Back Braces

[Jim_H]

Hi all,

Thought I'd start a thread on this topic. There are a lot of discussions on top bracing and thickness, but the back braces don't get much love (and perhaps rightfully so).

I'm curious what people think about the back braces and their role in the structural integrity of the guitar, and in the overall sound quality. Also, the choice of materials and glues being used.

All of the guitars I have built so far have had spruce back braces and fairly generic profiles...

Are back braces primarily structural?

Is stiffness and weight critical like it is with the top braces?

Is 'tall' better than 'wide' (or vice versa) ?

What about the back itself? As thin as possible without compromise? or is there a certain stiffness to weight to shoot for?

I have lots of bracing stock stashed. Most of it is either Red Spruce or Sitka that was acquired with 'bracing soundboards' as the primary thought. I also have a few pieces of nicely quartered mahogany that would make some nice braces. I'm thinking about using it, and I'm wondering what the implications are in terms of size and shape relative to a (presumably) lighter spruce brace.

[Darryl Young]

The lower 2 back braces have a lot to do with determing the resonant frequency of the back. If you build with a live back, the frequency of the back relative to the top frequency is important. Gore says if you get these too close the guitar can sound muddy. Get them too far apart and they don't couple well. So in that sense, the stiffness of the two lower braces and the thickness of the back are important.

While not as strong an impact compared to the top, using woods other then softwoods for braces adds weight and makes the back slower to react so changes the attack. That's why woods with high strenght to weight ratios are typically chosen. Again, it won't be a drastic change to use mahogany or some other hardwood it's just no ideal.

I wish I could add more info on the purpose of the upper back braces. They help maintain the radius shape of the back and help maintain the shape of the rim.

[tippie53]

Mostly they are structural but they do have the secondary response of the back frequency . I prefer a Martin style lower wider brace . Tradition nothing more.

[David L]

Would it not make a difference if you are one who subscribes to the "responsive" back or if you subscribe to the "reflective" back?

David L

[kencierp]

I've come to use and supply with our kits the 5/16" x 11/16" style -- seems the beam/joist effect on average holds the contour better. I have had pre-made 3/8"x3/4" flatten out before and after glue up.

I've used genuine Mahogany for back bracing on classical's and a couple of "D"s

I really like the idea of a double back with the inside layer suspended and only attached to the sides. To get an idea of how well this works play your guitar holding it against your body. Now hold the guitar is such a way that nothing contacts the back -- the increase in volume is significant -- I've not done any analysis of tone one versus the other. To me this concept has promise. I believe Ryan and McKnight have experimented and perhaps produced instruments with this design feature.