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Hi folks!

I'm working on numbers 6 and 7; will be working on the braces soon, and thought I'd voice a couple of things I'm wondering about.

First - I have 6 pieces of brace wood from Stewmac, and there's a lot of variation in grain tightness: 3 fairly tight and 3 fairly wide. I'm assuming there's probably not a lot of reason to choose one over the other for the tops, but in the spirit of obsessing over every detail, thought it may be worth exploring.

Second - back braces. Specifically, the lower two: some build with low and wide braces, some with tall and thin (like most of the other braces). Thoughts on why?

Thanks! - Bill

I wouldn't pay much attention to the grain spacing, however I would dimension them to be the same size and see which ones are the stiffest. I would use those ones for the top x.

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Pat

A guitar is basically a transducer (not an amplifier) - which is a mechanism for changing one form of energy into another. In the case of a guitar it's the mechanical force of the plucked string being turned into sound. Once you've release the string you cannot get more energy into it so all you can do is start to convert the energy you've got in the most effecient way. As far as transducers go, guitars are pretty bad at it. But then that's what give the guitar all it's flavors of tone.

I've been liking the low/wide braces for some time now. Most of the guitars I build I go for active/live/responsive backs, a back that helps in the production of tone, so to speak. If I am building a guitar that requires a reflective back, one that helps drive the top rather then work with it (so ta speak) then I use tall braces to stiffen up the back.

All guitars have 'reflective' backs but a so called live back is more flexible and as such robs the top of it's energy and so in exchange for volume you get more tonal color.

Don't worry about the grain count in braces. It's been shown that it doesn't correlate to stiffness, and even if it did you could simply shape the brace to the desired stiffness. If you had a procedure for measuring stiffness then you could find out. Most of us (it seems) just cut to near specification then flex them in our hands and use intuition to narrow them down to where we want them.

At number 6 and 7 you are just beginning to start making sense of all this stuff. Books have been written on it, it's that complicated.

If you want to sort out the bracing by stiffness, dimension them to the same size, and just drop them on your bench.
The ones with the highest pitched "ping" are the stiffest.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

It seems to me that Mr. Carruth's work and the work of others has suggested that spruce density maps reasonably well to stiffness, so if you do not wish to do deflection testing on dimensioned bracing as we do, that might be a quick way to match stiffness for the x braces and tone bars.

I use low wide braces across the widest parts of the back. I think this allows the back to "flatten out" when the humidity drops rather than crack. Many of the old guitars used low wide braces in the lower bout. Other than having one less type of brace to make, I don't know why the switch to "tall,thin".

Probably it, maybe just another attempt at efficiency in the production environment.

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Steve Smith
"Music is what feelings sound like"

I've use a bunch of different back brace schemes over the years. These days I've settled into using four cross braces on the back, with the lower two low and wide.

The supposed dichotomy between 'reflector' and 'active' backs is not all that clear cut. The only guitars I've seen that had backs that didn't move at acoustic frequencies are Ovations. Of course, building the back heavy and stiff does cut down it's contribution to the sound, and can help to 'keep the sound in the top', but it's a matter of degree rather than some absolute.

As far as I can tell, back movement only helps increase the output of the guitar in the lowest 'bass reflex' range. If it has a 'tap tone' that is somewhere near the 'main top' tap tone in pitch it can couple usefully with the top. This increases the output around the 'main air' pitch because the back can work with the top to pump air through the hole.

Above that range (say, upwards of 400 Hz or so?) motion of the back tends to 'steal' energy from the top, and thus reduces the power of the guitar. This is no altogether a bad thing, as the resulting 'dips' in the spectrum seem to contribute 'tone color' to the sound. The trick seem to be to limit the amount of power they consume. Probably the way to do that is to keep the band widths narrow, and the amplitude down. Mass in the back seems to help limit the loss; the usual dissipative mechanisms only cut in when it's moving. Low damping serves to keep the band widths of the resonances narrow. That supplies some rational for te use of dense, low damping woods, such as rosewood, for backs.

I use a 'tech' method of tap tuning, based on Chladni patterns, to 'tune' the top and back before assembling the box. I'm not looking for specific pitches per se, but rather find that the shapes of the patterns, and the relative frequencies can go some way to predicting how the guitar will work. The trick is to ge tthe top and back to work together.

When I switched back to using ladder bracing on backs I started out with tall and narrow braces throughout. As I tried to get the stiffness and mass of the backs to a point where they would work with the tops I found I had to take a lot of material off the lower two back braces to get the flexibility I wanted. The lower braces ended up looking pretty puny. So I started making them wider. At this point I typically end up with the upper two braces in the range of 5/16" wide by 1/2"-5/8"tall, while the lower two reverse that: 5/8" wide by 5/16"-3/8" tall in the center. The added mass doesn't seem to hurt, and the flexibility gives a strong tap tone on the assembled guitar in the range I'm after. Sometimes there's a reason behind tradition.....

Sometimes everything goes against reason. Resonant back with 10' radius and BRW laminated bracing.

Hans, I can’t tell if it’s the photo or if the lower two braces are shorter than the upper two. Is that the case? I hadn’t considered what clay mentions about impact to humidity swings, but it sounds reasonable. Is that what you were after with making them shorter or was it tonal?

Brad


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Brad, if you mean less tall, yes, they are shorter. I used to make them all the same height, then when tap tuning, take them down till the back had a pleasing ring. You may also notice the scalloping gets longer as the braces proceed to the rear.
As far as humidity swings, I never believed in making instruments to withstand large swings just to satisfy the lazy/un-knowing owner. A 10' radius does keep the flattening down and so does the bracing lamination.
Think I read somewhere that Larsons abandoned the wide/flat bracing on the back, under the bridge as un-necessary in later instruments.

Thank you everyone for your insightful responses, I appreciate it -
- Bill