Effect of a Wider Back Radius

[CMStewart]

I see that most guitars today have between a 12' and 15' back radius. I know that a smaller radius gives more strength to the back, which is good. But it seems that a bigger radius (like 18') would give a warmer sound, while still being strong enough. Maybe I'm wrong.

Is there a structural or sonic reason the radius on most guitar backs is so small? What sonic effect, if any, would a larger radius (18-20 feet) have on the guitar? I'm just looking to learn more about how these things work, and general precepts to think about. Also, let me know what kind of radius you all build with. I'm curious if I'm wrong about most guitars.

Colby

[Tim McKnight]

Colby, Those are good questions and you may get differences of opinions but your logic is on track. The flatter the back (and top) radii are [generally] equates to a bias in low frequency response. However, there are other factors that also contributes too such as plate thickness, bracing overall weight, mass and how much and more specifically where wood was removed during the voicing process.

Google "bowed saw music" and the way the saw is played directly correlates to the arching question you posed. The greater the arch in the saw blade the higher pitch notes can be played on the saw blade and the flatter the saw is arched will yield a lower pitch note. Hope that helps shed some light on your thread topic?

[Bruce Sexauer]

As it happens, I have been using an 18’ back radius for 25 years. Because the supply houses at that time only offered a 15’ dish, I learned to make my own.

[John Osthoff]

Originally Posted by Tim McKnight

Google "bowed saw music" and the way the saw is played directly correlates to the arching question you posed. The greater the arch in the saw blade the higher pitch notes can be played on the saw blade and the flatter the saw is arched will yield a lower pitch note. Hope that helps shed some light on your thread topic?

Respectfully Tim, I am not so sure this analogy works here. I tried a little experiment where I took a bar of metal and put a fixed bow in it. When I tap on it in its "fixed" state it had a fundamental pitch. Now when I bend it straighter, the bow is getting wider but the pitch goes up. I would think this has to do with the tension that the metal is under not the arc that is created.

I suppose some builders may put the arc in the braces by fixing both ends at points less then the length of the brace creating a brace that is in compression (and tension) and if were the case, I would agree that the higher arc creates higher pitch, but it would be due to the tension ( & compression) of the back rather than the radius of the arc.

I use a fairly dramatic 10' spherical radius on most of my guitars. I find that this adds a little comfort, strength and although I don't have real data on this, a bit more focus to the bass. The 10' radius backs also use the Gore/Gilet radial brace pattern. This allows me to adjust the fundamental (pitch) of the back, It is a finite adjustment and I can only lower the pitch by removing some of the height of the brace. I don't often have to mess with this adjustment unless I find the back is coupling and robbing energy from the top. I have also found that this configuration has a lower fundamental than the guitars that I build with a 15' or 18' radius but I assume this has more to do with the ladder bracing than the height of the arc.

[jmat]

Not to derail, how is dish measured. Something like a 15ft dish takes 15ft of length to offset a foot?

[nacluth]

Originally Posted by jmat

Not to derail, how is dish measured. Something like a 15ft dish takes 15ft of length to offset a foot?

It’s the bottom part of a sphere with a 15’ radius. So it’s the cut off bottom of a 30’ ball. The hardest part is building 3 story balls made of mdf.

[Guitars44me]

Originally Posted by nacluth

It’s the bottom part of a sphere with a 15’ radius. So it’s the cut off bottom of a 30’ ball. The hardest part is building 3 story balls made of mdf.

That sounds big enough to be difficult indeed

Paul

[CMStewart]

Originally Posted by Bruce Sexauer

As it happens, I have been using an 18’ back radius for 25 years. Because the supply houses at that time only offered a 15’ dish, I learned to make my own.

Hey Bruce, that is interesting. Sounds like that was a very intentional choice. What made you choose an 18' radius?

[CMStewart]

Originally Posted by John Osthoff

This allows me to adjust the fundamental (pitch) of the back, It is a finite adjustment and I can only lower the pitch by removing some of the height of the brace. I don't often have to mess with this adjustment unless I find the back is coupling and robbing energy from the top. \

Hey John, interesting thoughts. How are you able to tell that the back is robbing energy from the top?

[jmat]

Originally Posted by nacluth

It’s the bottom part of a sphere with a 15’ radius. So it’s the cut off bottom of a 30’ ball. The hardest part is building 3 story balls made of mdf.

Thanks! Makes sense, in fact now that you explained it nothing else could make sense. Back to the regular scheduled program and with apologies to the OP on the detour!

[justonwo]

The tighter radius gives you rounded notes. The larger radius gives you a flatter response.

[John Osthoff]

Originally Posted by CMStewart

Hey John, interesting thoughts. How are you able to tell that the back is robbing energy from the top?

I probably should have said not robbing too much from the top. When the back is lively, it will couple with the top. I can (and do) check this with spectrum analysis. But when we hear a wolf note, this often an indication of the back closely coupled (robbing too much energy) from the top. Ideally I would want to add some stiffness to the back, but sometimes shaving the back main brace will fix this. Other times I may need to mess with the top. Hope that helps.

[iim7V7IM7]

In science, an experiment with too many uncontrolled variables is referred to as “art”…

[John Osthoff]

I was recently working on creating better side profile templates for my guitars. Here is a screen shot from my software showing the dish (or bowl) with a 10 foot radius intersecting the side. I was able then to unroll the side to create the template. The template is laying out (sort of looks like where the fretboard would go)

Hopefully this image helps visualize the back radius.

[Tim McKnight]

John,

Maybe we are describing the same thing only differently or we are on totally different pages? So here is an experiment to try. Hold an unbraced top or back's long edge against your chest, then bow the plate, cross grain, with your hand while tapping it with your other hand and listen to the pitch change in relation to the amount of arch in the plate. The higher the arch (say 10' radius for example) the brighter the pitch (stiffer, higher frequency and brighter or treble biased) and the flatter the arch (say 60' radius for another example) the lower the pitch (looser, lower frequency and bass biased).

I just tried this with several tops and backs just now so I know I am not just blowing smoke. Same thing can be duplicated with a Stanley hand saw blade.