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Hello,

I'm still plugging away at my first build using the plans drawn up by Michael Payne for the OLF medium jumbo. I'm at the point where it's time to start working on the braces for the top and back. The plans call for a 15' radius but I don't have a radius dish of that size but I do have access to a dish with a 20' radius and I was thinking of using that one instead. How much of an impact would that make to the guitar's tone?

Neil:
Your dish will probably work out just fine. The effect on tone will be negligble and very few people would even notice the slight difference in esthetics.
Carry on....
Mikey

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The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

Thanks, Mikey. I was hoping you'd chime in given your experience with the OLF MJ plans. I suspect the flatter back might be more comfortable anyway. I've never really liked bowl-backed guitars!

BTW, this first build is certainly becoming a bit of consolations:



Worry not though, it's been patched up and it's my first so it's not going anywhere once it's done. That being said, it's almost bracing time:

Dig the clamp art. How much extra did Stewmac charge for that?

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Aaron Craig

LOL, those are actually YJ John's. They were a Xmas present from his kids; he's been letting me take up space at his workshop and he's the one who actually pushed me to do a build in the first place.

Hey Neil...I use 12 ft radius, cause that's the dish the guy had. I've yet in all my wanderings seen reference to it relating to tone, in any distinct way. I am on #4 so...the grain of salt.

the back radius has no tonal input . It is there for strength .

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

i dont mean to open a can of worms, but how can one possibly make that remark? i know the number of guitars youve built john, and i respect your knowledge, experience and talent. doesnt everything we do to a guitar shape its tone in some way? wouldnt a flatter (larger) radius loosen up a back plate? and vice versa? even if you think the back is just a reflector, you have to admit that back radius effects tone.

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sweat the small stuff.

I don't want to get hammered here, but I thought the back curve was for strength as well as to act las a better reflector than a flat back to bounce sound back at the top. I would think a conical or domed reflector would concentrate the sound waves back towards the center of the soundboard better, which is the loosest part of the plate, which will vibrate the most, and produce the most sound. I can't possibly have made all that up in my head, I know I heard it somewhere before.

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http://www.sixgunguitars.com

I also think it affects the tone. Everything affects the tone. Yhea, the radius is there for the ''strenght'', and that said, a ''stronger'' back means it is stiffer and tighter. So, a stiffer and tighter back will be more responsive and will loose less energy when reflecting the sound.

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Francis Richer, Montréal
Les Guitares F&M Guitars

No it has little to do with tone . I guess if you want to use the argument that anything you do to a guitar influences tone you can. There is no scientific evidence of the radius we see on guitars to influence the tone .
The radius aids in strength . I would love to see a blind study on many of the opinions we have developed over the years . I saw one blind study where 3 guitars were handed to a blind folded player . A D45 A Gibson J35 and a DX1. After a blind test he was then allowed to play them without knowing the results . Blindfolded he picked the DX1 the D 45 the J35 unblindfolded it was the D45 the J 35 and then the DX1.
Amazing what inferences will do to an opinion.

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

My vote is mostly for strength. With out a radius the back would flex more and crack or sag between the braces.
I'm sure it has some effect on tone because bending the wood into a radius changes the resonant frequency of the back....but any particular radius doesn't guarantee a particular resonant frequency because no two sets of wood are the same. It's totally hit or miss what effect it would have.

Those blind tests always seem a little pointless to me. Martins and Gibsons have too much variation within each model due to lack of fine-tuning compared with luthier-builts. Unless the test operator picked the best of the best of each model out of a large selection, there's too much randomness in the test to mean anything.

And anyway, people buy what they want... branding and eye-candy are part of that (and don't forget that some people will avoid eye-candy, seeing it as tacky and a waste of effort/money).

As for back radius tone, I would say both sides are right, but count me in the intuitive "everything affects tone" camp. I doubt it's possible to scientifically demonstrate why one back arching has more potential than the other. And it is only potential after all, the difference is bound to be so small that there will be plenty of specific cases where each is better than the other. But I can't imagine it not making a difference, so it's just up to the builder to choose his/her own beliefs and learn to optimize them. It's easier to judge the tone quality of completed guitars than individual components, so if you can intuitively choose a good combination, the tiny differences can build up to something more significant.

That said, I haven't made up my mind on back radius. It's going to be either spherical or cylindrical rather than flat, for humidity tolerance, but I'm still trying to make up my mind whether a straight line from tail block to head block offers better vibrational potential and resistance to neck resets, while accounting for the effect of a head block foot going to the back's upper transverse brace, potentially slightly thinner spherical back, and various possible bracing patterns... With ladder bracing, cylindrical seems much more natural. But I'm leaning toward spherical with an asterisk so far.

blind test can tell you a lot. Visual inference when having nothing to do with tone will influence the choice. I agree that "eye appeal is buy appeal" but when it gets to the crux of the matter , often opinion is accepted over facts .

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

The next question here might be...and I think it may be what Neil is asking... is...what's the tonal difference between the 15 ft. and 20 ft. radius.?


Michael

Most probably none, and even visually it will impossible to tell, you can even glue a back braced with a 15' radius onto a rim prepared for 20' and vice versa, it will fit like a glove. 15' and 20' are very close, and the slightest variation in RH will make the back relax one way or another.
The radius helps with RH changes and in avoiding cracks since it is traditionally ladder braced. Because it is under tension it is theoretically possible to use a thinner back for equal strength (to a flat one). However, for maximum strength, the back should be arched laterally and left flat longitudinally, like the section of a tube.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Once you glue wood cross grained you in theory make a hygrometer . As RH changes so will the back radius. I only built 118 guitars . Learned this craft from some very good luthiers. Not one ever said the back radius was anything more than a strength provider. I know many may disagree but the back is not as much a sound reflector as you may think. Laurent is 100% correct.
I hope Alan Carruth chimes in he has a way of explaining these details very well.

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

So a regular radius dish shouldn't be used when building a longitudinally flat back ? What do you use?

It seems that at the extremes, there are two ways to make a guitar - one that is relatively lightly built everywhere and the whole instrument vibrates and makes sound and at the opposite extreme, there's the Smallman type design where everything but the sounboard is built rigidly and heavily so that all (most) of the vibration is coming from the top. I imagine that the closer you are the the former type guitar, the more the radius of the dome might influence the sound. (p.s. I'm a total non-believer in "reflection" type of stuff for any type of design).

The reason I say that is this - the arch of the back should have an influence on its resonant frequency and that pitch will determine how it interacts with the frequency of the top. The tighter the radius, the higher the pitch should be, (and I italicize that because I've never actually tested it - it seems like a tighter radius should make it stiffer and raise the pitch but there may be something going on that I haven't thought of).

So my thinking is something like this - the pitch of the back resonance will influence the sound enough to where you can hear it. If you change the dome and it changes the back pitch, it should change timbre.

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http://www.birkonium.com CNC Products for Luthiers
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Andy - that's exactly what I was thinking as I read through this thread. It seems one cannot believe in tuning the back and top plates to play well together without also believing that the back arch plays a role as well since, as you pointed out, the arch of the back plate directly affects the pitch at which it will vibrate. At least that's my simple-minded take on it.

This is another one of those questions that doesn't really have an answer. Most folks that are building pick a radius or two and stay with it. Mine is 10'. Works for me.
Why do I do it? Because the first couple I made were a shallow radius and go concave in drier spells. A pretty hefty radius just plain looks better. Whether it is stiffer or not depends on several things, one of which is thickness of the back. Biggest question is whether the individual builder wants a reflective back or a resonant back. That's mostly dependent on the musical use.
I had not considered a cylindrical radius on the back (my tops are 15' cylinder), but that may be an interesting experiment. However, I wouldn't expect much difference as I mostly operate on an intuitive level and stop working a back when it sounds good and then work the whole box when it's together.

I made a go-bar base and a sanding sled with a 25' radius.
Seems to work nice,
especially for a weiss.
I agree that the tighter radius is good looking.
I don't about tone diff though,
but I would think that if you put a really tight radius, it might do something.
I have noticed the Ovations do have good projection.

I use a mostly cylindrical back plate and add a tiny longitudinal arch to prevent the back from caving in when it's too dry. I agree that it does not make much difference. There is a lot of over-thinking about back or top radius: no matter what the radius is, you all tune your plates, right? So any difference a tighter radius will make is compensated (intuitively I should add) by either thinning the plate further and/or bracing.

I don't use dishes, but a long board with a 5' (or so) lateral arch.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Very cool, Laurent!

Am I mistaken, or is the posting of political information prohibited on this forum?