A question about the profile of the soundboard

Henning · #1 10-23-2023, 08:13 AM

Hello, how important would you say that the profile (side view, longitudinal and lateral) of the soundboard is for producing the sound of a guitar, please?
I understand it's important for stability and structural strength
What if you make the top flat instead of convex?
Kind regards

Earl49 · #2 10-23-2023, 09:31 AM

Flat top guitars are not truly "flat". They usually have a radius but it is large -- 40 feet (12 meters) is a common number that I have seen. A curved surface is the strongest in nature, so the bracing and top thickness can be lightened up a bit if the top is domed slightly. Conversely, if you build it perfectly flat then everything needs to be slightly heavier. There are so many other factors that would affect tone. For example, the same guitar in humid conditions can swell and raise the entire bridge by a few mm. When dry it can sink several mm below the flat condition. Diagnosing a swollen or sunken top involves placing a straightedge across the lower bout at the bridge and seeing how much clearance occurs at the edges. Taylor has some tech sheets about diagnosing wet and dry guitars with good diagrams.

Most guitars sound better when dry, but that is probably due to increased internal damping characteristics of wetter wood and may have nothing to do with the curvature. You are asking a question that likely cannot be answered for sure. Perhaps some of our resident builders will contribute their opinions. I'm just a guitar nerd and certainly not a luthier.

Henning · #3 10-23-2023, 09:40 AM

Thank you Earl49, i think you enlightened the matter. Regards

redir · #4 10-23-2023, 10:24 AM

I have built several true flat tops before and the only thing I can say is they sound different than the ones I built with arch tops. Kind of hard to explain the tone. The string tension ends up pulling an arch into the top anyway. But you do have to build the geometry of the neck angle and the top into it when you build a truly flat top.

John Arnold · #5 10-23-2023, 10:48 AM

String tension imposes torque on the bridge. If the top was not stiffer above the bridge, this torque would result in an S shape, not a smooth radius. This stiffness above the bridge comes from the X bracing, combined with the fact that the guitar is narrower in that area.
With that in mind, IMHO the best shape for a top is such that it is flat below the bridge and has a positive radius above the bridge. This creates a shape such that the bridge is tilted back slightly when the guitar is unstrung. Under tension, the bridge rotates a couple of degrees (due to the torque), and the bridge will then essentially be level with the rim. IMHO, this is the best of both worlds....a looser feel associated with true flat tops, combined with the superior structure and stronger harmonics that are attributes of a radiused top.

Henning · #6 10-23-2023, 02:22 PM

Thank you, guys, just want to add that it's said (or rather I've read) that a guitar with an arched top sounds and projects better. Whatever that really might be.

Fathand · #7 10-23-2023, 02:33 PM

Quote:

Originally Posted by John Arnold

With that in mind, IMHO the best shape for a top is such that it is flat below the bridge and has a positive radius above the bridge. This creates a shape such that the bridge is tilted back slightly when the guitar is unstrung. Under tension, the bridge rotates a couple of degrees (due to the torque), and the bridge will then essentially be level with the rim. IMHO, this is the best of both worlds....a looser feel associated with true flat tops, combined with the superior structure and stronger harmonics that are attributes of a radiused top.

I think I'd like to try this on a current build that is in early stages. Is this accomplished by leaving the braces un-radiused south of the bridge plate?

Alan Carruth · #8 10-23-2023, 03:13 PM

John Arnold wrote:
"IMHO the best shape for a top is such that it is flat below the bridge and has a positive radius above the bridge."

Some Classical makers who build on a solera, such as Greg Byers, use that system. The edge of the rim is flat, and they dome the area in front of the bridge, gluing in the fan bracing on the solera. Greg makes a very nice guitar, and that may be part of the reason?

I don't think it's accidental that the most successful and widely copied top brace schemes, fan bracing and X bracing, both concentrate wood between the bridge and the sound hole. Structurally and acoustically it's key.

John Arnold · #9 10-23-2023, 05:49 PM

Quote:

Originally Posted by Fathand

I think I'd like to try this on a current build that is in early stages. Is this accomplished by leaving the braces un-radiused south of the bridge plate?

Yes. I don't use a radius dish. I use a 25 foot radius on the upper part of the X braces, which are flat below the bridge. With the flat section, the overall rise in the top is roughly the same as a 45 foot radius. I have shaped cauls to place under the top when gluing the X braces. The tone bars and finger braces are glued flat. The upper transverse brace is a 50 foot radius, which is also glued in using a matching caul.

Fathand · #10 10-24-2023, 05:48 AM

Quote:

Originally Posted by John Arnold

Yes. I don't use a radius dish. I use a 25 foot radius on the upper part of the X braces, which are flat below the bridge. With the flat section, the overall rise in the top is roughly the same as a 45 foot radius. I have shaped cauls to place under the top when gluing the X braces. The tone bars and finger braces are glued flat. The upper transverse brace is a 50 foot radius, which is also glued in using a matching caul.

Thank you. I don't use a dish either. I made radius sanding sticks that work well, maybe a bit shallower arch than yours. I'll be give it a try.

Henning · #11 10-24-2023, 08:19 AM

Here is why an arched top is superior according to David Russell Young: it is stronger, hence less likely to crack either from mishandling or a sudden drop in humidity. An arched top is more stable than a flat top, so the playing action can be set lower. Arching prevents the top from distorting the top into a severe S-shape. It also has a superior strength to weight ratio and produces a more sensitive, louder, more sustained and usually a better balanced tone.

Alan Carruth · #12 10-24-2023, 10:11 AM

I live in New England; 'guitar hell'. With all the weather changes, and low temperatures in the winter, we used to see lots of top cracks around February back when I did more repairs. A bit of a dome in the top gives it someplace to go before it cracks. It doesn't matter how good it sounds if the top is so cracked up you won't/can't play it.