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Ok, so the main purpose of a curved top is structural stability. As far as I know all domed top guitars have convex tops.

I've recently discovered that arabic ouds have concave tops for this same reason.
I wonder what would the acoustic implications be if one decided to dome the guitar top backwards - ie concave.

I know that structurally it would probably require a floating fingerboard over the soundbox for playable action.

Just wondering if anyone ever tried something like that and if it's worth exploring.

Shoot, I bet every builder has made a concave top.
Not intentionally though.

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I suspect a concave top would not provide the same structural enhancement as a convex top due to the direction of the forces. The strings are trying to fold the guitar in half, a convex arch will oppose this force while a concave arch does not. It still can be made to work, the real reason for the arch is not structural, it's for better tolerance of fluctuations in humidity. There is some structural benefit from the arch, so a concave top would probably need to be a little thicker or braced a little heavier, so it would not be quite as loud.

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Have you heard an oud?

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I know this doesn't address your question but it got me thinking about my experience with an oud.

I once had to repair an oud that had been purchased in the middle east then brought to WI. Cracks everywhere. The hide glue stunk of so badly of rotting animal it almost made me gag. That has been seared in my memory. I'll never forget.

I thought ours were ladder braced and gut strung? No wonder they don't sound like martins.
It would be an easy enough experiment anyway...

Plenty of Guitars that have concave Tops. Pretty much every flat, flat Top that I've come across displays a concave soundboard - probably from humidity issues but they never seem to recover. Pretty much all original Romantic guitars have concave soundboards and many of those are still doing fine. The degree of concavity is slight though.

Probably not concave like you mean, but Grit Laskin did this one. I read once he was trying to replicate the string to top angle of a harp.
http://pinterest.com/pin/27373510205519660/

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Mike Lindstrom

Lutes and ouds have longitudinally scooped soundboards and are straight from side to side. The ladder braces are straight but the perimeter of the bowl which the soundboard is glued to is relieved to form a scoop about 4-5 mm deep at the midpoint of the soundboard. The bridge is much farther back on the soundboard than on a guitar, so the scoop achieves somewhat of the same thing as a domed soundboard in that the back of the bridge is higher than the front, which is what the string tension wants to do to the soundboard. Another reason the top is scooped is to give more clearance for playing action. There is no saddle on these instruments and the height of the strings off the soundboard is about 4-5mm, IIRC. With a flat top a lute or oud would be fairly awkward to play with such little clearance between the strings and the top at the area of plucking, or picking in the case of the oud.

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Roger Knox. I have to take issue with your statement about the major function of the arched top. My old teacher and boss was one of the first guitarmakers put a substantial arch in the X-brace (a 24' radius) of his steel string guitars. He'd already been doing it for years before I started with him in 1979. His expressed intent for doing so was to counteract the top distortion caused by string tension on flat tops--i.e. the belly behind the bridge, the collapsing in front of it.

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michael propsom
www.propsomguitars.com

As I said in my previous post, the arched top provides some structural benefit, so some builders will choose to use it for that reason. It is not necessary for structural reasons, guitars with flat tops have been built for years, and many are still around.
However, many of those guitars built with a flat top go concave with high humidity. Arching the top solves the problem by changing the effects of humidity variation to variation in the amount of curvature instead of going from flat to concave. Flat also leaves no room for drying, so cracking could also be a problem.
One of the major reasons for arched tops is to avoid concave tops, because they look bad.
That's my opinion, feel free to agree or form your own, this is sort of a retorical question.

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I know I have !!!

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