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I am working on a 12X12, type A style, 18 3/4" scale mandola.
The location of the UTB leaves me with little room for the sound-hole.
I am considering using sound-ports on both sides (aprox. 1" dia.) or a 2" dia. hole very near the heel-block.

Any opinions or horror stories would be appreciated.

Bob

Hi,
Maybe you've heard about this Helmholz resonance law :
"The ideal size of the sound hole should be the surface area of a circle, which radius would be the 1/4 of the radius of a sphere that would have the same volume as the soundbox"

Easier : take you soundbox volume V
Calculate (V / 4.188) then take the cubic root (^(1/3) )
Then divide per 4
You get the radius of a round sound hole.
If you divide your sound hole in several ports, the total surface must be equivalent.

I've read that Helmholtz's research was based on violins, so the same range of frequencies as the mandolin.
For a guitar frequency range, it is reported that you must apply a correction of an average of 17%

Of course, this is pure theory and I have not build enough instruments to have a critical opinion on that theory.

Last word, for those who wish to calculate precisely the surface area of any odd shape : the trick is use Inkscape.

_________________
Christophe Mineau
Let's make chips ! - http://www.labellenote.fr

Christophe:
That is the first time that I have heard of it but it gives me a place to start. It looks like I will have to dig out my old log tables to figure a cube-root but it will work.

I have discovered that the fret board will cover the preferred part of the soundboard so I will have to use the sides.
Next time, I will do more planning before I brace the soundboard.

Thanks for the lead.

Bob

Hi Bob,
Don't you have the possibility to put a fretboard extender over the hole ? like on a F5 ?

You could also go for ff holes ?

To come back ti the sqare root, you don't need your log tables anymore you have online calcs like this one:
http://www.picalc.com/

Bye,
Christophe

_________________
Christophe Mineau
Let's make chips ! - http://www.labellenote.fr

Christophe:
The calculation requires a cube root which is not on my calculator but I will give you the long process that I used so that someone else can use it as a guide (or a bad example).

I weighed the soundboard template and a measured piece of the same sheet of plywood. the area was calculated based on weight.
I averaged the height of the ribs and calculated the volume of the box.
I used your formula and calculated the cube root by iterations and divided by four.
The result seems low at 1.7675" dia. or 2.45 square inches.

Thank you for your help.

Bob

Hi Bob,
Take care, as I said, this is only theory, so just a starting point.
I will try to calculate it on the different mandos I have to see where it is exactly.

For the square root, take the online calculator I gave you the link above
http://www.picalc.com/
For square root, you elevate at power 1/3 (sorry my mathemetical english is approximative)
for instance cubic root of 3 : type 3 Xy ( 1 / 3 ) =
And for the surface calculation, it is a bit for the geeks, but I explain how I do it with Inkscape software on my site :
http://www.labellenote.fr/articles.php?lng=en&pg=273
(sorry, still in Franch, I will translate that today)

_________________
Christophe Mineau
Let's make chips ! - http://www.labellenote.fr

Are you basing this instrument on an existing plan, or is it your design? As irritating it is to hear it, drawing it all out in full size before you start cutting wood usually avoids this type of problems...

For now, it sounds like a shorter fingerboard should solve your problem.

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Rian Gitar og Mandolin

Arnt:
You are correct.
I am waiting for an order of spruce and decided to go ahead with a three piece soundboard. I do not want to put "F" holes across the soundboard joints as they are fairly narrow strips on either side. I have decided to put one inch diameter holes on both sides and extend the reinforcing veneer to allow an additional hole to be drilled on each side. I intend to string it up in white and alter it as required.

Thanks for your help.

Bob

Violin builders won't finalize the f-hole shape and size until the instrument is done. They then tweak them to adjust the instruments overall sound.

_________________
Another day, another dollar.

I would guestimate a slight undersized f holes as above and enlarge if it did not look right when you eyeballed it ,just before final sanding and prior to finishing.. Wasn/t there an article abt making mandola/s in a recent issue of GAL?. You might want to check the GAL website and read the article/photo even if your not a GAL member. Guild of american luthiers .