Any solutions for over-sanding live back (I follow the Gore/Gilet design books)?

[gdebinche]

Any solutions for over-sanding live back (I follow the Gore/Gilet design books)?
Hi, this is about the design parameters for the vibrational stiffness frequency (VSF) values suggested in table 4.5-2 of the design book of Gore and Gilet. My problem is that I over-sanded my back (sepele wood) for the live back design of the steel string guitar. The target thickness was 2.35mm but I end up with 2.1mm (my bad on a new sanding drum machine). From eq. 4.5-7, the 2.1 mm thickness would theoretically shift the suggested value of VSF value f of table 4.5-2 from 55 to 49.5 (say 50). I see two ways (?) to still attempt reaching the 4 semi-tones difference design goal between the top and the back:
1) lower the VSF suggested value of 75 for the top (soundboard) by the same amount as for the back, that is reducing f from 75 to 70 (I acknowledge that this is an intuitive guess), and adopt the corresponding thickness for the top from eq 4.5-7, or,
2) keep f=75 as VSF value and its corresponding thickness for the top and instead adopt the correction strategies exposed in section 22.3 of book 2 of Gore and Gilet. In this case, I would not apply the brace central scalloping of the lower bout to increase stiffness of the back.
Any idea if this could work ? Any other ways ?(Please don't suggest buying another back blank!)
Thanks
Mario

[kiwigeo]

Without thinking too hard about this if you've gone a bit thin on your back then try making the lower cross brace higher than specified in The Books to increase stiffness and raise T1 (1,3) peak. If frequency peak ends up too high you still have the option of lowering it by scalloping out central section of the brace.

[gdebinche]

HI Martin,
With or without scalopping the raised lower cross?
Mario

[kiwigeo]

HI Martin,
With or without scalopping the raised lower cross?
Mario

Fit a higher than specified lower cross brace and complete the box. Tap test and see where the main back frequency sits and trim (down) if necessary by scalloping.

[gdebinche]

Thanks Martin for your suggestion, it make sense. Mario
I am still curious about the relationship between top and back vibrational stiffness frequency (but I understand that they were determined from experiences). Or, the error tolerance on target thickness to still comply to the design requierement.

[kiwigeo]

Thanks Martin for your suggestion, it make sense. Mario
I am still curious about the relationship between top and back vibrational stiffness frequency (but I understand that they were determined from experiences). Or, the error tolerance on target thickness to still comply to the design requierement.

The vibrational stiffness values are constants that have been derived from large numbers of measurements. If you start changing this constant you're basically adjusting the formula to fit what you'd like the end result to be

[Trevor Gore]

If you're following the book process (back on sides, then close the box with the top) you have the opportunity to tune the back B(1,1) resonance before gluing the top on.

Make the main back brace a few millimeters taller than normal and don't scallop it. Glue the back on the sides and flush trim it. With the "boat" out of the mould, hang it from a finger under the head block and tap the back over the brace. You should see a tall peak in the response which is the B(1,1) mode. Shave the main back brace until the B(1,1) is slightly above the target T(1,1)3 frequency. Then proceed to close the box in the normal way. The T(1,1)3 comes in a few Hz higher than the B(1,1) so you have scope to do some brace scalloping to trim it to target if you wish. See the last paragraph, Build p 12-11.

[kiwigeo]

LOL...was just about to edit previous post to note that Trevor would eventually chime in with sagely advice

[gdebinche]

Thanks, both of you!
Trevor, do I also raise the back cross braces if I raise the main back brace as you recommand?
Mario

[kiwigeo]

The lower back brace is the most important one - the lower bout is most of the activity when the back is vibrating

[gdebinche]

Ok. I was not expecting so quick answers to my 'problem' ! You are really of great help and generous of your expertise.
Mario

[kiwigeo]

You are really of great help and generous of your expertise.
Mario

That's what this forum is all about. You're also lucky that you're using Trevors books....he answers all questions regarding same.