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Forgive me if the answer to this question is overly obvious.
If one chooses to build an active back vs a reflective then how much bracing on the back does one really need?
I understand the stresses on the sound board and thus why which braces are where, but how much is the back actually stressed?

Has anyone tried bridge type bracing, where the horizontal braces are basically free in the middle and glued at the ends of the braces?

Are the back braces more for damage prevention?

Again sorry for the tradition challenging inquiry.
B


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Interesting question.
For a "normal" thickness of 15' radius back (eg. 2.5mm EIR) I've found I need the ladder bracing, 4 braces, 8mm wide and around 16-18mm high to give a frequency around 4 semitones higher than the main top frequency (OM guitar) which would be regarded as an active back.
The brace form and help maintain the radius of a back, with all that implies (stiffness, resistance to sting pull and resistance to movement with humidity changes.)
Not tried the bracing mentioned.

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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.

I've been using four ladder braces on backs as well, for several years now. I have found the Martin system works well. The two upper braces are more or less 5/16" wide by 5/8" tall, and the two lower braces are 1/2" to 5/8" wide and end up below 3/8" in the center. With four narrow braces I was trimming more wood off the lower ones than I was comfortable doing to get the back to work the way I wanted. This is an 'active' back, with the 'main back' tap tone close enough in pitch to the 'main top' to couple effectively.

Search for Kasha style bracing.

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Jim Watts
http://jameswattsguitars.com

Trevor Gore shows how to brace a live back in his book.

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It will probably be alright.

A live back is not dependent or strictly connected to the bracing pattern.

I've been building with a live back and tuning the back after the box is closed -- thanks, Audacity!

I use a bracing pattern derived from Gore/Gilet: two "normal" transverse braces (one in the upper bout, one in the waist) and a third transverse brace in the lower bout that's tall on the "east" and "west" sides of center, but about ⅔ height for around 4" in the middle. Then four short braces (with ends that taper to zero and are not tucked) in a star pattern pointing to the center of the lower bout brace.

To tune the back, I can easily shave the already-lowered center of the lower bout brace to get the back resonance 4 semi-tones (~25% in frequency) above the soundboard resonance.

The back bracing needs to be stiff enough so that its resonant frequency is not too close or lower than the tops frequency. So even though the back is not under string tension it still needs to be stiffer than the top to get its resonant frequency up. A reflective back is stiff enough and the resonant frequency of the back is high enough that the top and back are not coupled. The back has no to very little effect on the top. A guitar with a reflective back is usually a bit louder as the top is a better sound producer for the audience than the back and in the reflective case the back is not stealing energy.

Like Hans, I follow Trevor Gore's design and usually build a guitar with an active back four semi tones from the top resonance. That is close enough to couple but far enough away to not have too much of an effect on the tops resonance. An active back guitar while not quite as loud tends to have a more complex voice from the combination of the back and top.

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http://www.Harvestmoonguitars.com

John, when playing a guitar with an active back, do you need to be careful not to let the back touch you to avoid damping its vibration?

I usually use 3 or 4 ladder braces, 1/4" to 1/2" wide depending on how troublesome the back wood seems (higher expansion wood needs more glue area for the braces to hold on in high humidity stress).

I notch them into the linings at 1/8" to 3/16" tall. It would be an interesting research project to try various notch heights and chart the effect on resonant frequency and see how the mode shapes change (chladni testing). I suspect a given change in height at the ends has more effect than the same change in the middle, and that taller ends push the node ring inward and reduce the amplitude of the vibration... to some extent, making the back vibrate as if it were smaller.

On a related note, I'm not sure if there is an ideal amount of back area to have active. Should you try and get the whole thing vibrating, or make the upper and waist braces really stiff so only the round-ish area of the lower bout vibrates? I make the upper brace stiff for neck stability, and everything below that active. But most live back patterns (Martin, Gore, and lower bout X patterns) seem to go with the latter approach.

And is it better to have more mass vibrating at low amplitude, or less mass at high amplitude? My style should result in the former, whereas lower-bout-only with the brace ends carved low should be the latter. Density of the back wood you're using may determine which is better.

I've always kind of wanted to have a signature back bracing pattern rather than the boring old ladder braces, but so far haven't come up with one. Wood is much stiffer along the grain than across. Braces in-line with the back grain don't do much to prevent the wood inbetween from feeling squishy and easy to split. Squishiness is primarily determined by distance along the grain to the nearest brace. Ladder bracing gives the minimum distance between braces at all points for a given amount of brace wood, while also being the easiest to construct, so it really is well suited to the task.

EDIT: Just noticed I never really answered the question... I don't think the back really needs any bracing, structurally speaking, though I'd be a bit uncomfortable without even an upper bout brace. I have a small guitar that I built years ago with just two back braces, and the lower one peeled up at the ends during its first winter. I never got around to fixing it, so that brace isn't adding much strength. I've thought about reaching in there and plucking it out entirely, but for some reason it never buzzes unless I actually tap on the back, and the guitar sounds great, so why change it?

I do not think it really requires extra care. The back will happily vibrate (you can feel it) unless pulled really close and tight to the body.

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http://www.Harvestmoonguitars.com

I have to disagree a little. A couple of years ago i made a guitar that sounded OK. Not great, just OK. As i was playing it late one night it suddenly sounded MUCH richer and louder. I got very excited. I sat up and paid more attention. It was gone. Disappointed, i slouched back into my chhair. It was back!

Yep, you guessed it. It was my generously sized belly dampening the back. There was a noticeable difference when the back was free to when it was touching my gut.

So, at least on one guitar, my experience is that an active back can be effected by playing style.

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I have seen guitars where the 'main back' pitch was lower then the top pitch, and they can sound OK. Higher is better, though.

As I may have mentioned, I've used a number of different back brace patterns over the year, and all of them could yield an 'active' back. The tend to sound a bit different, but not hugely so. For me, what matters the most is how predictable the thing is. I do 'free' plate tuning using the Chladni pattern method, so I've been trying to find the back brace scheme that allows me to tune the back off the box and get it 'right' most reliably. Ladder bracing seems to work about the best in that respect.

Changing the back pitch can alter the top pitch, since the two are part of a system of coupled oscillators. The changes tend to be 'small', though. OTOH, changing the back pitch can have a fairly large effect on the 'main air' frequency, which will tend to drop lower as the top and back pitches match more closely.

Getting the top and back too close in pitch can give rise to some amazing 'wolf' notes, so it is to be avoided. That's one reason to have the back start out higher in pitch than the top. I have measured some pitch drop in the top with 'playing in'. If the top starts out a little higher in pitch than the back it can shift into 'wolf' territory over time. Having the top just below the back pitch might risk a wolf initially, but tends to get better over time rather than worse.