Official Luthiers Forum!

I'm sure this has been asked before but in all seriousness I'm just trying to get different viewpoints on the topic.

I know that typically we increase the radius of the back of a guitar to be more curved than the top. Case in point the typical 25' top paired with a 15' back and variations on this theme from various makers etc...

I'm curious about what the differences would be with a top that is heavily radiused to something like a 15' and if the back were 25'?

I realize it will still sound like a guitar I'm just curious what ya'll have noticed in the sound if this is done.

Top frequency would rise, and back frequency would drop, comparatively speaking, for the same wood, thicknesses and brace dimensions.
Haven't built like that, so I don't know how it would sound.
I did increase my "standard" top radius and decrease the back radius about 6 guitars ago and found I prefer the sound of the guitars since then, so I'm not likely to try it, but maybe someone here has.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I agree with Colin, but I would also add it makes getting the neck and body to match with the correct geometry harder. Around 30' the upper bout is closest to the correct angle. One can work around a tighter radius, it would just take a bit more work.

_________________
http://www.Harvestmoonguitars.com

From an Ervin Symogi article:
"Because of the dynamics of the guitar, tonewoods for faces need to be different than tonewoods for backs, if the instrument is to have the best and most even sound. The best guitar faces are made of high quality musical instrument grade softwoods such as spruce and cedar. The best guitar backs are made of high quality hardwoods such as rosewood, ebony, maple, walnut, koa, mahogany or any of a number of other suitable body woods. The consensus among luthiers is that face and backwoods need to be chosen from woods of differing densities because the resonant frequency of the back needs to be higher than the resonant frequency of the face, by at least a tone. The best wisdom on this matter is that if there is too great or too small a gap separating the fundamental resonant frequencies of the top and the back, then guitars have an uneven tone. That is, the sound becomes an uneven mixture of loud and quiet notes. Likewise, if the face and the back are most active at the same frequency or frequencies they’ll act in tandem to reinforce certain notes, but leave others weak. It does not matter what the sides are made out of, except that guitars in which the back and side woods don’t match are considered to look too strange and generally won’t be saleable: backs and sides need to match for aesthetic reasons."

As Colin pointed out, the top frequency would rise and the back frequency would drop, so depending on the woods you are using it may create too small of a "gap" between the fundamental resonant frequencies of the plates.
With the typical construction, the back being a denser wood and being more strongly arched, the frequency separation is maintained.

Not really relevant to your question but to my eyes it would look rather odd! On steel strings I use a 3 m radius for the back and have grown to rather like this fairly high arching.

Dave

Being in the US I do a 10' back and also like the look.

_________________
http://www.Harvestmoonguitars.com

I'm making an early Stauffer guitar now with a back radius about 8 foot, 2400mm. I guess the original has a flat belly. The maker I got the plans from said that he put a slight radius on it. With bracing, I wonder how much the radius means. Stiffness can be adjusted.

_________________
Why be normal?

Dan it is worth saying that no question is silly! It is one of the purposes of this forum to challenge conventional wisdom.

It is my understanding that an arched back provides stiffness to the box structure which will help to maintain it’s integrity over the long term.

Again as I understand it arching of both plates means that both plates can better handle changes in humidity. They can increase or decrease the arching with moisture content changes without serious damage. Which particular radius does the job is one of those real open questions!

So building in an arch seems worth doing on that basis. Effects on sound, i8well who knows - apart from what people above have said.

Dave

You might be aware of the Vega and Howe-Orme cylinder top designs of about 100 years ago. They sound great, and loud. But the geometry woould make the build a bit more challenging than a nearly flat top.