Official Luthiers Forum!

I have built radius dishes that are 15' and 28' for the back and top. After doing that I see some recommend 12' and 25' . So I'm wondering how much difference there would be in sound , strength or whatever if the materials, braces etc are the same. I guess I'm wondering how the different s in radius dishes makes.
I'm new to building but did do a search and didn't find much information to help me. Part of what I'm wondering is should I reduce the radius dishes I have made.
Any help will be appreciated. Thank you, Chuck

12' vs 15' radius back, with same materials, thickness, braces etc, the difference will be that the vibrational frequecy will go up because the back panel stiffness increases.
This enables the the builder to reduce the thickness of the back, hence reducing weight and giving higher mopolole mobility (or back response), while maintaining a similar frequency.

_________________
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.

That's a great new term. Will have to think of something to apply it to...

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

There may be other considerations re: dish radius change that are not acoustic in nature, such as how the builder establishes a true and untapered/wedged fret board extension over the upper bout. We use a 28' radius dish, plus 28' radius X, tone bars, fingers, & bridge plate coupled with a 60' radius upper transverse brace glued on the flat to establish the correct angle on the extension...a change in dish radius would alter that relationship for those using this approach to establishing body/neck extension geometry. If a builder is using another method (elevated extension, body sanded to angle, 25' radius with flat UTB and flattened upper bout), then a top dish radius change would mean just minor changes to sanding shim thickness, wedge taper, etc.

My other thought is whether minor variation in stiffness of top or back would obviate that small change in radius, or - said another way - the hopeful statement that "...this is so if all other things are equal..." does not actually enforce equality in the workshop.

_________________
A constellation only takes shape when one maps the whole.
- Beth Brower

If you have the radius dishes don't bother to change them. It is a relatively small change, and not necessarily for the better. Because the increased bend in the plates increases stiffness, you can wind up with a "tight" sounding instrument unless you thin the plates to compensate, which can cause some loss of "headroom". For a fingerstyle instrument that is no big deal.
Some builders use a true "flat top" and allow the string tension to pull some arch into it. A 28 ft radius is plenty, especially for a new builder. As Woodie mentioned, a tighter radius can create a whole new set of considerations for fitting the neck.

I used 10' radius on top (cylindrical) with laminated bracing and 10' dished backs with laminated bracing and never had a loss of "headroom" or tight sound. I never used thin plates...

I honestly don't think switching between those radii would make any discernible difference in sound that you could appreciate. Maybe to a set of perfect 12 year old ears.

I would agree that the consideration should be structural with the 28' top being more user friendly in regard to fitting the neck and fretboard extension properly.

I use 30' on the top only because when I started in the 90's I thought that was what Martin used and I have adapted to it.

For what it's worth 15' on the back is what I use.

_________________
It's not what you don't know that hurts you, it's what you do know that's wrong.

Haans,
Since you didn't use thin plates you probably didn't loose any "headroom". I try to avoid thin plates on steel stringed instruments. They may seem louder, but to me have an unpleasant timbre.

I have used cylindrical tops on octave mandolins. About 3/8ths to 1/2 inch bend in 7 . The bend being unidirectional seems to make them work differently.

dang touch screen keyboard!

_________________
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.

This may or may not be applicable, but if you're looking to tune the back to the top following Trevor's method, it's really hard with a 15 foot back and a tighter radius goes a long way. I use 30' and 15' now but plan to switch to 30/12'.


Sent from my iPhone using Tapatalk

Call the Martin factory and they will tell you they produce "flat top" guitars -- the reality is that they contour the main sound-board braces to 52' radius, this is necessary to prevent a concave appearance and add a measure of insurance against cracks due to humidity fluctuations. The back bracing is contoured to about 15'. Years ago the engineers gave me that dimension in inches (can't remember exactly) but it's so close to 15' it does not matter.

The way I understand it, all other things being equal -- a tighter the top brace contour will increase treble response and reduce perceived bass response.

As for neck set angle, its a huge construction consideration and can be accurately processed in several different ways. Admittedly I am a fan of Martin guitars, their factory employee environment and especially the quality ethics. So I have always tried to copy what I could. Here's what I learned from them regarding the issue of neck angle:

http://www.kennethmichaelguitars.com/neckangle.html

_________________
Ken Cierp

http://www.kennethmichaelguitars.com/

If you already have the dishes done - just go with what you have. The 1 or 2 foot radius difference doesnt make much difference.

In the future - if you want to experiment, then have at it - but you can do that without a radius dish. You can shave the braces and then clamp the braces to the plate and it will pull it to shape. Its easy enough to use chalk to fit the sides to the new back radius.