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I'm thinking that if an OM model guitar is built to the depth of a dreadnaught, the soundhole size should be increased to 4 inches rather than 3 and 15/16ths inches.

Is there something obvious that I'm missing, or something deep and mysterious that I'm overlooking due to ignorance?

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Old growth, shmold growth!

You might have missed the fact that 1/16th is like, really tiny and probably wont make a huge difference. But what do I know?


Edited to add the one word in bold I forgot that changed the whole meaning of my sarcastic post.

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On a serious note, on my Koa dread that I finished in August, I was building it, and an all koa OM at the same time. One night while preparing to cut the rosette for the OM, I thought it would be a swell idea to cut both tops at the same time, so I did. Problem is, I forgot to adjust for the fact that the dread should have a bigger soundhole than the OM. I had both tops clamped to a piece of plywood, drilled my guide hole for each one, then proceeded to go back and forth between the two guitars cutting the channels for the rosettes. It was only months later, when I was about to put the finish on the all koa dread that I realized something strange was afoot. I measured the soundhole and sure enough, it was 3.75" across instead of 4". Well, there was not much room between the edge of the soundhole and the rosette, so I sanded off a tiny bit and left it. Total diameter when I was done was 3.80" across. I didnt want to make the space between the hole and the rosette too small. Might look odd. Anyway, sort of a small sound hole for a dread, but I think it worked out well based on the sound of the guitar.

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An increase in depth will lower the main air resonant frequency. An increase in soundhole area will raise it. Go from there.

Pat

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It's interesting that increasing the depth of the box does _not_ drop the 'main air' pitch enough to matter unless you really make it a lot deeper. Fred Dickens did an experiment on this about 25 (!) years ago, making a guitar that was 6" deep, and cutting the sides down by an inch at a time until he got it too shallow to go any further. The 'main air' pitch rose by (drum roll) 7%, iirc, just over a semitone. The reason for this is a bit complex, so bear with me.

We're all familiar with the 'Helmholtz' air resonance, the 'rum jug' tone, and the 'main air' resonance looks just like that. It's not the same thing, though: a real Helmholtz resonance requirs a box with _rigid_ walls. Actually, the 'main air' resonance we see on guitars is one half of the 'bass reflex couple' between the 'Helmholtz' resonance you would have had in a rigid box and the 'main top' resonance. This is similar to what happens in a 'bass reflex' speaker cabinet: the speaker drives air in and out of the port, and pressure changes in the cabinet push on the speaker. In the case of a speaker, you 'tune' the port so that the Helmholtz resonance has the same pitch as the 'free' speaker, to get the maximum coupling. When you put the cabinet together, you end up with _two_ peaks in the output, neither of which is at the frequency you tuned things to. That's because the coupling pushes them apart in pitch, and the seperation betweeen the two peaks is a measure of how strongly the 'air' and 'speaker' are coupled.

In the low range, the guitar works in much the same way, except that the 'free' top and the 'Helmholtz' resonance are more like a musical fifth apart to begin with. Still they're close enough to couple pretty well, and, on most guitars, the 'main air' peak and 'main top' peak are an octave or so apart.

So, making the box deeper with the same size soundhole does, indeed, drop the pitch the 'Helmholtz' resonance would have been at, but it also changes the strength of the coupling between that and the 'main top' resonance. A given top displacement causes less pressure change inside the box when it's deeper, and it's the pressure change that couples the 'air' and 'top' modes. With the weaker coupling, the 'main air' pitch is not shifted down as much as it would have been from the 'real Helmholtz' frequency. The reduced freuqncy drop just about balances out the lower Helmholtz' pitch, and the 'main air' frequency doesn't change to speak of unless you really alter the depth a lot; say by a couple of inches. Interestingly, what _can_ drop in pitch is the 'main top frequency, which is, of course, also shifted upward less than it would have been when you make the box deeper.

The end result, then, is that you changed the box depth, and all that happened is that the _top_ pitch changed. Confusing, isn't it?

@Alan
Sounds to me like you are describing a situation in which the enlarged box decouples the top from the internal airmass slightly, allowing a freer vibration and increased bass response as a result, since the reduced "cushioning" of the air allows greater monopole movement, and a drop in pitch to the top vibration accordingly.
Maybe?
In that case the enlarged soundhole would have a minimal effect, I think.
Might it result in a more "open" sound to open the hole by the 1/16th inch? I think in a bigger box more brightness would be a good thing, so long as the resulting tone wasn't too "scooped", unless you wanted a scooped tone.

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Being new to guitar building, I'm only getting about half of this talk. But, has anyone tried to gradually enlarge the sound hole of a particular guitar by increments and judged the sound or air frequency-thing?

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Adam

Pretty much as Alan said, and the sides and back are in the "equation" also.

Yes. Enlarge the hole and the air resonance rises. It's a good way of getting off a wolf tone on the air resonance if you happen to land there. You only need to move the resonance by 1-2 Hz to fix things. However, exactly how much larger you go depends on the strength of coupling with the top, back, sides etc. So if you try it, go gently. 1-2mm increase in diameter will get you off a wolf tone. Expect of the order of 1Hz change in the air resonance per 1mm change in sound hole diameter. These small changes don't alter the guitar's characteristic voice by very much.

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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

By wolf tone, you mean some sort of undesirable tone or dissonant frequencies, I'm guessing? If what you are saying is true, then wouldn't it be best to leave the sound hole a good, say, 1/16" smaller on the diameter and slowly sand the material away on a completed guitar to get it to have the "right" tones?

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Adam

Yes. There's a couple of conditions (at least) necessary to get a "wolf tone". The first is that you have built a guitar that is rather responsive (i.e. high average admittance aka high monopole mobility) and the second is that one of the main low order resonances happens to land on a scale tone. In the case of the main air resonance this would be, say, landing directly on G at 98Hz. At a resonance you have high admittance of the string energy into the guitar, so it just rushes in and you get a "bark" from the note and no sustain, because the string energy drains too quickly. It can easily happen for the main top resonance if that, say, happens to fall exactly at 185Hz, which is right on F#.
Don't worry, it's true! And yes, you could do that. You just have to make sure your rosette stays looking "right" as you do it and then pitch the resonance exactly between scale tones. It's one of the reasons I do rosettes like this http://www.goreguitars.com.au/main/page_10929_40.html

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http://www.goreguitars.com.au

Just to add o what Trevor said, some other parameters of the soundboard do seem to have an effect, besides the actual pitches.

I was just comparing two of my recent builds, one with an air pitch (strung) of 97 the other 98. Without strings the pitches are about 3 Hz higher.
The first one has a heavier, and also slightly less stiff, soundboard, than the other. It's basically dense, thin, less efficient spruce vs. lighter and thicker spruce. The main top modes are about the same, 200 and a bit.

The heavy one has a clearly worse 6th string G than the other. This is not a particularly easy fix as it is below 98 already, and I don't want to soften the soundboard anymore.

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I hope Alan Chimes in again I found the the hole size will do more to effect the guitar than depth. The smaller hole seems go provide more bass . Also the hole diameter of an OM or 000 14 fret Martin is 3 7/8 inch

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John Hall
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As Alan and Trvor state, there is more to the air frequency than just the holesize. I used a 4 1/4" soundhole on a 000 body and the main air frequency is still 98 Hz. I've read where some folks have stated an oversized soundhole may add a little power to the guitar. Some have stated it makes it less "boomy" so will mic better.

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Formerly known as Adaboy.......

John,

That's what I've found too. Seems it takes a large change in depth to get any audible change; you can get a noticeable change in the body resonance with a relatively small change in the soundhole size.

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

Enlarging the soundhole can do several things that can change the sound. I'm pretty sure Trevor goes into some of this in his book (I only read it once, and I'm a slow study), and some of it I'm less sure about than I'd like to be, so take this with the proverbial grain of salt.

The obvious thing is the a bigger hole allows the air to flow in and out more easily. That's one reason for the higher Helmholtz resonance pitch. There's less 'edge' in proportion to the 'area', too, so the loss is lower (one reason for the increased flow), and the 'Q' value of the 'air' resonance (and maybe the 'main top' as well, since it's part of the 'bass reflex couple'?) tends to be higher with a larger hole. This can make a 'wolf' worse, if the resonance happens to be at a played pitch. That implies that making the hole smaller might be a better solution for a 'wolf', but that's harder than enlarging it, of course. Anyway, in some cases altering the 'Q' of a resonance can change the way the guitar sounds, even if the pitch is not altered. In one instance, when I put a paper rose in a Baroque-style guitar I found that the pitch of the 'main air' resonance had not changed, but the drag of all those edges had dropped the 'main air' peak height and 'Q' down to almost nothing. The 'presence' and 'weight' of the bass had given way to a more 'lively' and 'Baroque' timbre.

One thing I'd like to investigate is the 'upper cutoff frequency' of the soundhole. As you make the hole larger it becomes better at raditaing sound at lower frequencies (think of it like a loudspeaker). At some frequency it theoretically becomes 100% efficient, and that's the 'upper cutoff frequency'. In theory, you might not be able to sustain a resonance inside the box above that pitch (which might be a good way to find it). It's also been pointed out (in relation to the similar 'coinicidence frequency' of a plate) that lowering this cutoff frequency can increase the output in the low frequency range. At any rate, I suspect that this has a lot to do with the 'brightness' of guitars with larger holes. I'll note that the UCF of the soundhole sizes we use puts them right in the range of the 'singer's formant', around 3kHz, where the ear is most sensitive, so this could have a lot to do with 'articulation' of the sound as well.

As always, plenty to think about...

Oh yeah...

Soundhole size does seem to have more of an effect than body depth for sure. Body depth alters the Q of the air resonance more than it's pitch. I wish Fred had talked more about that when he told me about his body depth experiment: I didn't know enough at the time to ask, drat it.

I made an insert for the soundhole of one of my test mules that had the hole off center. Putting it in with the hole 'up' vs 'down' made a detectable difference in the 'main air' pitch, for about 1/4" difference in actual hole location. Allen, in his article on air modes in 'American Lutherie' #1 talked about soundhole location and Helmholtz pitch. It's a bigger effect than you might think.

I'm just going to make the hole 4 inches instead of 3 and 7/8 inches and see what happens with the deeper body.

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Old growth, shmold growth!

You are talking about OM style guitars, correct?

Do you mean deeper body and larger soundhole in the same guitar, or some deeper bodies and some larger soundholes?

Have you done deeper body and larger soundhole in one guitar?

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Old growth, shmold growth!

For sure...

One gem of info that sticks in my mind is that "the optimum sound hole radius for maximum volume is one quarter of the radius of the sphere that has the same volume as the body". Don't ask me where that comes from, I can't remember, but I'd love the reference if anyone knows. It matches better for typical sound hole sizes in typical classical guitars than SS guitars, with the holes being too small rather than too large.

Regarding higher frequency sound: other than air resonances, for a sound wave to escape the sound hole it first has to establish itself inside the guitar, which seems to happen first when the half wavelength of the sound matches the body depth and this occurs at around 1.5kHz. If you average the frequency responses of lots of guitars so that all the randomly occurring higher order peaks in the frequency responses cancel each other out, you can see this lift in output at ~ 1.5kHz (and you can see the wave formations in virtual ripple tank experiments).

Alan, is this part of your "singer's formant" phenomenon?

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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: I think I recall seeing that formula on the MIMF but I can't bring back the name of the author or the name of the thread itself.
Tom

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

There was a big discussion several years ago and as a result I planned a 000 short scale with a 4 1/4" soundhole as an experiment. I'm finally getting the body bound now.

Todd, good to hear of your experience, that's encouraging as that is the effect I am hoping for. This is a stage guitar so balance is more important than volume. We'll see.

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Steve Smith
"Music is what feelings sound like"

I am relatively new at this I know ,, so take this witha grain of salt . The purple heart build I have is 4-3/4" depth with a 4" sound hole . I strung the guitar and tuned it and went to a friends who is also a builder . he played it and made recomendations ( " mainly the action" so I have some set up work to do . Also working on the frets a bit , not quite as level as need be .)

Anyway , main point is , the guitar sounded very good, good sound good sustain , good volume . I can see where the " precisness" of the bridge compensation makes a huge diffrence .
I cannot see where an 1/8" inch on a sound hole would make any noticable diffrence . The Volume of air diffrence you are moving is so small .

just my thunk . and again , im a newbie .

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Trevor:
'Formant theory' comes out of research on speech. As I understand it, if you plot out the spectra of a bunch of people saying the same vowel sound, you'll see that they all have peaks at the same frequencies. It's the frequency spectrum that differentiates 'o' from 'ah'. Sopranos, with a higher vocal pitch, have fewer partials within each formant than basses, and that's why it's sometimes harder to understand women than men, and children are even worse.

Consonants also have formants, often in that 2-4 kHz band where the ear is most sensitive.

It's been found that good soloists tend to produce more power in the 3 kHz range than 'normal' singers, and the same singers will have less power in that range when singing in an ensemble than when singing a solo. This 'singer's formant' is one thing that helps a singer 'carry' over the sound of the orchestra. Apparently, the only orchestral instrument that has much output in the 3kHz band is the violin.

It looks to me as though guitar soundhole size is chosen in part to maximise the output in that 3kHz range, which will help with 'projection' and 'clarity'. Maybe.

I'd be interested in that radius formula. Could it have come from Helmholtz? Maybe I can dig something out if I have time.

Thanks Alan. My rather badly posed question was about whether you thought the 1.5kHz lift fitted a singers formant. I think Jansson did a lot on singer's formants. I should go look it up! I've not been able to find any higher frequency "formants" as such in guitars, other than the 1.5kHz lift as previously mentioned and the 4.0kHz lift which I think is that coincidence frequency thing.

I've searched "On the sensations of tone" previously, looking for the radius formula. Didn't find it, but it's a big book!

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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

Maybe it was this post (about 2/3 down)--
Robert Callaghan - 02:21pm Aug 2, 1998
in this thread--
http://www.mimf.com/archives/body_size.htm

But still no book reference for the 1/4 nugget.

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David Malicky