sound hole placement, based on tuning fork tests

[David Ford]

I've been testing my hand built box with a tuning fork, looking for bridge placement sites.  I have found a few areas that vibrate very well with nice tone.  Next question would be, where to place the sound holes, since some of these placement sites are mirrored on the opposite end of the box.

One spot is 1.5" away from the longitudinal edge.  The surprising part is that there are two bands that vibrate well, both at almost exactly 1/3 of the box length, so, one at 1/3, and one at 2/3, with sound production better towards the ends than at the center.  I'm intrigued by that 1/3 value, thought the sweets spots are not exactly aligned on opposing sides of the box.  Maybe off by 1/4".

When I place the tuning fork at any of these positions, the other points vibrate well.  At the center of the box is sort of dead.

Question is, would these vibrating bands also be a good place for a sound hole, or, should I let them resonate and add to the sound, and place the sound hole offset from there.
Box is 8x12x3, so, bands are at 4" and 8" of length.

Must be a standing wave going on.  I used an A turning fork.  Hope other note values don't shift the optimal points.

Please see attached photo.

[jim]

One thing you have to keep in mind is , when you add the bridge and depending on string tensions, it will change everything.

[David Ford]

figured that.  shoot, now I can't find that long discussion thread that I was referred to, regarding all this.

[billy freedom]

dr oakroot says you only need a 3/8" soundhole or two 1/4" soundholes. he used algebra to figure this out. i pot a soundhole on the side of the box leaving the full face to ring like a drum. look at your drawing say it was a frame with a skin stretched on it those elipses are the sweet spots of the drum head. just like a round drum it sounds nice hit close to the edge. hit the center it sounds like crap. i dont know who said a bridge sounds best in the center but the solution is obvious even to a 4 year old hitting a cooking pot with a spoon.

[billy freedom]

http://handmademusic.ning.com/photo/bridge-placement?context=user

 

[Wade]

 

This kind of stuff always interested me. What Billy says makes sense. My best sounding cbg's bridge tend to be 1/3 or even less on the end of the body. As an artist I'm picturing the cbg surface as a pool of water and the bridge as an area where you would drop a stone or pebble into the water and forming ripples or waves. having the bridge not dead center should give more variety to the waves and allow some to travel further on the body before cancelling each other or dying out. Just my observation/imagination getting away on me again!

 

Wade

[jim]

If you really wanna get the over thinkins, heres a calculator i made based on helmholtz formula   http://www.mediafire.com/?b2s117ccjkbgvwu#1    enjoy