X-braced Back?

[Juntunen Guitars]

I was talking to another luthier a few months ago and he was showing me an acoustic he was working on where he put an X-brace on the back as well as the top. He was only doing it for support since he doesn't radius his braces other than running it over a jointer while holding up one end of the brace so he couldn't share much on how it affected tone so I'm wondering if anyone here has done this or heard of it being done before and can shed some light on it. Another builder I talked to said he didn't believe the back bracing really affects the tone at all but wasn't sure since it's not really a topic many people work on compared to what can be done with top braces. I know I had heard of one maker using an X-brace on the back but I can't remember who it was, I believe this was back in the 40's or something around there but I could be wrong.

[Mattia]

A bunch has been written about this, and lots and lots of builders use various variations on X-bracing for backs. Search the MIMF.com archives and Luthiersforum.com for starters. Al Carruth has written about it, Rick Turner, Ervin Somogyi, to name but a few. I'm guessing Trevor Gore addresses it in his new book as well.

There are two basic schools of thought with regard to tuning backs: the 'reflective' or stiff back, which acts as a reflector for sound, or the 'active' back which is tuned to within a tone or semitone of the top (main tap), and should create a kind of 'bass reflex' type effect - standard sized guitars only really support notes down to low A or low G based on Helmholtz resonance, so the 'reflex' in an active back can provide extra support to the lower registers.

I've used single and double X brace patterns on my backs, mostly because they help hold the back in shape on my larger jumbo sizes, and my feeling is they make for relatively easy to tune backs. Although given my minimal experience I'd take anything I say on that subject with a healthy tablespoon of salt

[Juntunen Guitars]

I've heard of Somogyi before. I'll look at those websites. I tried doing some google searches but kept coming up with next to nothing or people saying don't do it but never a reason why. Thanks

[Prostheta]

This is something I will be researching as I want to build a couple of acoustics myself. Both a guitar and a bass. I have found precious little useful information about bracing at an achievable amateur level for basses, except for the usual swathes of contradictory and anecdotal info of course. I presume that given the limited box size for an ABG, looking into the "active back" approach would be quite relevant in my case?

[Mattia]

Pretty much yes (re: ABG's). I recommend sniffing through (browsing, not searching) the MIMF archives, although I'm not sure how accessible they are right at the moment, as well as posting a few questions either there and/or at the OLF (luthiersforum.com). I believe Mark Swanson has written a few thing on the subject, and information on acoustic baritone guitars (from, among others, Dave Berkowitz and again, Al Carruth. most of it in various online forums) may be of use/applicable to design considerations for acoustic bass guitars. RM Motolla also has a few designs up and a bit of discussion on various design choices on his website:

http://liutaiomottola.com/instruments/Tinozza.htm

The Big Red Books also have a few interesting tidbits on acoustics and bracing/bracing design. I've never seen anyone actively discourage X braces on backs, just a few folks who have tried it expressing either a strong preference for the X, or a slight preference for 'traditional' bracing without hating it, because they find it easier to tune/control the backs as part of their overall system/approach. Most folks have just never tried it.

I will note that I have little interest in and pay almost no attention to forums that focus on vintage instrument reconstruction as an end in itself; people building Martin copies will not be terribly interested in X braced backs, and may be more likely to dismiss them as a 'bad idea' if they believe the 'Holy Grail' exists, and is shaped like a pre-war Martin Guitar. Not all acoustic guitar builders are all that progressive in terms of experimentation and willingness to try new things. Counts double for most classical builders. Same sort of thing that you see in the Electric world with slavish reproductions of Gibson Les Pauls (including all the stupid design choices) or early Fenders. Not my thing.

Edited April 17, 2012 by Mattia

[verhoevenc]

X-braced back Terz I built:

My purpose here was two fold (based on some video lectures by Somogyi). I wanted to increase the cross dipole of the top so that it was louder in close proximity (although at the expense of the sound 'traveling') as a terz is a very intimate setting guitar. I also did a strong X-braced back to work as a 'reflector' of sound instead of my usual ladder braced back that acts more 'coupled' with the top. Both were considerations, IMO, to have the guitar be loud nearby. I believe it worked as the thing's a cannon for it's size.

Chris

[Prostheta]

I found that interesting, if a little esoteric for my bare-basics level of understanding on acoustic builds. I completely agree about the anachronistic tendencies of some builder's forums, however the fact they are unwittingly committing these vintage specs on pioneering instruments to record is a good thing. They certainly do not supplant progress, which is an innately positive movement. As far as LPs go for example, I think Matsumoku developed a great instrument in the PE. Note the use of a laminated Birch neck....

The incorporation of Nomex and carbon fibre in lightening and stiffening tops is fascinating, if a little specialised for my current abilities and resources but it shows that there is plenty of room to move onwards. I'm wondering if making bracings from a three-piece laminate of wood-CF-wood might yield benefits in terms of both of these beneficial characteristics. From the outset, I would rather just make a simple junker acoustic to get the foundation principles down. The moulds and jigs made in the process make the second and subsequent instruments an order of magnitude easier.

X-bracing the back of the instruments I have in mind sounds like a potential negative if tuning the front and back is complicated because of it.

[YetzerHarah]

Yeah, I was more than a little annoyed that Bob Benedetto's book all but ignored the subject, but I guess you can't win them all.

[Mattia]

To be fair to Bob, in archtop construction, shaping the arch is much, much, much, much more important to controlling tone than the choice of bracing pattern - archtop backs aren't braced, in general, and tops either go with a simple X or simple parallel bar. Shape and height of the carve, recurve and thicknesses however are an entirely different issue. Can't really compare acoustic arch tops to flattop guitars in terms of construction and sound generation. Application and distribution of forces are very different.

[YetzerHarah]

Don't get me wrong, it is a great & very in depth book, but I still feel like that was a glaring omission (the only one I noticed in the book, I might add).

[Mattia]

Not sure it is - arch tops aren't tuned by fiddling with the bracing so much as by adjusting the carve of the top. And Benedetto never aimed to provide a be-all, end-all solution to archtop building, just a solid how-to guide explaining how to build an archtop in the classic tradition, complete with a plan.

[Juntunen Guitars]

Don't get me wrong, it is a great & very in depth book, but I still feel like that was a glaring omission (the only one I noticed in the book, I might add).

If you want to look into tuning braces read these articles