Official Luthiers Forum!

Howdy folks,
I was wondering how you decide what size soundhole to use on any given guitar.
More specifically, how big of a hole would you use on a top with the following dimensions:

Length: 19-1/4"

Upper bout: 11-1/4"

Waist: 9-1/2"

Lower bout: 14-3/4"

Thanks in advance,
Phillip

_________________
Regards,
Phillip Patton

http://www.pattonblades.com

The bitterness of poor quality lingers long after the sweetness of low price has faded.

https://hoosierbladesmith.wordpress.com

I think, more importantly you need to know the volume of the box. Alan Carruth has commented before on effects of soundhole size on "main air" resonance, but as I understand it you determine the volume of the box and then find the sphere with the same volume. The radius of the soundhole should be 1/4 the radius of the sphere, for the Helmholtz resonance to be maximum. (...don't have any first hand evidence to back this up.)

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Dave
Milton, ON

Martin uses 3 3/4" on their OO guitars, which are in the ballpark of yours. But there are other factors, especially the stiffness of the braced top and back -- their main vibration frequencies affect the 'main air' frequency. Guitars usually sound best with a 'main air' frequency in the range of ~90 to ~105 Hz (~F# to ~G#), so the low E string notes have some oomph. A more flexy top or back will result in a lower 'main air' frequency. Smaller holes give a lower 'main air' frequency (sizing is best done experimentally, as it takes some pretty sophisticated modeling to make accurate predictions -- Helmholtz's formulas are for rigid spheres). I recently finished a GA built lighter than I've done before, and the 'main air' freq came out pretty low (~86 Hz), so I enlarged the soundhole and added a sideport to raise it.

Also the depth of the guitar is a factor, as that affects the internal volume, tho IIRC, Alan Carruth has data that shows depth has complex effects (I don't remember the specifics, except that it can end up making little difference to the 'main air' freq). Other factors are the kind of sound you are looking for, ease of reaching through the soundhole, and aesthetics.

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David Malicky

3 3/4 is 95mm. OM are about 98mm. Gibson L-00s which have a wider and more depth have about 100mm. OMs which are even larger are only 98mm however. It is maybe interesting that a classical guitar which is pretty much the same volume as a 00 (at least at the quick look), has a hole of only 3 1/3 (85mm). Classicals use 90mm only when the box is at its max, not far from an OM.

I've been wondering about all this since I want to build a pseudo 00 using my classical mold. I guess I will use the standard MArtin hole size, after I'll try to figure out the volume with more precision.

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Build log

Definitely don't want to dispute what has just been said. But we went through this same discussion in great detail over at the MIMF not too long ago and I came out convinced that I didn't need to obsess about it and that it made a lot of sense to make it big enough to get your hand inside. Time will tell but for this and other factors I'm trying 4 1/4" on my latest OM. If it's a bust then I'll bind it

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Steve Smith
"Music is what feelings sound like"

Philip: Measure your forearm diameter,add 1/2 inch, transfer measurement to appropriate place on top. Gives you a bit of room to get at the braces to do some tuning after the bbox is together. On a slightly more serious note 3.750 to 4 inches should work fine.Have fun.
Tom

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A person who has never made a mistake has never made anything!!!

smaller hole will allow more Bass ,larger hole kills bass. 3 3/4 would be a good size .

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Would a maple b&s guitar get more bass from a smaller hole?

Jeff

Thanks for all the replies, fellas! Sounds complicated.

I just finished a guitar with the soundboard dimensions I mentioned, and the soundhole is 3-1/4". I've noticed that the bass strings are kind of muted, or muffled. Or just not as "chimey" as I'd like. The body is fairly deep, 4-3/8" at the tail end, and 3-3/8" at the heel block. Sounds like it would benefit from expanding the soundhole and/or adding a soundport. Yes?

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Regards,
Phillip Patton

http://www.pattonblades.com

The bitterness of poor quality lingers long after the sweetness of low price has faded.

https://hoosierbladesmith.wordpress.com

So, what would a bass string sound like with a bigger hole?

_________________
Regards,
Phillip Patton

http://www.pattonblades.com

The bitterness of poor quality lingers long after the sweetness of low price has faded.

https://hoosierbladesmith.wordpress.com

Dave - when I tried that formula on the OLF OM it gave me a soundhole diameter of very close to 3 inches. That's quite a bit smaller than the drawing show. Where did you see this formula? Is it supposed to apply to steel string guitars?

Phil

It's hard to say for certain without knowing how heavily or lightly built your box is, or measuring the 'main air' frequency (not at all hard). Your weak bass could be any of 3 issues: 'main air' resonance may be too low, or it may be too high, or the hole is just so small that it's not letting much sound out. Given that most OOs use a larger hole than 3 1/4, enlarging is probably in the right direction.

Here's an easy way to check the 'main air' frequency: hum into the soundhole. It will hum back to you at the 'main air' freq. Check the note of your hum, hopefully on the low E string.

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David Malicky

There is NO way you can calculate the main air frequency in advance of making the guitar. With some experience you might be able to predict it, but that's a different animal. The exact frequency doesn't matter, except that notes close to that pitch will tend to be stronger, and notes below it will tend to have less energy in the fundamental than you might like. The best a beginner can do is either copy an existing design, or try something and see if it works. It never hurts to make it a little small, particularly if you make the inner rosette ring a good distance away form the hole to allow for later enlargement, just in case.

Dave Stewart & Alan -listen to what they have written .

Mike

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Mike Collins

Well, I tried the humming-in-the-soundhole method, and it didn't seem to work. Probably operator error... But I did try tapping on the bridge with my finger while muting the strings, and it registered on my chromatic tuner as G#. I added a soundport to the upper bout, and the "tap note" is now between A and B flat. The bass notes sound noticeably better, and the G string sounds MUCH better. I think I'll expand the soundport a little, and see if I can get the bass strings to improve some more.

Thanks again for the input everyone, much appreciated. Carry on.

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Regards,
Phillip Patton

http://www.pattonblades.com

The bitterness of poor quality lingers long after the sweetness of low price has faded.

https://hoosierbladesmith.wordpress.com

Phillip: Check the MIMF and see what folks there have to say and read Mario P. in particular.

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A person who has never made a mistake has never made anything!!!

Great to hear of the improvements. Yes, tapping with a chromatic tuner is a good method -- that will home in on the 'main air' freq. A to A# is pretty high (~113 Hz) -- that's not to say a larger hole won't improve the overall sound, but the lowest bass notes may start to fall away. If so, a further step would be to shave braces and/or sand the top/back, which will bring the 'main air' freq back down.

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David Malicky

I'm pretty happy with the way it sounds now, so I don't think I'll shave the braces any. But I will try to do a better job carving the braces on the next one.

Here's another question: Would a steel string need a different size hole than a nylon, assuming the same size body?

_________________
Regards,
Phillip Patton

http://www.pattonblades.com

The bitterness of poor quality lingers long after the sweetness of low price has faded.

https://hoosierbladesmith.wordpress.com