage monitor, if you will. Made a big difference at all volume levels.

Best,

Michael

_________________
Live to Play, Play to Live

Awesome posts! Thanks everybody. I really appreciate it.

I think when it came to tailpiece thoughts, I was thinking of a violin. I mean, it's small, very loud, expressive, great tone. BUT, it is bowed, so that most likely excites the stings more. Well I just bowed my guitar, and it really wasn't much too much louder, but it did sound pretty good, like a cello.

Again, thanks for the food for thought. When I feel like I have the process down a little more I'll start experimenting. Mostly tyring to get the upper bout more active (it just irks me that it's there and really serves "no" purpose), and if that involves moving the sound hole, making the guitar smaller, making the fretboard detached, so be it. It'll be fun. But for now, I feel like I should get the good old standard guitar down first and resist the temptation to experiment. The old standard seems to work pretty good!