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I built an over-sized baritone ukulele (22 inch scale) and the sound seems congested, almost a vibrato. The soundhole looks small on this body (65 mm dia) and the sound seems quieter than I would have expected for a mahogany sound-board. It does sound a bit bassy to me.

Is there a formula or ratio to estimate the appropriate area required for a given body volume. Could I augment the area using a soundport.

Any leads would be appreciated.

Bob

Yes, there is; but you won't like it.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

LOL

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Gee, Trevor, does it require the use of a special character set?

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

I think it's time for a 'no kidding' sound hole design thread

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Steve Smith
"Music is what feelings sound like"

All kidding aside ! Yea Wud said that ! , Ive wondered that myself . This is a good qustion that could use input from a pro !

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A close approximation would be would bea start.

Trevor:
This soundboard has falcate bracing so I was hoping for sound quality equal to my falcate tenor.

Todd:
Is "oh yea" measured in millimeters or feet?

Bob

Furlongs per fortnight

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Steve Smith
"Music is what feelings sound like"

I'm guessing it's in varas, furlongs per fortnight is a velocity, fer cryin out loud!

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

It is a velocity and also the units of measure preferred when I am running. You could try to clock me in mph but that would require scientific notation. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

but seriuosly folks.......
This might at least be a good place to start
http://www.phys.unsw.edu.au/jw/Helmholtz.html
(about 2/3 down the page re guitars)

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Dave
Milton, ON

heed helmholtz

Reparing a prewar Gibson bari uke, all mahog of course. Soundhole size is 2.91" (74mm). Mabey a good starting point?

The problem with all the simple formulae is that they assume a rigid vessel, which a guitar isn't. If it were rigid it would make no sound at all. So the simple formulae are not even close in terms of being approximate. Once you include the compliance of the top, back and sides you get closer, but also very complicated very quickly, and rapidly head towards a multi-degree-of-freedom model of low frequency guitar function, because the air movement (the "Helmholtz" effect) couples with the top, back and sides movement.

Chapters 1 and 2 of the Design book cover this stuff and the sorts of usable approximations you can make if you don't want to get into (too) heavy mathematics. But that's 150 pages.

So Bob, probably time to take the plunge...

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Trevor:
The simple formula puts me at the edge of my circle of cometence. The most practical approach for me involves a tapered water glass, some 80 grit sandpaper and re-tuning every hundred thou of diameter change, maybe more often.

They say that cutting edge technology is indistinguishable from magic. I will abandon science in favour of art.

Thanks for the help.

Bob

Trial and error has worked well on many occasions! Go for it!

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Yeah...but I keep cutting and the hole is still too big

Just save that hole and install it on the next (larger) instrument. I keep a whole stack of holes next to my bag of kerfs, it takes up almos zero shop space. I'll send you a few small ones if you PM me your address.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Willie Nelson came up with his own formula I think.

One of my students made a 12-string reso once that had a very small soundhole as part of the 'artistic' design. As I feared, it was choked. After a couple of experiments he decided to add a side port. He reinforced the area where he wanted the port, and started cutting away with a burr in a handshaft. He kept the thing strung, and just checked it every so often to see how it sounded. At first he got big improvements in sound for each step, but after a while it stopped getting better and got a little worse. At that point he stopped cutting, and made an insert to finish off the edge that took it back to the size that worked best. It was a bit of a pain to make, but sounded good and ended up looking as though he meant it, which is, after all, what counts. His next design featured a much larger hole...

I think its actually 20 rods to the hoghead...
cheers
Charliewood

I think you're right, but it's probably not adviseable to try to duplicate it.

Alan described a great method, and one that I've used on a mandolin's "F" holes, but would work just fine on anything. I built the mandolin with slightly smaller than normal F holes, then after having played it a few weeks or so, began to enlarge the F holes, with the instrument strung to pitch. As Alan noted, the improvements came quick and early, then tapered-off, then a period of no further changes, and finally a decline in tone quality. At that point, I simply bound the F holes and called it good!

You can easily do the same with a round soundhole, and just bind it if you go too large. Or if you have a 3 ring rosette, cut the soundhole to remove the first ring, and see where that leads to.

What I have found is that there -is- a large enough "window" where the soundhole size doesn't really take too much from the quality bank. Too small, and it'll sound choked, or like there's a bunch of sound inside the instrument that doesn't relate to what you hear out front; too large, and you lose much of the depth of the bottom end, and can kill projection. But the good news is that there's a plenty-large range that doesn't really gain or take away. For me, on most guitars, that means I can make the soundhole plenty large enough to get my hand inside... <lol> My personal favorite guitar was 4-9/16", and my next one will be at least that large.

Also, keep in mind that a larger soundhole, even if we bind it back to physically shrink its area, will also affect the soundboard's stiffness, and this was especially pronounced with the mandolin's F holes(and quite likely was a leading factor when the tone went backwards, as it never completely recovered even after binding), but even on guitars, I'll brace just a wee bit differently when sizing soundholes at their extremes.

Thanks for the help.
I sanded the sound hole from 2.583" dia. to 3.021" dia. in six steps. The most tedious part of the job was tuning the strings after each step. I did not approach the rosette (no rosette) but when I started sanding the end of the fretboard, I had to add a trace of oval to the sound-hole.

The change in tone followed Al's description except I stopped when it sounded "pretty good". The instrument looks more normal now and there is improved treble notes. The four strings sound more separated now.

I will post a picture if I can figure out how.

Bob

I wonder how Mario Maccaferri did it?

I seriously considered a Maccaferri style bridge on this instrument.

Bob