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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 19, 2011 at 3:44am

Hey Oily, I have about a dozen Nodes I would be glad to loan you if you want to try those in different places as well!

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Permalink Reply by Ron "Oily" Sprague on November 19, 2011 at 4:35am

Mark,

You tell me where you want me to put 'em, and I...

Wait. Ermmmm.

Please send me an 8 x 10 color glossy photograph with arrows and diagrams and caption explaining what each one is...

Sorry. Arlo moment.

Draw me a pic with your preferred node placement (oh God, what have I done...)

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Permalink Reply by Ron "Oily" Sprague on November 19, 2011 at 4:37am

Andrew Fowle said:

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!!!

Andrew,

It would. I was only half-serious. It's the other half that worries me.

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Permalink Reply by MichaelS Country Boy Guitars on November 19, 2011 at 5:23am

Soundhole in the side works well, it leaves the top unaltered and it projects the sound up to the player, if you put it in the top side. Cutting a soundhole anywhere will not change the inside volume, that is fixed by the size of your box. Now go make 3 guitars and have fun with them.

Andrew Fowle said:

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 David Ford on November 19, 2011 at 9:37am

I think all Andrew was asking was what you node. So, tell him what you node.

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Permalink Reply by Mark Bliss on November 19, 2011 at 1:22pm

Every time I write what I node, I gnu something new, then what I node makes me feel like a toad, since what I node was old and what I know now is gnu, and what I write wasnt right!

Anyway, I think its been beat really. I have thrown my opinion out there again and again on this. In summary, It just doesnt make enough difference to make too big a deal of it in my opinion. Some improvement in sound projection at certain frequencys, Dont make it too big, as the soundboard is your best volume producer. The side or top is an option, but then only you might hear the difference.

Dont sweat it too much, build it, play it till its thread bare. Have a blast, thats what its all about. I can sympathise with the desire for knowledge, I want to know too. But there is no better answer available. Just too many variables for even a reasonably correct evaluation, even if you tried to build three or more identical test specimins. I could blow the dust off my DB meter, and do some testing. At best it would be a time killing distraction in my opinion. And I have far more opinions than time. I'd far rather experiment with a violin style top plate, with f-holes, bass bar and tone bar and see what happens. Just sayin'........ archtop CBG or something way out there. Time to go back to the bridge placement thread and dish out some funky ideas.

I strongly suspect your best sound hole bet is the typical smallish ( think proportionally) hole in the corner is a good comprimise. Otherwise go for a good "look." Think more "port" than soundhole maybe. Dont obsess over this, its just not that critical!

Damn, now I sound like Michael. (The watcher)

Appologies for the spelling, I have an excuse, and now I am going to go look for it........

Regards and as always....

Have fun!

Mark

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Permalink Reply by Andrew Fowle on November 20, 2011 at 8:26am

Guys, I appreciate that for many, CBG building is done purely for the fun of CBG building. Not much knowledge is required to make something you can play and enjoy, and a good way to understand what works and what doesn't is to make lots of them - point taken. However I have neither the time, space, money, resources, nor inclination to go to the trouble of making a whole bunch of instruments to answer one question about why CBGs (which others have already made) were made the way they are.

I used to ask these sorts of questions on luthiers forums but all I got was a good talking down to for not being a luthier.

Michael, so you are saying that a sound hole is a way of directing/steering the sound, and that putting it in the top side directs it towards the player?

And I'm still a bit confused about whats been said about the reason for having a sound hole. I initialy had the impression that it was there to let the sound out, therefore being the source of the instrument's "loudness". I've since learnt that most of the sound comes from the sound board, and that cutting a hole in it is likely to actually diminish its ability to reproduce sound.

So I guess what I'm really asking here is:

"Are sound holes there to increase volume (loudness), or are they there to affect the tone in some way by changing the way the soundboard moves?"

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Permalink Reply by Mark Bliss on November 20, 2011 at 1:15pm

One of the biggest "myths" of acoustic guitars is that the sound "comes from" the sound hole.

"Are sound holes there to increase volume (loudness), or are they there to affect the tone in some way by changing the way the soundboard moves?"

The simplest and shortest way I can explain it is to say that everything varies. Depending on frequency and amplitude the top, back sides and sound waves themselves can do anything from compliment each other to basically cancel each other out. Therefore there are frequencys and amplitudes where a hole does little or nothing, and others where a hole may free up the top and minimize these "cancellations". It is because of this that a hole can NEVER be the perfect size or solution, and will ALWAYS be a comprimise. My previous comments and advise are based on the thoughts that keeping in mind that the hole eliminates valuable surface area, and also effects the tops freedom of movement and natural frequency.......

There are just too many variables to give you a pat or standardized answer beyond what I have already said! And given the limitations of the acoustic properties of these small bodies, it just isnt going to have that great of an effect, as opposed to a concert jumbo sized flat top or something where there is a great deal more room for "idealizing" the combination of things.

As always, JMHO!

Mark

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Permalink Reply by MichaelS Country Boy Guitars on November 20, 2011 at 4:12pm

I think the simplest way to say it is that the sound hole is there to allow the air in the box to move in and out, like an air pump if that makes any sense. If you have no hole it will still work but opening a hole allows the lid to move free'er. But as Mark points out our little box's probably don't move enough to really benefit either way. If you play a cbg before you cut the holes they usually sound almost as good (some better) than when you cut or drill a hole. You will be safe if you make a nice looking f hole or small round hole in the face, just don't make a giant hole and you will be OK.

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Permalink Reply by David Ford on November 20, 2011 at 4:45pm

put a couple of small holes today in the side that faces up towards me (where did I get that idea, Michael?).

Anyway, I had to lean close to hear any difference over the whole sound of the box. What I did hear was a more.....don't know how to say it.....acoustic.....sound.....free air environment sound. Real sound. The sound of just the box top, while leaning close, seems constrained in comparison. It was like the difference between listening to a cd and then listening to someone play live. Though the quality was very indistinct without leaning in.

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Permalink Reply by David Ford on November 20, 2011 at 4:50pm

Just listened to it again. I can hear more bass and fuller range of tone out of the hole than just coming from the box top.

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Permalink Reply by Andrew Fowle on November 20, 2011 at 7:35pm

Ok, ok, I think I get it now. The hole itself is only to free up the movement of the soundboard, and cutting said hole could potentially affect tone, volume, both or neither. The reason one cuts a hole in an acoustic instrument is in the hope that it will do good things for the sound board, not out of a hope that the hole itself will act like some kind of invisible speaker cone.

How am I doing?