Live Back question

[ozziebluesman]

Hi Trevor,

I would like to have a go at your live back design. In the book you say thickness your back to about 3mm. How critical is that measurement. I have started an OM build with some nice quilted mahogany, When I thicknessed the back I was going to go with the standard four ladder brace design and have thicknessed it to 2.5 mm. Is that too thin for a live back design? If so, I will make sure I plan it in one of my next builds.

Thanks for your fabulous books, they are a great source for me to learn and be able to build a better guitar.

Cheers

Alan

[Trevor Gore]

Alan, the backs should be thicknessed using the same methodology as thicknessing the tops. Measure the Elong, Ecross and G using the tap test method, then use the infamous Equ. 4.5-7, but this time with a vibrational stiffness factor of f=55.

If you use very dense back wood, (> ~900Kg/m^3) you'll need to knock some mass out of the back (make it thinner) or it's monopole mobility will be too low for it to couple effectively, and you won't get a very live back. However, if you do that, you'll also need to raise the brace heights to get the stiffness back and a high enough resonant frequency [T(1,1)3]... So, until you get the feel of things a bit, stick with wood in the normal density range for back woods (~600-800kg/m^3).

[ozziebluesman]

Thanks Trevor! Better do some more study of the books me thinks!

Cheers

Alan