Official Luthiers Forum!

John: If you are going for active backs I suggest that a copy of Trevor's design book would be a great asset. It's covered quite well in the book. Not sure of your experience so am just suggesting.
Tom

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A person who has never made a mistake has never made anything!!!

Don't overthink it.

I use the wider lower

wood stiffness is in relation to the cube of the height. It is just what I prefer. I have heard great guitars and lousy guitars using similar brace schemes. I like my backs more bouncy for lack of a better term and since I am very traditional in my building I like them. If I am sending a guitar where it has huge RH swings I will opt to the higher stiffer brace.

Worry more about the top , my 2 cents

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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

It's my opinion that all backs are more or less 'active' at some low frequency, and become more 'reflective' at a higher pitch. The question is where you want the transition to take place, and how you define/measure that. From what I can see, Trevor tends to go with a stiffer back than I like.

Also, with the 'free' plate tuning that I do I feel that a certain 'balance' between the plate ad the bracing is likely to make a better guitar. I find I can't get things to work the way I like when the plate is too thin; putting on tall bracing to get the right stiffness in that case just throws things further out of balance.

There are lots of different ways of making guitars, and they all work for somebody. You're going to have to figure out how to make the guitars you like, and that will be the 'right way' for you.

While at times I have tried to build reflective backs, I must say that not one guitar did not have at least some interaction on the lower frequencies. There certainly is no off/on switch as far as this is concerned. Trying to get a handle on understanding the complexities of guitars is one of the things that keep me captivated.
Tom

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A person who has never made a mistake has never made anything!!!

It's possible to get the same back stiffness (measured as operational deflection under a load in the middle of the lower bout) using a stiff panel and light bracing or a floppy panel and stiffer bracing, but the floppy panel and stiffer bracing will usually be the lower mass of the two.

My definition of a live back is where you can see a significant T(1,1)3 peak in the top's tap response. If you can't see that peak, it doesn't mean that the back is rigid, it just means that it is not light enough and/or flexible enough for that peak to show up. For it to show up, a mobility criterium needs to be satisfied (i.e. a low enough combination of stiffness and mass) and a resonance criterium has to be satisfied (i.e. the right combination of mass and stiffness that its resonant frequency is close enough to the top's resonant frequency). Then you get sufficient coupling between the top and the back for the T(1,1)3 peak to show. If you don't want the T(1,1)3 peak to show, stiff and heavy will do that for you. There is no black and white cut-off point between the two states; one merges into the other.

Neither option (live, non-live) is "better" than the other. It depends on how you want the guitar to sound and how it will be used.

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

http://www.goreguitars.com.au