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Something I've been thinking about and perhaps, you've already come across this, as I doubt anything I think will be original in this field. I was thinking that say with a top of back, that say it the top was made to say a 30 foot radius and the rims to a 25 foot radius would the act of clamping the top down whilst gluing place the top under stress so that it contained some stored energy and would act sort of like a spring adding to the sound? I sent this as a pm to a member, but then thought I'd post it here as others may have thought the same or know the answer.

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Formerly JJH

I learn more from my mistakes than my successes

I would think that it would raise the pitch of the main tap tone the same way bending a saw raises the pitch. I asked a similar question about stringing up changing the pitch and Alan Carruth chimed in that in his experience the increase in tension behind the bridge is offset by the slackening in front of the bridge and it turns out a wash. I read somewhere else from some classical maker that he can indeed measure an increase in main top tone pitch after stringing up.

YMMV

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There are lot's of builders who use radiused tops and a flat rim. Huss & Dalton does this on their traditional series guitars (25 foot radius/flat rim). John Arnold also does this and said "The advantage with the flat rim is that it allows for free vibration of the top." on the UMGF. This makes sense to me because the guitar acts like an air pump and the top has the greatest potential for movement around the perimeter(because the rest of the top will have to move up and down also). Speaker cones are not just a cone attched to a ridged rim, all the ones I've ever seen have a ring that seperates the cone from the rim to allow the cone to move in and out. It's the same reason people thin around the perimeter. I'm still putting the finish on my first but I used a 30' radiused top and a totally flat rim.

Also I'm not sure about how much stress there is. I heard someone(Arnold I believe) say the top relaxes after gluing fairly quick and it isn't an issue, that's totally based on my fuzzy memory though. I noticed when notching the rim and gluing that the top went with zero problems.

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Jeremy Douglas

Sorry, but I've got to throw some practical considerations in here. The difference between 25' radius and 30' radius arcs is small. And when you consider this is only present on something the width of the lining plus side (maybe 1/2" total) you are probably only talking about a few thousandths of an inch, if that.

Benedetto talks about this in his Archtop book...he said that some makers force the top over the braces...building in some additional tension. I'd expect more tension in the top to equate to more treble response in the guitar...just as tightening a string would do.

With regard to Benedetto, I believe he mentions that archtops built like this will sound better for a few years and then the wood will "take a set" and the guitar will lose some of the punch that it initially had.

I would think that something similar may happen on an acoustic.

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I noted that too and it must be because of the x-brace. The back on the other hand, which is typically ladder braced, needed force to get it into position.

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What you are discussing is also known as pre stressing . usually the "stress" will dissipate as the wood cells accept the imposed shape. It is an accepted practice .
john

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John Hall
blues creek guitars
Authorized CF Martin Repair
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You Don't know what you don't know until you know it

The act of gluing a flat top to a set of arched braces is a form of prestressing. Adding an additional shape induced stress a half inch wide is nothing. In faxt the amount of inisial distortion is only between .5 to 1.5 deggrees of deflection and is sprread evenly across the thinner cross sections of the bracing so it really is not consentrated in the glue joint. Just like with the braces the top assembly will in short time equalize. the argument of imposed stress once was used to discourage the use of assembly molds . Now it is common assumption that once the box is closed the stress is restrained and the assembly equalized.

I believe the OP asked about the back too. No one glues the back to a flat rim do they?

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Jeremy Douglas

I cannot recall where I recently read that one of the famous classical builders years ago would stress the top. I think it was something like: just before attaching the back, he jacked the guitar the long way in some kind of frame.