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Free Plate Tuning and Spectralysis
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=35982		 Page 1 of 1
Author:  Jmc2010 [ Sat Mar 31, 2012 11:20 pm ]
Post subject:  Free Plate Tuning and Spectralysis
I'm finally getting to the thicknessing part of the WRC top for my first build. By the time I got the saw marks out of the top, and got them relatively flat with the drum sander, I was already at 3.1mm. By the time I got them within .1 of a mm, I was down to 3mm. I tried to check the Chladni patterns on these to verify the frequencies, but the only one that I could get to give me a good pattern was the F Log/Cross. So, can some experienced eyes tell me if I am on the right track with what I have listed as the frequencies?

Also, if you notice, the target thickness using the Gore/Gilet system is 3.28mm. I am at 3 right now.. :( This is a master grade top from Shane that is very tight grained, and has a wonderful tone. Do you think I would be ok to still use it? It is going to be for an OM "sized" guitar. I really hope this top isn't waste. When I did the spectral analysis on them, using the system described in the Trevor Gore book, here was the outcome:

Length 535.00 mm
Width 196.00 mm
Thickness 3.00 mm
Mass 116.00 grams
ρ 368.7 kg/m3

fL 47.70 Hz
fC 94.10 Hz
fLC 37.68 Hz

Target L 0.49 metres 492.10 mm
Target W 0.38 metres 380.66 mm
Target H Top 0.003283 metres 3.28 mm
Targer H B&S 0.002407 metres 2.41 mm

Here are some screen shots of the charts:

F Long:
Image

F Cross:
Image

F Long/Cross:
Image
Author:  woodrat [ Sun Apr 01, 2012 12:53 am ]
Post subject:  Re: Free Plate Tuning and Spectralysis
Jmc2010, You will need to consider that at 10% less in thickness you are considerably less than 10% less stiff....probably in the order of 30% less stiff because of the cubing effect....Thats something you will need to consider carefully as you brace it...

John
Author:  Ken Franklin [ Sun Apr 01, 2012 1:33 am ]
Post subject:  Re: Free Plate Tuning and Spectralysis
Jason, I can't help you with the graphs but I can tell you that I have built some very successful cedar small jumbo guitars at that thickness. As long as your rosette is in I'd say you are fine. If not I'd save the top for a little smaller instrument or be extremely careful when leveling the rosette.
Author:  Trevor Gore [ Sun Apr 01, 2012 1:46 am ]
Post subject:  Re: Free Plate Tuning and Spectralysis
Jason,

What did you calculate the Young's moduli to be?

BTW, I'm just finishing an OM sized guitar in WRC (my usual medium sized shape) and the top thickness for that was computed at 3.3mm from a long grain YM of 5.6GPa, so I'd say you're in the right ball park. Bracing is my normal falcate style and it'll be a 180Hz guitar.
Author:  Jmc2010 [ Sun Apr 01, 2012 8:07 am ]
Post subject:  Re: Free Plate Tuning and Spectralysis
Trevor Gore wrote:
Jason,

What did you calculate the Young's moduli to be?

BTW, I'm just finishing an OM sized guitar in WRC (my usual medium sized shape) and the top thickness for that was computed at 3.3mm from a long grain YM of 5.6GPa, so I'd say you're in the right ball park. Bracing is my normal falcate style and it'll be a 180Hz guitar.


Hey Trevor, here is what all the YM was:

E Long 7190160616.80 Pascals 7.19 Gpa
E Cross 504069288.66 Pascals 0.50 Gpa
G Long/Cross 777688838.26 Pascals 0.78 Gpa

Problem is this.. The math is calling for a 3.28mm top, and I only have 3mm. Is there any way to salvage this?
Author:  Trevor Gore [ Sun Apr 01, 2012 9:01 am ]
Post subject:  Re: Free Plate Tuning and Spectralysis
Jason, I worked all your figures and got 3.26mm, basically same as you, so there is no disagreement on the maths. The Ecross on the wood I had was ~0.8GPa.

It is possible to use the wood at 3.00mm, but you will need to beef up the bracing to put you back in the "normal" ballpark for bridge rotation, assuming you're wanting a guitar with a T(1,1)2 of ~180Hz.

Now, with a low-ish E cross, you will likely see some bracing "print-through" if your braces are aligned with the top's grain at any point. You will also have to make a judgement as to how much taller to make the braces, which will be difficult for you this early in your building career. So my advice at this stage would be to put this top aside for another guitar, either a small body SS (it would be OK for a guitar ~470mm body length by ~360 lower bout, provided you didn't loose much thickness installing the rosette) built to the scantlings suggested in the book, or, with a bit more experience, figure out how much more brace height you'll need for an OM sized guitar, or reserve it for a classical.
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