Replies to This Discussion

Permalink Reply by Rand Moore on July 14, 2011 at 10:20am

Hi Matt,

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.

 

-Rand.

 

Permalink Reply by Matt Quinney on July 15, 2011 at 12:07am

Helps very much, Thanks!

Permalink Reply by Loco on October 11, 2011 at 2:53pm

 Thanks ! this helps perfectly for my first build

Permalink Reply by MichaelS Country Boy Guitars on October 13, 2011 at 2:28pm

Just to add some less scientific data, big holes equal higher tones or sound and smaller holes equal lower tones. Or so I have read somewhere on the world wide web.  By the way, no holes in the face works too if you drill some in the sides to let the air move, don't know how the size works for the side holes if its the same or not.