Taff would have a better answer but sound holes also let the sound out. you can go too big but also too small. if the sound is good to you and the volume is fine then dont mess with it. enjoy and maybe experiment with the next one...and the one after that...and the one after that...
Hi Michael, thanks Tim.
Rule of thumb? There proberebly is a math that would help get it right every time. But I cant count too well so that's no good to me.
In acoustic guitars this ratio was worked out back in the day of the first guitars and the soundhole size does change with the sizes of and volume of the body, classical guitar 3 1/4" to large steel string 4'' diameter, give or take.
I'll try to keep this short. When the soundhole is in proper proportion to the body size the sound you hear will sound more bass if made smaller and sound more treble if larger. If covered up the sound is muffled/restricted. The sound you hear then would be coming off of the tops face and the strings. You miss much of the harmonics and overtones in the soundwaves bouncing around in the body.
Before building my first cbg [a commission] I made 3 others experimenting with sound hole size and body size. Having found what worked I could then play with sizes and shapes and still be in the ball park. Michael, I would not change what you have but modify the next one and look for improved output and tone.
I have not read this anywhere but my thinking is that it may also depend on how responsive the top is, how quick or efficient it is at producing soundwaves, as to what size soundhole to use. An example might be, if the top is too thick or overly braced to produce strong resonant frequencies in he body, then the soundhole may not have the same effect on that sound.
Another example is if I put a lighted candle in front of one of my guitars [if built as responsive as possible] tapping on the tops face will force enough air through the ideal sized soundholehole to blow the candle out. Not really relevant to CBG, just thought I'd add it to show how things work, even if on a smaller scale.
Hope this helps Taff