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Hi Everyone,
               We concern ourselves a lot with the soundboard bracing and thickness' etc. ,but not a whole lot with the back,,,,,or so it seems to me.

I'm interested in hearing from others on this subject ,i.e. what thickness , bracing etc .

There are some who build with a thick back designed to resonate as little as possible, leaving the soundboard only, to deliver their desired sound. (the Smallman approach )

Others ,such as Mr. Carruth employ a cross brace set up. I recall him saying that this is so he can tune the back to the top ??

A more common approach is to use ladder bracing , but even this ladder bracing comes in a couple of forms. Those that have a wide ,low bracing profile for the lower two braces, and those that install 5/16" X 5/8" type braces for all four of the braces .

What type do others use and at what back thickness .??

I also recall Mr. Carruth ( I think ) mentioning that the wrong back resonance can introduce some unwanted wolf notes to the instrument .

It seems to me that a very firm back will give the guitar more projection . I have played guitars like this and they are louder , but also with an altered tone, to a back built to resonate.

Appreciate all your views !

Regards to all,, KiwiCraig

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CRAIG LAWRENCE of AUSTRALIA
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So far I've just used the two narrow and two wide configuration.

Ron

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Graig I have been using this pattern on my SJ backs for a while now I am in no way an expert on the physics of the backs response due to this pattern but my mid tones and highs seem to be brighter and the guitars are louder than with conventional ladder bracing.


There is an other lateral brace in the upper bout not shown in the picture.

Another benifit I get from this pattern is it supports the dome shape in the lower bout better. and I can brace a bit lighter.MichaelP39003.3831944444

Interesting....

Michael, how do you do the xbrace and the lower lateral brace? Are they all notched to fit all together or just the X and the lateral brace is butted up to the X brace with a cap over everything?

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I would also like to Hear Paul Woolson's input on this subject. He has a unique design as well....

Oh Paul........

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"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Craig,
    I use thin (1/4" or a little thinner) tall braces on my backs for all four braces. I dont see the need for the two thick braces on the lower bout of a back like Martin uses. I like to have a thin (.090") flexible back that is lightweight. I dont have a reason for this philosophy, it just seems right to me. I am interested to hear other peoples take on this.

Homeboy

For my classicals, I prefer thin backs -- in the 0.080" to 0.085" range -- to thicker ones, and I use ladder bracing that's not too tall.

I too have played guitars (classicals) with thick backs and, as a player, I don't care for them. They might have great projection, but to me they feel cold and lifeless. I like a guitar that resonates, and vibrates against me as I play it, and a thinner back provides this. To me, this type of guitar feels more "alive" and it's just more fun to play.

Best,

Michael

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My current thinking, which could change at any time, is that the 'main back' resonant mode is the only one that actually can add much to the power output of the guitar. It works best if you can get it to be about a semitone higher than the 'main top' mode, once things are all together and settled in. Other than at that particular frequency, I think you want the back to move as little as possible. There are all sorts of ways to get this result, and each of them will have some effect on the tone other than the bass and mid-range enhancement that the 'main mode' tuning achieves.

I think that, outside of that limited contribution, the back has more to do with 'color' than actual power output. It's not negligable; far from it, but there are plenty of good reasons for paying more attention to the top.

and I can brace a bit lighter.

Sorry to point this out, but if you measure all the braces on the back, you'll find you have a lot more bracing, and much less open back area, than on a conventionally braced back.

Not saying one is better than the other, but I am merely pointing out that being lighter is not a benefit of the X brace system for backs, as it is indeed not so.

[QUOTE=Mario] and I can brace a bit lighter.

Sorry to point this out, but if you measure all the braces on the back, you'll find you have a lot more bracing, and much less open back area, than on a conventionally braced back.

Not saying one is better than the other, but I am merely pointing out that being lighter is not a benefit of the X brace system for backs, as it is indeed not so.[/QUOTE]

Yup ( ) the shortest (and probably the lightest) brace you can put on the back is one that is straight across from side to side.

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I notch the x portionx butt the laderial struts and cap all with a disc shaped cap thentrim to the braces and shape with a finger plane.

I did not mead or say that I had less bracing area. I ment I can brace with thinner shorter profiles. My weight is actually up a little. but not as much as you may think.

Thanks for the input guys.

Ron, I too , have used the two wide lower braces, but wonder if I'm missing out on some available bass responce because of it. Benedetto says he acheives a larger bass with the lighter back ,but if it's too light ,his treble suffers.

Thanks for your plan Michael. A very cool looking brace set up. What dimensions are those (star) braces ,and what is your back thickness. I can clearly see why this bracing type would have the guitar project more , as it would lesson the back's movement . Have you noticed any less bass reponse with this firmer back ?

Like Michael McBroom, I find guitars with a heavily braced back ( read inert) ,to be cold and a little lifeless to the player. Just can't feel that thump on your chest when your've hit low "E" ! , but realise there is more projection out front.

It would seem that a looser back would slow down the air movement as it is cushioned, and thereby give more of a bass response but to the detriment of volume , and perhaps treble. I guess there is a balance one has to arrive at by way of volume/projection verses tonal response ??

Thanks also to Mr. Carruth . How do you arrive at having your back a semitone higher than the top 'main mode" I.m guessing you start off with an initial back thickness and thin down the braces until you reach your desired resonance . What thickness do you start off with?

I have discovered that a ladder braced back built to Cumpiano's dimensions ( 2.8m.m.thick,,,.11 ") delivers a main back resonance at least a semitone lower than the main top resonance . On checking a Martin Auditorium , the back resonance was higher,,,,, a lot higher. I found it lacking in the bass dept.

Your input and views/ suggestions are very much appreciated.

Regards, Craig Lawrence

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CRAIG LAWRENCE of AUSTRALIA
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The contribution of the back is quite different with classical guitars than steel string.

A classical player will not be holding the back of the guitar against the body dampening the back as it is not how a classica; guitar is held or played whereas a steel string player is more apt to be standing playing with a strap with the back of the guitar resting against the abdomen.

For a classical guitar, tonal color is usually more the contribution of the back than projection, at least for Spanish school construction. Spanish school construction uses a back that is typically 2-2.5mm in thickness (~.080-.100"), ladder braced with taller thin cross braces.

In Smallman (Australian school) instruments the back is typically thicker and carved or at least more heavily domed which would increase projection as a reflecting surface as opposed to adding color as a resonating surface.

Craig:
I 'free' tune my backs to get the pitch right, and generally don't have to do much adjusting after they're together. As usual, there's a certain amount of experience embodied in this: you have to work with a particular bracing system for a while to know how it's going to come out. OTOH, because it's a system based on measurements of pitchs and patterns it's fairly easy to teach people to use it, which is not generally true with 'tap tone' systems.

The semitone difference was something that Fred Dickens worked out many years ago. He noted that many classical guitars he liked had that relationship, and was able to model it in electrical circuit software on the computer at Bell Labs where he worked. Other people have found the same thing in other ways. I'll note that less than a semitone spacing can give 'wolf' notes, and that it also works pretty well to have the back a semitone lower than the top. Note, though, that the top pitch often drops as much as a half semitone in the first moth of playing in, so having the back lower means the gap is closing, inviting that wolf.