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Here is the latest Luthier Tips du Jour video. In this video I invite Trevor Gore to answer a viewer's question about the difference between using 3 or 4 back braces.
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You can absolutely get a live back with four braces, been doing it since the books came out.

I was shivering just looking at that picture of Antarctica.

But on classical guitars? That was Stephanie's question.

Can be done, but you need pretty skinny braces. Fleta was just about the only "big name" classical luthier to use 4 back braces, specifically, it seems, to make non-live backs on his guitars. Fleta chose to use a strong diagonal dipole, obtained with asymmetric top bracing, to extend the response in the T(1,1)3 frequency area.

So I think I got the right answer out, even though, by the look of the video, I hadn't slept for a week (jet lag!!)

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

With four back braces I make the lower two low and wide, as in Martins. You get the structural strength you need with more mass and not so much added stiffness.

Correct! (of course. It's Al C!)

Most people (at least on this site) are aware that brace stiffness runs with the cube of the brace depth (rectangular section). So make a brace twice as tall and it will be 8 times as stiff.

Fewer people, I suspect, are aware of the fact that double the span of a centre-loaded beam and you get eight times the deflection for the same load, so an inverse cube rule. Going on the width of a classical guitar (~360mm) compared with the width of a dreadnought guitar (~405mm), the D brace is only ~70% as stiff for the same brace cross section. But from the vibrational point of view (which is, after all, what we're interested in) it will also be carrying more mass, because of the extra length of the brace and the extra mass in the panel. All up, with the increased span and thickness of the panel, the mass increase will be of the order of ~30%. A quick calc. shows you need about 90% extra brace stiffness in a D to get get back to the same vibrational performance as the smaller classical, which (phew!) tells you why Ds have four back braces and classicals have three.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au