Acoustic Guitar Construction

Hello all,

Finding this forum has been quite the jackpot! I built my first classical 25 years ago with a flat (non-domed) top. I'm about to start on my second guitar and hopefully continue to build more regularly (both classical and steel string).

Not having several dozen guitars under my belt, I'm relying on the wisdom of the crowds to help me guesstimate the parameters that go into a clear, well-balanced sound. Top thickness, bracing patterns, materials, plantilla... all of those will obviously affect tone in some way. But what I'm really wondering about is doming vs flat construction. It seems to take a lot of extra work to radius the braces and glue them in a radius dish or other contoured mold. Then there's the question of how a domed top needs to be aligned to the neck angle - Bogdanovich's plans dome the top but also tilt the lower bout back a bit so that the bridge ends up at the proper height relative to the plane of the neck. Others seem to alter the neck angle back to counter the bulge of the dome. I read in another thread on this forum that Bob Taylor uses essentially flat tops in his guitars.

Has anybody directly compared flat vs domed tops, particularly on classical designs? Do we know for a fact that flat tops are at greater risk for bulging behind the bridge over time? Is there a notable sound difference?

I'm feeling a bit overwhelmed with all of the parameter choices to be made. It's not like a computer simulation where I can tweak a design parameter and get immediate feedback - once I make a choice it'll be months before I hear the result.

Thanks in advance - good to meet y'all!

-Larry

Hi Larry,

Welcome -- well there is a lot of baloney published covering the need/use of domed/radiused sound boards. And there are millions of great modern guitars constructed with and without a dome. Contrary to popular belief vintage guitars with domes were not constructed that way -- the doming occurred over time do to the pull/tension of the strings -- a very natural consequence.

However, there are two very good reasons for putting a very slight dome in the main braces (X and tone bars for steel strings- lateral for classicals) first it helps prevent the top from looking concave -- second and in my view most important, a domed top has slightly more surface area relative to the perimeter than does a perfectly flat top. This feature (you have to think about it) is helpful in preventing cracks in the sound board do to humidity fluctuations. Not because its stronger but because the added surface area allows for more shrinkage, the sound board will sink a little rather than split along the grain. The dome top is not any stronger as a matter of fact in relation to the pull of the strings a dome is a reverse arch -- kind of like jerking out the king stone. Just look at a flat bed trailer or bridge. The weight is carried by the hump (pushing down) not the other way around.

I think that those who have built classicals using a dished solera will testify that once the plate with bracing is released from the clamps if just flattens out anyway or worse curls. And why would it not? There is nothing structural relative to placement or logic that would tend to force the thin fan braces into holding a curved position.

And yes the element of doming does come into play in regard to action, set-up and the bridge height target line. The thing most often over looked in this detail is the fact the both the bridge thickness and saddle height can be adjusted so its not quite so critical (stressful for some) as it tends to be portrayed.

Try reading this
http://www.kennethmichaelguitars.com/neckangle.html
to get a feel for the neck angle and elevation logic.

Hope this helps a little.

Thanks for such a thoughtful reply, Ken. A local amateur (Alastair Fordyce) has pointed out the same thing - that when taking a guitar to a drier climate, the domed top gives a little room for flattening. He's actually seen a guitar top go concave with too little humidity (the outside of the top contracted while the transverse bracing held the inside of the top at a constant width.)

The article you posted confirmed what I thought, too, about how doming and tilting the top affect bridge height.

Lastly, another posting made elsewhere on this forum (http://www.grevenguitars.com/pdfs/VoicingtheGuitar.pdf) suggests that the tension from doming (we are, after all, gluing braces in while the top is in tension) increases rigidity and shifts the tonal spectrum toward the treble, all else being equal. And it's the "all else being equal" that I believe the rest of Greven's article questions - there are other parameters that can be adjusted (the stiffness of the material itself, thickness, and bracing) as well.

I'll start a thread on the "builds" board as soon as I've made a bit of progress. Thank again!

I will be following this thread as I'm beginning a build bassed on the Bogdanovich book

Here is an interesting class that Mike Haney did for I think for the North Atlantic Luthier Association on using a dome on the top. I am only posting as it gives what I think is a representative description of why and how people use a dome. I watched this a couple of yarns ago. It took me a bit after I saw this post to track down that distant memory.

John

Ken am I reading that perhaps it's not necessary to Dome the top? As this will happen over time due to string tension. Another question. I know that KMG carries the dished out sanding bowls, but is it possible to use my MegaMold to do this same thing?

Oops I was only thinking of the sides not the bracing