Back Radius

[colin north]

I was curious as to your choice of 10m back radius for the cutaway S/S in the book, but could not find the reason for it mentioned in the book.
As you are a man who obviously does nothing without a reason, what is the rational behind it?
If I've missed it in the book (and I'm still looking, but I get dizzy at times so have to lay off), apologies!

[colin north]

P.S. Sorry 3 m radius back.
Im talking about a live back. I understand we are looking for a back with an appropriate response.
Tightening the radius (as opposed to a 15' radius) will of course increase the frequency response for same bracing h & w, and back thickness.
But also increased back thickness and bracing height would have the same effect, why the radius decrease?
(Can you tell I only have a 15' and 25' radius dishes!!?)
10 m radius for top - I understand the reasons you use this.

[Trevor Gore]

The back radius is 3m (~10 feet), so I hope that's your typo, not mine!!

When I first started building, like everyone else, I more-or-less copied the Martin pattern. They had a cross-wise arch of ~15 feet (~5m) held by the ladder bracing and some type of tighter compound curve lengthwise which gives both the depth taper and the lengthwise arch. When I moved to building with dishes, I obviously wanted just one radius. I took a few measurements, computed some sort of average radius and that turned out to be ~3m. I built with it, and using the brace sizes that I wanted and the panel thicknesses that seemed right (based on experience) I ended up with the panel frequency (i.e. the T(1,1)3 frequency) that I wanted, with the one tuneable brace. It all worked out functionally and it looked fine, so that's why it's 3m. I use the same radius on the classical backs, too. The reasoning isn't in the book, so I hope you get over the dizzy spells!

[Trevor Gore]

Looks like our posts crossed!

If you want to use your 15' dish, I'd go stiffer braces rather than thicker panel. The reason for that is that the thicker panel would increase the mass and reduce the panel's mobility, so it doesn't couple as well.
I've made a few guitars using Brosimum Rubescens (bloodwood), which has a density of ~1000kg/m^3. I find I have to make those panels thinner just to get the mass down and up the brace stiffness, otherwise they're just too heavy to move. The standard back panel "vibrational stiffness value" works fine over the density range ~650kg/m^3 to ~850kg/m^3, which covers the majority of timbers. Once you get into the really dense stuff you have to take some mass out to get the mobility back up.

[colin north]

Thanks heavens for that, my wife keeps making fun of me because I can't find anything in the fridge except the beer (and the hard shellac of course!)