Does box ornamentation effect resonance?

Replies to This Discussion

Permalink Reply by The Phrygian Kid on August 17, 2012 at 9:48am

Omg. I guess it's time to stop putting all that metal shit on my soundboards??
Thanks mate, someone had to say it one day :)

Ps I don't think soundholes are so much about letting energy leave the box more freely as they are about changing the shape of the resonating top. Look at a violin and try putting ur bridge between some soundholes....

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Permalink Reply by Ron "Oily" Sprague on August 17, 2012 at 10:32am

Vinyl,

Decoration glued, screwed, etc. to the box changing the box top resonance: technically, it does, but not so's you'd notice much. If you're talking about corner edging, then don't worry about it, as those are attached at nodes where the top is already attached to the sides, where the top doesn't freely resonate. If something like a sink strainer or grommets, then those are essentially sound holes, and their placement, size, and number can theoretically affect top resonance. If you're just gluing bits of sparkly gewgaws to the top, then yes, this will theoretically affect the resonance modes of the top. However, read on...

There's been numerous threads on CBN discussing top resonance, soundhole size, and soundhole placement in great detail (I know, I've contributed many electrons to 'em). Here's a summary:

Optimum soundhole size: yes, there is. It depends somewhat on the size of your resonating chamber (the box). For typical 9" x 7" x 1-1/2" boxes, something between the sizes of a US quarter to US 50 cent piece ( say 1 to 1-1/2" diameter) is sufficient. If you want to emphasize bass frequencies, then either go toward the smaller size (i've done some 1/8 -1/4 " holes a la Ovation, but located in the box side facing the player, on a couple of mine that worked well, but these are also 12" x 10-1/2" x 3-3/4" hand built boxes - bugger resonating chamber, more air volume), or locate the soundhole in the upper bout nearest your thickest string. Larger soundholes seem to emphasize higher frequencies, which may seem counterintuitive to most people. Which also brings up optimum soundhole location: again, technically, yes, there are some; there has been some empirical testing of this, which is ongoing. Most boxes I've built tap and tuning fork resonate really well about 1/3 to 1/2 of the distance in from the edge toward the center, with a dead spot almost directly dead center. Interestingly, that is where a center soundhole works well, for reasons that are too longwinded to go into here, but for which there has been research done on commercial instruments. The other places that resonate well that I described above are where soundholes work well, and are typically where most people place them.

The top thickness also comes into play, but that's a whole 'nother can of worms. Many builders report great sounding results by using the bottom of a commercial cigar box, because 1) it is thinner than the top, allowing it to vibrate marginally better, and 2) they desire to preserve the top graphics. Even paper covering on wooden boxes can very slightly dampen resonance, but many people build great gits out of paper covered boxes. I prefer the tone of exposed wood. The box in your pic looks a great size, but also looks paper covered; is it all wood? It'd be a shame to strip all that decoration off.

Scale length, string gauges, wound versus unwound, and open tunings further complicate the issue. Newbies often ask what the right string gauge for a CBG is, and most people come back either with the gauge or gauges they use on their own gits, or tell the questioner to experiment with different gauges and tunings to see what works best on the box in question. I am one of the latter.

Then there's bridge and nut height, placement, compensation, etc, that come into play to build a great sounding git. By this point, people start to get either frustrated, or want to become professional luthiers ;-).

It's really easy, as you can see, to overthink and overanalyze this, because ultimately these are just cigar boxes we're dealing with. Quite honestly, unless you just simply enjoy the geekiness of the stuff I described above, it's simpler to just build it. If the top (or bottom) of your box is sufficiently thin to vibrate ( assuming this is an acoustic build), then you should be OK. For your box, I'd seriously think about putting a mini humbucker in it; then all the acoustic fluff described above starts to go away. But that's just me.

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Permalink Reply by Randy S. Bretz on August 17, 2012 at 10:38am

lol

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Permalink Reply by Ron "Oily" Sprague on August 17, 2012 at 10:51am

Yeah, just read through the similar discussion on Chat. It does make you laugh. ;-)

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Permalink Reply by Ron "Oily" Sprague on August 17, 2012 at 10:40am

The Kid's got some great looking and sounding gits with metal shit all over 'em. And he's right about the soundhole affecting the shape of the resonating top; on a flat top, it affects the resonating nodes and modes, and on a curved violin top, it changes the shape to further enhance specific node and mode frequencies.

Listen to him, he knows his chit (metallic and otherwise).

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Permalink Reply by The Phrygian Kid on August 17, 2012 at 8:30pm

Aw thanks. But Ive never built up soundholes with added metal. I never want to say anything on anyone's pics cos it'd get real personal real fast as we've all seen before but I think all you grommet and bolt guys have lost the plot personally. But each to their own, for some the appearance of the thing is priority one and I respect that, it's an art with a pinch of science not the othe way around, so I don't think there's ever a definitive 'better' :)

Let us consider a hypothetical cbg for a second and let's start off with no soundhole. We have a thin rectangular expanse of woodLike substance and we fix it on all four edges to a rigid frame then we apply pressure (torsion) at a midpoint with strings via a bridge an get the whole lot singing.
Now lets imagine we punch in a circular soundhole between bridge and neck join. If youre seeing it like i am it should be apparent that cutting this hole creates a much different shape in terms of the resonating board, because the lengths of fibrous wooden material that make up the board are no longer longer attaché at each end to this rigid frame... The bits of wood upon which the bridge stands no longer reach the neck join right? Back up to before we drill the hole and cut a pair in between the bridge and the edges like violin holes... Again, you are affecting the connections between these wood Fibres significantly because the portion where the bridge stands is again somewhat decoupled from the four sided rigid frame of the box. In essence you are sabotaging the lids structural integrity... IMHO this is what soundholes are really about. Now do your grommets not somewhat undo all this work by intruding more 'rigid frame' to the thing? Perhaps they don't in that you can now cut even more or even bigger holes, but I suspect there's a point of diminishing returns where you're just reducing the size of the board.

Sorry to ramble, it might make sense to some bugger :)

(and it's still only half as long as an oily fool rant :p hehe)

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Permalink Reply by Vinylhed on August 17, 2012 at 8:36pm

I was with you right up to "Aw thanks..."

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Permalink Reply by Clock The Wolf on August 17, 2012 at 8:42pm

"all you grommet and bolt guys"

???!!!!!!???!!!!!!???!!!!!!

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Permalink Reply by Ron "Oily" Sprague on August 17, 2012 at 11:57pm

Yeah, it is. :-p But I do follow what you're saying. It fits perfectly with the reason for a center soundhole on a standard instrument: it's not there to "let the sound out," but instead introduces a resonance node at a point ahead of the bridge where the top flexes most. To utilize this, and also to control the location of various resonance nodes and their resultant frequencies, builders have resorted to various types of top bracing. The center sound holes on standard acoustic gits are designed a certain size to let the widest range of those frequencies be projected by the pumping of the soundbox's internal air volume by the soundboard. It's a speaker.

On a CBG, using a common box size, you're dealing with air volumes and top sizes roughly a third of that of a standard instrument (I could do the calcs, but I'm nodding off, and it's past 11PM here in Michigan...maybe tomorrow). A standard soundhole is roughly 3-1/4 to 3-5/8" in diameter, so the soundhole for a CBG should be...Uh-huh. About 1 inch, or slightly more, in diameter. Ongoing empirical studies with varying soundhole size templates, and theoretical Helmholtz calculations, show convergence around that number for typical resonant guitar string frequencies, on common cigar box sizes.

Metal grommets increase the rigidity of the soundboard at those points where they are let in, causing different resonance nodes in the top, as the Kid is alluding to. *sigh* It might be "easier" for you to look up Chladni patterns, Helmholtz calculations, read numerous published acoustic theory and empirical studies related to guitars, etc. But I guarantee, having done those things myself, it'd be a damned sight faster for you to just build the box you envision. I bet your customer will love what you come up with.

Or you could glue a bunch of bolts, nuts, and grommets to your ugliest first-pass-before-you-damage-the-nice-one box, and call it Frankengit >:-E. In typical perverse fashion, it would probably sound better acoustically than the pretty one that you carefully crafted. And keep it for yourself ;-).

It'd definitely look kewl if you were to wood-burn ol' Boris' face on it...

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Permalink Reply by RTZGUITARS on August 17, 2012 at 11:04am

You've open Pandoras Box Vinyl, Everything effect the volume thus eveything effect tone. How loud you can get a box guitar is always the issue. A cigar box has a small amout of air space or volume so it already limited, you wil never make an acoustic CBG as loud as a Gibson Jumbo. My personal view is metal does not belong on an acoustic guitar. Having said that some bridges trapeze, clam shell are part of a style and o.k. The more metal on a box  corners, grommits,bolts,drainscreens, add wieght and KILL VIBRATION vibration is volume and tone, I have said it before Martin and Gibson don't use bolts for Bridges and nuts they use material that transfer vibration a 3/8 bolt does not.

The history of the CBG is to use whats is laying around (to make something out of nothin) If you want an ornate guitar build it, put everything including the kitchen sink on it and we have seen that- Bedpan-

But if you want the best sound a box will give you, your gonna need alot of boxes, its endless.  

We got people on this site that make great sound with very little, Voicing is the most important part of building.

Were Gonna Need A Lot Of Boxes!!!!!!!                    

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Permalink Reply by Vinylhed on August 17, 2012 at 11:07am

Cheers, Kid!

Oily, I don't know where to start! That's a full and lengthy response, thanks for your time and expertise. The bottom and sides of the box are wood, but the lid is a little thick and I suspect just board. This doesn't worry me too much, as I've read comments to the effect that they still sound okay. I made a guitar for my nephew last year from a similarly designed box, and it sounded just fine acoustically (despite no sound hole) and through the piezo

Good point about the corners, I should have thought of that.

You're quite right about the dangers of over thinking things, but this is a commission, so I'm feeling the pressure to make a guitar that works well and looks the part. I'm not sure I'm up for cutting violin style sound holes, and grommets can cover up jagged edges. So, I'll probably add a couple of those. 

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Permalink Reply by Ron "Oily" Sprague on August 17, 2012 at 11:13am

Hmmm. What about a single cigar shaped soundhole...you can do a pretty nice job of intricate soundhole shapes, just by taking your time with a coping saw, and cleanup with files and sandpaper.

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