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How do You know what is the ultimate size of the soundhole,I have read about Helmholtz


heory but it doesnt give me any clue on how to achieve the right size.More volume in the Box-larger soundhole?
Please any helpful insights are welcome.
Amateur Swedish builder with sawdust in his gray hair..

A guitar does exhibit behavior similiar to a Helmholtz resonator, but not quite close enough to use his equations.
The size of the sound hole affects the "main air" (or Helmholtz)resonance frequency, smaller hole>lower resonance frequency. The body size, shape, and location of the soundhole also affect the main air resonance frequency, so there's no simple way to determine an optimum soundhole size.

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

Very generally: the larger the soundhole the higher the tuning of the air resonance. Conversely, a smaller soundhole will lower tuning.

Also, a larger soundhole will be less of an impediment to the various soundwaves bouncing around the inside of the box to come out, and might be perceived as louder. IME, smaller soundholes seem to lower the Q of the resonance as well as the frequency, making the overall frequency response smoother. I've not actually tested this - just a casual observation.

RE formulas: technically a guitar is a 'neckless' Helmholtz resonator. The formulas people use for building speakers etc will not work. I've seen the math for neckless helmoltz resonators before and it's scary calculus, and it wouldn't work right anyway since a guitar box is not a rigid vessel. However, there are a couple simplified rule of thumb type formulas floating around. I've used them to get in the ballpark when making experimental zithers and what not. For the most part sticking within the bounds of tradition will yield good results. Trial and error is a powerful tool.

Forgive my ignorance, but does a lower resonance frequency translate to a deeper sounding instrument? (All things being equal of course. Which they can't be of course. )

Yes, that is correct. Seems backwards to me as well!

edit: Well, maybe not a deeper sounding instrument, but one with better response in the lower frequencies. Maybe that's the same thing.

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

Not necessarily. In fact, probably not. A higher tuned main air resonance may well be perceived subjectively as more grunt. It depends, of course.

OTOH, lower tuning will help the low E string sound all the way to open E.

This is where "tuning" and "frequencies" and, well, guitar theory in its entirety just makes me want to pull my hair out. I read tens of thousands of words and can't make any concrete beliefs.

Nah, I knew it couldn't be that simple. I still wish someone would just write a bullet point list of things I should believe about relevant guitar building theory and I'll just settle for that for life! Do [this] and [this] will happen. etc, etc

You need to read the Gore/Gilet book(s). It isn't cheap or simple, but it does explain everything about how a guitar works.

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

They're on my list to Santa

I had some help with this problem on the mandolin board. It was something about sound hole size and placement. Using his calculation, I came out with a number and it is starting to tap with a ring.

Bob

Even just get the first book...

It's one of those 'on the one hand' sorts of things. Making the hole bigger will raise the pitch of the 'main air' resonance (usually, all else equal, all that stuff...). That might tend to make the sound less 'bassy'. OTOH, making the hole bigger also makes the low pitched main air resonance stronger (see above), so it might become more bassy. It comes down to a matter of balance: how much of what did you have to begin with, and how did it change.

The story goes that FDR once said he wanted to meet a one-armed economist, because every time he asked one a question the answer always started out with: "Well, on the one hand..." The perils of an inexact science.

Hope no one minds me resurrecting this thread... I'd like a little more information about soundhole size and its effect, namely, how much of a pitch change we are talking about...

To be honest, I've cut a rosette a bit too small, and it got me thinking. If, rather than the 65mm soundhole diameter I was planning, I could only have one of 61mm or so, how much of a change could that conceivably make? Based on the respective volumes of the baritone uke plan I used as reference, and the dimensions of the body I'm building, I reckon 63-65 is the kind of range I should be aiming for.

I know Alan has already said that a larger diameter *could* result in more bass, but I still have this idea in my head that with such a small body, a bit of a bass boost could be useful and that a smaller soundhole *could* help!?

Would +/-2 or 3 or 4mm be enough to drastically change [or deteriorate] the sound? Should a smaller soundhole be coupled with a soundport that could be opened out if need be?

Thanks in advance for any advice!

I changed the soundhole radius on a falcate-braced dred with a very responsive top. The soundhole was originally 4" and the bass was great (it's a dred) but the trebles were a bit much - to my ear there were way too many harmonics and made it kind of "jangly". I reduced the soundhole size to 3 3/4" and the sound became much more balanced. I was surprised that the sound changed as much as it did but I suspect that a guitar with a less responsive top may not be as sensitive to the changes. That said, it is only a sample size of 1

I may go back and increase the hole size a 1/16" to try to get a bit more of that treble action back but am undecided. Reducing the soundhole size is a real pain in the butt and I don't want to do it again.

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Steve Smith
"Music is what feelings sound like"

Yeah, reducing soundhole diameter doesn't sound at all like fun!

This is going to be a pretty responsive top... Well, it's going to be pretty thin anyway...
1/4"/6mm difference helped decrease jangly trebles. Noted! Thanks for that.

I know it could seem like I'm just trying to save having to make a new rosette but something is telling me this might have been a happy accident.

This is going to be tuned up to open G for the foreseeable future and it is absolutely tiny. My assumption is that trebles shouldn't be much of a problem. I always thought it would be the bass that might be lacking.

I don't know about taking a specific measurement from my experiment but I do think there are two things to be noted:
1. Soundhole size can make a difference.
2. If you're experimenting, Make it smaller first

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Steve Smith
"Music is what feelings sound like"

Sorry, yeah, I was going to type a disclaimer to avoid confusion but didn't!
It was just nice to hear your observation to put things into context a little.

Maybe another way to phrase my general question is: how much of a change in diameter do you need to make an audible effect?

Wright showed in his thesis, and others have said the same, that the exact pitches of resonances don't matter too much in people's perception of the sound except for notes close to the resonant pitches. That is: switching the 'main air' resonance from, say, 101 Hz to 105 Hz probably won't make a noticeable difference in the timbre, and it's possible that even a switch up to 120 would not. All of those pitches are between played notes, so the 'air' resonance won't reinforce any one note very strongly. OTOH, if you move the 'air' resonance from 101 down to 98, you may get a huge change in perception, because now you're at the pitch of G on the low E string, and you're likely to have a 'wolf' note. Then again; maybe not. I have a 'test mule' Classical that has it's 'main air' mode at 98 Hz, and even though it causes the fundamental of the note to 'warble, most people just don't notice until I point it out. In that case it the effect seems to be 'masked' by steady higher partials, and rendered insignificant. At any rate, having a resonance on a played pitch will tend to make that note stronger than the ones around it, even if the difference is not a problem.

About all you can do is try something and see what happens in your case. If you can characterize the resonant behavior befroe hand you might be able to make some predictions about wehat coul happen with a particular change, at least within limits. Reducing the hole size with some sort of insert can also give you a sort of reading: if making the hole 1/4" smaller moves you away from the sound you want, it's more likely that enlarging the hole will move it in the right direction. But maybe not.

Always happy to clear things up.....

Hmm... Clear as, well, clear as it can be I suppose!
Seriously though, thanks for that explanation, Alan!

Can someone just take the responsibility of the decision away from me and tell me what to do? Only kidding.

I suppose I should do some reading about baritone ukuleles. The guitar I'm building fits perfectly in a baritone uke case, but is a little slimmer, hence my guess of 63-65mm... Anyone experimented with soundholes that size?

EDIT: I just thought, having reread your first paragraph: Does that mean I should be ok making it a few mm smaller, as long as I avoid a whole note?

Problem is, you want know if it will be on a whole note till you build it.......

Gore gives techniques in his book for altering the main air resonance frequency without modifying the size of the soundhole.....adding mass to the perimeter, etc.

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Formerly known as Adaboy.......

Also check this thread, if you haven't already. (Data for a classical guitar, 4 DOF modelling and direct measurement)

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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

To determine the soundhole radius, aim for approximately half the soundhole diameter. This approach works well in normal circumstances, although subject to round-off error.

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Tim Allen
"Never hurry, never rest."

Thanks Daryl, I'll bear that in mind.
Thanks Trevor, I'm off to read that now!
And.. er.. Hi Tim.

Maybe I should cut a 60mm soundhole and leave myself up to 2mm for opening out with a knife. That should leave enough room to move 1.7hz from any problem note. (Excuse my terminology, it will all improve once I get my hands on a copy of "the book"!

I'm dying this top so I'll have to make all those adjustments before-hand. I assume the thin coat of oil won't change the resonant pitch too much.

I may glue in a patch for a soundport just in case I need one. Seem like a bad idea?

I reckon the pitch is going to somewhere around 155.56hz or E3 with a 64/65mm soundhole (not very scientific but "standard" baritone Uke is just slightly lower than that. So if I hit 155.56hz *exactly, would a 1.7hz change in pitch be enough to remove the wolf note?

So kind of everyone for helping!

Cheers,
Nick

The measurements and modelling were done on a full sized classical. Pretty hard to say how the sensitivities would change as things are scaled down. You only need to shift things about 2 Hz to get off a scale tone (if, indeed, you have built something with sufficient monopole mobility for it to matter), so if you leave 2mm, that should be enough. Unless you're really good with a knife, use a sanding drum (powered if you're confident (what I do), by hand if you're not) to open up the sound hole. Re-measure your resonances after every 0.5mm of diameter change and be sure to keep it round!

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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