Altering the value of F

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JamesO
Kauri
Posts: 34
Joined: Wed Sep 09, 2015 12:27 pm
Location: Fresno, CA

Altering the value of F

Post by JamesO » Sat Nov 07, 2015 6:26 pm

Hi all

My apologies for not updating my results regarding the cross grain testing. I've made my mallet, but had an exam earlier this week and haven't done any re-testing yet.

As I was driving around this week, thinking about this approach to building, a question came to mind. When we adjust the value of F from 75 for a steel string guitar to some other value for working tops, do we also adjust the value for the back? If yes, do we adjust it equally? If I adjust from 75 to 70 for the top, would I adjust from 55 to 50 for the back? I noticed that the difference between the two values for F in a steel string guitar is 20, while the difference for a classical is 10.

I ask because a number of the builders I admire take a Smallman type approach of using very thin plates over a rigid structure, and I was wondering how Trevor's method could apply to their approach.

Would the target frequencies of a voiced top or closed body stay relatively the same as what the book suggests, or would those also tend to change?

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Trevor Gore
Blackwood
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Re: Altering the value of F

Post by Trevor Gore » Tue Nov 10, 2015 9:43 pm

When Equ. 4.5-7 was "calibrated", I chose f values that corresponded to panel thicknesses in the typical range for the types of guitar listed. With respect to live back panels, depending on how stiff you make the back bracing, from the standard bracing with a cap on, to fully scalloped in the centre, the range of back frequencies available runs from ~200Hz to ~300Hz, which is plenty of scope for most situations. I've only felt the need to go to a different bracing scheme on the back when I have used very dense, but relatively low Youngs modulus material, when I have used stiffer bracing to ensure a non-live back. There is no specific relationship that I observe between the f value differences between top and back.

Of course, you can use any value of f that suits your purpose, making adjustments to the bracing as well, if you so wish. That's one of the reasons the f number is a variable, you can select to suit your purpose but still know you can get the same result next time.

The backs of real Smallmans that I have seen are non-live, but the stiffness is obtained by using a thick laminated panel similar in profile to the back of an archtop guitar. There are no back braces.

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