I'm currently working on 4 "paddle box" style guitars/strummers/whatever.  I'm at the stage of shaping the tops and trying to decide on sound hole size and design.  I've looked online at a lot of technical discussions on Helmholtz resonation and such, all of which is way over my head.  I'm looking for a simple formula to get a general idea of calculating sound hole size to box capacity (if there is such a thing).

The second part of this concerns soundhole shape and whether that affects sound.  Most flat-top guitars have round holes.  Arch-top and bowed instruments (violins, cellos, etc) favor f-holes, and then there are luthiers that get creative and carve very elaborate designs.  Is it just personal preference?  Obviously round holes are easiest to make, but I'm trying to take my builds to the next level in terms of both workmanship and performance.

Any input is greatly appreciated!

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Yep, that's me - data analysis glutton. But you're right - the rationale behind it is to see if there are some practical rules of thumb, that can then be tested against real builds. Must have been something they put in the Houston water supply in the 60's! I'll let y 'all know if there are any useful trends - and just as importantly, if there is nothing there. As Edison said when it was put to him that he'd gone down way too many unprofitable paths on the way to the light bulb: "Oh, I wouldn't say that. I now know 999 ways how not to make a light bulb."

I'm a big believer in testing multiple, high risk hypotheses on the chance that one of them might actually turn out useful. It frustrates my bosses no end >-:E
Don't want to spoil the party but affecting Helmholtz resonance is not the only effect of soundholes - and it may not even be the most important effect in terms of the overall sound of the instrument. Scientific research (I'll dig out the papers if need be) has shown that the biggest source of radiated sound energy from a guitar is the soundboard (ie. not the soundholes). The tone of the soundboard is intimately connected to the manner in which it resonates - and more specifically to the modes of vibration. Those modes are determined by the shape and dimensions of the soundboard - including the shape and size of any holes in it.

So I'm not saying Helmholtz resonance isn't a contributor to guitar sound - I'm just saying there's a lot more going on so don't get fooled into thinking it will give you a way to calculate a guitar's tone.

Soundboard vibration is something that's difficult if not impossible to determine by calculation. Observing and measuring the vibration of soundboards is tricky enough, typically involving laser holography. So unfortunately we're back to that old favourite trial and error - plus of course any wisdom we might draw from traditonal instrument design.

You are, of course, correct. Don't worry about any bubble bursting; I'm not trying to find the CBG Theory of Everything.There's certainly more to a successful acoustic / electric instrument build than sound hole design, and the thickness of the soundboard is paramount. You and I have read lots of the same papers;-). Only 3 reasons I'm doing this: 1) I can't find anywhere on the Web where anyone has attempted to collate this material for people who don't like math, for CBG-specific application - even the guitar sites where it is used kinda give you the formula, some discussion, and then leave it to the perhaps non-specialist reader to try to work out how to use it; 2) it seems like a fun thing to do that might actually have a weeny bit of utility - or it may not- I don't actually mind that much if it doesn't work, because at least when the question comes up for the umpteenth time, someone could point back and say, " Yeah, this was looked at, but it turned out to be blind alley;" 3) no one else seemed to want to ;-). Maybe this has been done by some of the commercial guitar companies, and if it has, and has utility, I can understand why they might wanna closely guard it. If it has been done and failed, I could see the same rationale applying.

Not trying to do a PhD on this stuff, but if this info did have some utility, it might save us from just a smidgen of trial and error - or alternatively, provide someone with another wild-ass experiment to try.

Well, and Hal did ask...
I'm curious if anything ever came of the spreadsheet?

I recently built a 3-stringer with a 3/8 thick soundboard and (4) 1/2 inch holes near the corners and it belts out plenty of volume. It is tuned to DAD and sounds awesome without the amp 

As long as air can move in and out of the box, the soundboard can sing, and string size is something you should play with until you find each guitar's "voice". 

Find where you want a hole. And drill or cut it. Start small and test drill bigger test till you like what you hear. Next time you know how big a hole to drill , but where sould you drill same place or try a new place your ears will  tell you what works.

The warmest tone comes from the round hole,,next best is th F holes,,,the black one is a bit dull sounding,,,thats what ive found with my first few builds



Mike -

Are all four guitars equal in all other ways? (Box size, wood thickness, scale, bridge height, string size, etc.?)  Do you notice any difference in volume or prjection?  Looks like there is a plate on top of the black box (lower right corner) that may be reducing the soundboard vibration accounting for the duller tone?  The black one has the most artistic ones - I may have to steal them, LOL.  Nice job.

yeah i like the sound holes in the black one,,,ill try them again,,,,,you never know,,






It looks like this thread came back to life, so I'll just add to it instead of starting a new one.  I'm trying to find a reference that derives the ratio of soundhole radius to the radius of the "equivalent sphere" for maximum loudness.  I've seen this ratio referred to as 1/4 in a couple of places, but the information is given with the caveat that "I don't pretend to understand the math, but I've been told, ..."

I've found a bunch of places where the Helmholtz resonance frequency is derived, various ways to affect it, fudge factors to correct for the fact that the guitar body isn't rigid, etc.  But I haven't been able to find a reference that derives the ratio for maximum loudness. 

I know these are CBGs, and "there are no rules", and there are all sorts of empirical ways to determine the optimal hole-to-box ratio.  So this is kind of intellectual curiosity on my part.  But there is a practical element as well. 

For any box, if the urban legend is true, there's going to be a soundhole diameter that gives the maximum loudness when you play it unplugged.  That soundhole radius to volume ratio will correspond to a particular Helmholtz resonance frequency, which, for best results, should be close to the pitch frequency of the second-lowest string on the instrument (at least that's what's done on regular guitars - it should certainly be a higher frequency than the lowest string, and biased towards the bass).  Which means that, for any box, there's a configuration of strings and tuning that will give not only the loudest, but also the best balanced sound.  It would be cool to know (sort of), in advance, what sort of instrument a particular box is best suited for if it's to be played unplugged.

Or maybe it's just intellectual wanking... :)  In any case, does anyone know if this urban legend of a ratio of 1/4 between the radii of the soundhole and the "equivalent sphere" is true?  And, if it is, can anyone point me to the reference where it's derived?


Hi all,

I thought the hole itself doesn't affect the sound of the guitar. It is more the actual top (soundboard) of the guitar that generates the sound by vibrating like a drum skin. Usually, up to 80% of the sound is created from two vibration points to either side, and just below the bridge.

If you want to see which part of your soundboard is generating most of the sound - sprinkle some flour or fine sand all over the soundboard and pluck some open notes. You can then see the flour/sand settle into areas where there is no movement, and be bounced away from areas with lots of movement.

The trick is then not to damage or change the areas of maximum movement on the soundboard too much, as this will drastically affect the sound of the guitar.

Then again, if your guitar is TOO bright and you want to dampen the sound down a bit, you can stick bits of blu-tack or plasticine under the soundboard at these vibration points to dampen them a bit. Classical guitarists do this quite often.

If you want more bass resonance I will design the sound hole smaller, and to decrease bass resonance I will increase sound hole size, regardless of sound hole shape. Keep in mind wood type, grain, cellulose density, and thickness also plays a role in tone, but to recap, smaller sound hole=more bass, bigger sound hole=decrease in bass.


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