Has anyone any information on how much the soundhole shape and location influences the final sound? Round, f-holes, multiple holes, - does it make any difference for CBGs other than aesthetics?
Is there some kind ratio of sound hole size vs box volume for optimal sound?
This is a fairly common question and I recall a thread about this in the Super Advanced Cigar Box Guitar Building discussion group where Herb Berwald supplied us with the following formula:
Formula for calculating sound hole diameter
1.) Calculate box volume(V) = Length * Width * Height
2.) Solve for the radius of a sphere of the same volume: radius = ((3*V)/4*pie)^(1/3)
3.) Divide the radius by 4.
4.) This resulting value is the new radius of the sound hole.
5.) So, 2 x radius is the new sound hole diameter.
Then another CBN member (who had a cigar box with the following dimensions: Length=8.25", Width=5.5", and Height=2.75"), solved the problem using the same formula condensed into one big equation using the following numbers:
(2 * (((3 * 8.25 * 5.5 * 2.75) / (4 * 3.14))^(1 / 3))) / 4 = 1.55023303
So, a sound hole about 1.5" in diameter is "optimal" according to this formula.
I also remember an even older discussion where they decided a neck thru CBG would best be served by two U.S. quarter sized holes on each side of the neck, mainly because the neck would block a single hole, but I think they also used some formula.
For more complicated sound hole shapes, I think you will need to go a Google search using key words like "F-hole", "Size" and "Formula" as a first guess.
I tend to make sound holes on the small size, and when I tap on the back of the sound box I expect to feel a puff of air on my cheek (as I hold the box to my cheek and tap on it). If the puff of air is indistinct, I think its because the hole is too big. But, that is my personal rule of thumb, without much scientific support. Also, I usually make my box guitars with home made boxes and 2mm ply veneer sound boards, so this method may not me as applicable to cigar boxes with thicker plywood tops used a sound boards.
Hope this helps.
Thanks ! this helps perfectly for my first build