Tapping panels for thickness?

[chrisbaer]

Hello all! Quick question (hah) - how would you interpret these results? Do they make sense?

I'm tapping the back panels for a new guitar. They are slightly figured walnut, 4mm thick, 217mm x 605mm, mass 297g.

If I had to guess, I'd read the results:

fL = 38
fC = 98
fLC = 52

It seems like this roughly comports with the values in Trevor's book, the Walnut being less dense than Rosewood.
I have a spreadsheet with some of Trevor's formulae and have plugged the above data into the Youngs Mod Dynamic Method tab... but I'm not sure which formula to use to calculate a final thickness. This is for a Gore-style J45, using plans from the book.

What do you think? Thanks so much!!

Chris

[chrisbaer]

Ok, rereading, I realize the formula I'm looking for is 4.5-7. One confusion I have is what is the variable "p"(square root) in the top line?

Elong/Ecross? Density?

I think I'm understanding the rest.

Thanks!

[Trevor Gore]

Chris,

The "p" is ρ (Greek "rho") and is the symbol for density. See Fig 4.5-4.

There's more stuff on coding up Equ. 4.5-7 here and here. The typos have been fixed in the 2nd edition.

[chrisbaer]

Well, after some trial and error, I got some results!
Using numbers from the book and the threads you pointed out, I was able to come close to your target thickness for Englemann #1, so I think my equations work.
However, 2.3mm for a back seems a leeeeetle too thin. So perhaps my frequency measurements are faulty.

Walnut back thickness.png (22.31 KiB) Viewed 10899 times

Just a point of clarification... When using the Youngs Mod Dynamic Method formula, the actual measurements of the panel being tested are used, but when using formula 4.5-7, the dimensions of the guitar you intend to build are used, correct?

I'll try again with the Redwood top panels I have - they are much more uniform than the walnut back panels.

Thanks!

Chris

[Trevor Gore]

Chris, the density seems very low for walnut at 395.9kg/m^3. ~570 kg/m^3 is fairly typical. Having said that, 2.3mm for a back panel thickness is fairly reasonable for a live back.

Just a point of clarification... When using the Youngs Mod Dynamic Method formula, the actual measurements of the panel being tested are used, but when using formula 4.5-7, the dimensions of the guitar you intend to build are used, correct?

Correct. Be sure to use the right dimensions in the right places. Panel dimension for Equ. 4.5-2, 3 & 4; box dimensions for Equ. 4.5-7.

[chrisbaer]

Well, I tapped the Redwood top I have... mixed results. I'm intending on building a Gore-style J45 (very light X-bracing).

Redwood Top Thickness.png (22.1 KiB) Viewed 10855 times

I tried several different tapping methods and got pretty consistent results.

This is a piece of tempered Redwood. It's very well quartered, very light and very stiff.

The panel itself is already thinner than the formula recommends for a steel string guitar.

So, either: 1) my measurements are off (FL,FC,FLC), 2) I can build, but take very little off the panels, 3) this isn't a suitable top for a steel stringed guitar.

Thanks for taking the time to help, I really appreciate it!

(As an aside, Trevor, my wife was in Sydney on business a couple weeks ago and thoroughly enjoyed it! She only really had a day to see the city before holing up in offices, but it was enough to spark the urge to come back someday!)

Chris

[Trevor Gore]

That's very low density and low stiffness redwood. Normally I'd expect Elong to be in the 10 GPa region. So if it feels stiff, as you say, it suggests something is wrong somewhere. If not, figure out the mass that the top panel would be and see if the low density compensates for the lack of stiffness. You may be able to use it for a small SS guitar (low a and b) where thinner panels can be used.