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8 String Tenor (Read 4531 times)
Matt Blacka
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MAKAIO Ukuleles Australia

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8 String Tenor
Jun 21st, 2009 at 2:33pm
 
I played an 8 string tenor uke the other day, and loved the richness of the paired strings.

I was just wondering if anyone here has had any experience in soundboard bracing designs for an 8 string uke? I assume to maintain good sound quality that the soundboard is left the same sort of thickness as a 4 string uke, but additional fan braces are added?

I don't particulalrly wan't to have to do a heap of deflection calculations to work out the bracing sizes if I can help it.

Thanks,

Matt
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unkabob
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Re: 8 String Tenor
Reply #1 - Jun 21st, 2009 at 6:48pm
 
Matt:
I believe that the eight string is sometimes called a "tarro patch". That may help in your research.

I was impressed by the evenness of fretting the double strings.

Bob
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Re: 8 String Tenor
Reply #2 - Jun 23rd, 2009 at 3:54am
 
Hey Matt,


I don't think you'd need to change the bracing much for 8 strings...Assuming you use a typical 3 fan brace style.  Maybe just make the fan braces a bit taller and extend them to the edge of the sound board.  I would add popsicle stick braces around the soundhole too.
Dammit....now I want to build one
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Matt Blacka
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MAKAIO Ukuleles Australia

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Re: 8 String Tenor
Reply #3 - Jun 23rd, 2009 at 1:13pm
 
Thanks guys,

I looked at the left brain lutherie site and there's a bit of guidance there about total string tension for different uke designs. No surprise that an 8 string has about double the string tension as a 4 string.

I wasn't sure whether to go for about 5 fan braces (instead of the usual 3) and make them similar or slightly lighter than the normal bracing, or if I should just beef up the standard 3. I might try to remember back to my engineering mechanics classes and spend some time calculating bending stresses and deflections. Maybe I can actually contribute something then, instead of just asking.

I bet Mike or Asa has some reasonable ideas on this to..... Wink

Matt
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konacat
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Re: 8 String Tenor
Reply #4 - Jun 24th, 2009 at 6:30am
 
Matt,

You can check out my previous post about the cube rule. I hope it helps.

http://www.hanalima.com/cgi-bin/yabb2/YaBB.pl?num=1217748481/4#4

Philip
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Don_Orgeman
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Re: 8 String Tenor
Reply #5 - Jun 24th, 2009 at 10:59am
 
Philip:

Your math rule is correct.  Of a rectangular shaped brace with width (W) and height (H) the resistance to bending is controlled by the moment of inertia.  The moment of inertia for a rectangular shape is calculated I =( W X H cubed) / 3.  So, if you make a brace twice as high you get 8 times the resistance to bending.  Based on this formula, the brace should only have to be 25 percent higher if the forces are doubled.

Notice that the width of the brace is linear, so doubling the width of the brace doubles the bending force the brace can resist.

Don
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Matt Blacka
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MAKAIO Ukuleles Australia

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Re: 8 String Tenor
Reply #6 - Jun 27th, 2009 at 11:14pm
 
To delve deeper into this idea, I went back to my structural engineering class notes, and set up a model for calculating soundboard deflections. It is a simplification of reality, but includes all the import mechanical factors. It includes the stiffness and moments of inertia for both the soundboard and the fan braces, with the braces having parabolic scalloping.

To check out how well it worked, I initially calculated the soundboard deflections for a standard 4 string tenor (23.9 kg string force), with scalloped 3 fan bracing as per the Hana Lima ‘Ia plans. I included a soundboard thickness of 1.5 mm, and the soundboard and braces were all assumed to be Engelmann Spruce. The resulting deflections calculated were a sag of 0.27 mm in front of the bridge and a hump of 0.09 mm behind the bridge. I quickly checked these values against some numbers that David Hurd listed on the Australian luthiers forum for some of his tenor ukes, and he quoted 0.3 – 0.38 mm deflection in front of the bridge.

Based on this, it seems that the soundboard model works pretty well. At least it is predicting deflection in the right ball park, and should give a good indication of relative deflection changes for different string forces.

For an 8 string uke (48 kg string force), with the same scalloped 3 fan braces, the model calculated a deflection sag of 0.55 mm in front of the bridge, and 0.18 mm hump behind the bridge. To reduce the deflection back to a similar level as was predicted for a 4 string uke, the 3 fan braces would each have to be approximately 1.5 mm higher along the full length of the brace. So it seems that its not really worth adding any additional braces, but just to have the braces very slightly higher.

Matt
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MAKAIO Ukuleles, Australia www.makaioukuleles.blogspot.com
 
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