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sound hole placement, based on tuning fork tests

Can't figure out how to see my posts on the resonator box group, so I'm reposting here.

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.

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.

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Permalink Reply by Mark Bliss on November 7, 2011 at 7:49pm

Good response and points from Mark.

And no reason to be annoyed, the humor is not about " tedious geeks" nor any feeling there even exists a "Joe Everyman who shouldn't go fuddling his mind with complicated stuff" (At least on my part.) I think what you are detecting is a little bit of inside humor from a few of us who kind of go "Uh-oh, another one of those threads" when its really the same few of us who rather enjoy them.......

Tedious geeks indeed..... LOL! (Guilty)

In summary to some of the above great information and advise, keep in mind those nodes, anti nodes, sound hole positions and "ideal" size calculations will vary with frequency and intensity of input and that every solution will be at best a comprimise over the intended range of use. And given the size and volume limitations inherent to a CBG there just arent that many options as far as acoustic performance. Michaels side soundhole point does work, but then I left one with no soundhole as an experiment, and I'll tell you it may not project quite as well as another, but yet..... Doesnt have much negative effect really. (I plan to cut a hole just to compare back to back when I get some time.) Get the top freed up and moving first and foremost, and take advantage of every square inch of surface you can.

I still have to recommend getting the fundementals of good construction dialed in for a well functioning instrument first, before getting too particular about some of these details. But it never hurts to expand ones knowledge. Unless you are spending so much time on gleaning information your building and playing suffers for it!

Hey, keep it fun!

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Permalink Reply by Ron "Oily" Sprague on November 7, 2011 at 9:27pm

Inside every "tedious geek" is a Joe Everyman.

Inside every Joe Everyman is...a mathematical physics engineering acoustical scientific genius, who learns with his hands.

Inside a box of Cracker Jacks is...a whimsical instrument that wants to be a CBG.

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Permalink Reply by David Ford on November 8, 2011 at 1:16pm

Well, at least I like physics! ;-)

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Permalink Reply by David Ford on November 8, 2011 at 1:49pm

I also have limited time and limited tools. So, trying to optimize my efforts.

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Permalink Reply by MichaelS Country Boy Guitars on November 8, 2011 at 1:51pm

Top (soundboard) tips to think about: Less junk on the lid lets the soundboard move free'er . Less hole = more soundboard to vibrate. Screw or glue the lid shut, it helps transmit the sound better, I don't know why but try it it works. More airspace in box can help, less neck or other stuff in the box = more airspace. Good hardwood neck will effect sound/tone and sustain more than you may think. Thin lid good, thick lid less good (no box is bad, just less good).

These are the results of a Joe Everyman learning with his hands, but I don'tmind when someone else gives the mathmatical reasons.

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Permalink Reply by Mark on November 8, 2011 at 2:06pm

Mark Bliss said:

Good response and points from Mark.
And no reason to be annoyed, the humor is not about " tedious geeks" nor any feeling there even exists a "Joe Everyman who shouldn't go fuddling his mind with complicated stuff" (At least on my part.) I think what you are detecting is a little bit of inside humor from a few of us who kind of go "Uh-oh, another one of those threads" when its really the same few of us who rather enjoy them.......

No offence taken. And I'm sorry if it sounded like I was being a spoilsport. I guess this place would be pretty sad and dull if there wasn't room for a bit of humour.

In summary to some of the above great information and advise, keep in mind those nodes, anti nodes, sound hole positions and "ideal" size calculations will vary with frequency and intensity of input and that every solution will be at best a compromise over the intended range of use.

The bit about every solution being a compromise is absolutely bang on. For any guitar, whether it's a CBG or some multi-thousand dollar custom shop special, there will be features that are approximations or compromises. Just no way around it in the real world.

One point that maybe worth clarifying though. The positions of nodes and anti-nodes don't change with frequency - they are features of the modes which are in turn a feature of the geometry and structure of the soundboard. I believe that what can change with frequency is the degree to which a particular mode is active. So it's possible that at one end of the tonal range a certain set of nodes and anti-nodes will be important and that at the other end of the tonal range a different set of nodes and anti-nodes will dominate. Perhaps it's a minor issue in practice but (being a self confessed tedious geek) I like to try to be exact if I can.

Thanks to everyone who pitches ideas into these discussions - I find this sort of debate a great help in motivating me to keep exploring this subject.

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Permalink Reply by David Ford on November 8, 2011 at 2:45pm

I suspect screwing it shut allows everything to sustain or transmit through the entire box, and not dissipate in some loose vibrating flap.

MichaelS Country Boy Guitars said:

Top (soundboard) tips to think about: Less junk on the lid lets the soundboard move free'er . Less hole = more soundboard to vibrate. Screw or glue the lid shut, it helps transmit the sound better, I don't know why but try it it works. More airspace in box can help, less neck or other stuff in the box = more airspace. Good hardwood neck will effect sound/tone and sustain more than you may think. Thin lid good, thick lid less good (no box is bad, just less good).

These are the results of a Joe Everyman learning with his hands, but I don'tmind when someone else gives the mathmatical reasons.

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Permalink Reply by Mark Bliss on November 9, 2011 at 1:03am

Sorry, I got totally distracted, Oily has me thinking of ways to get a resonator cone into a Cracker Jack box.......

I re-read my blatherings and see that I never said anything about nodes/antinodes moving per-say, just changing. The patterns may move around some however by the addition of bracing, neck mounting, drilling holes...... as opposed to before those additions.

I also forgot to add, (and said several times before the last times there were in-depth sound hole discussion threads) that in my opinion proper F-holes as used in traditional instruments (and some adaptations of F-holes in these home-made instruments) are located and function as much or more to effect the the sound board/top movement charactaristics than as "sound holes."

Ooh, that was a sticky paragraph, could have worded that more simply! Two bits of irony then I will move on. First: how Davids diagram looks like those used to describe (and predict) nodes. Second: I got a guitar newsletter simultaniously to the start of this discussion that was titled "The three biggest guitar myths" and myth number one? That the guitars sound comes primarily from the soundhole. Just a funny coincidence.

Bottom line, as interesting as all this is I still suggest and advise-dont worry too much about the soundhole question, other than making one way too large its a fairly minor issue. In fact (last thing I promise!) consider this one last thing. If you install a simple piezo disc inside the top, having no soundhole at all MIGHT reduce the tendency to squeel and feedback at higher volumes.

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Permalink Reply by Ron "Oily" Sprague on November 9, 2011 at 1:45am

MB,

Seeee the pretty shiny theeennngg...

>:-E

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Permalink Reply by Andrew Fowle on November 18, 2011 at 6:23pm

So if more soundboard freely vibrating = more volume, and more air inside the box = more volume, does cutting a sound hole out of it decrease volume? Is it purely there for tone?

What effect would it have to place a sound hole in the side as opposed to in the front (or not have one at all)?

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Permalink Reply by Ron "Oily" Sprague on November 18, 2011 at 8:19pm

Andrew,

That's at least three builds you have to make and play, to tell us the answer:

1) Sound hole in the side

2) Sound hole in the front

3) No sound hole

Yep, sounds like somebody (probably me...after Christmas) is gonna have to do the Torres thing, and build identical-seeming boxes as templates, with a detachable, bolt-on neck, and detachable tops and sides with sound holes in various places, and of various sizes, just to arrive empirically at what we all already know or suspect to be true.

Damn.

This is gonna be fun! >:-E Means I get to buy a db meter, heh-heh.

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Permalink Reply by Andrew Fowle on November 18, 2011 at 8:25pm

I figured it would be so much quicker and easier to ask folks who may have already made CBGs in such a manner for their feedback than to begin building three instruments from scratch. However, if I've inspired you to do so, please don't let me stop you!!!