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I've been digging into the "main resonance modes" for about a year now;
Why is a dreadnaught sound hole ~3.90" diameter?[/b]

The main air resonance of 6 of my dreadnaughts [ T(1,1)1 ] is so very close to 6th String Open E (82.4hz) that it dampens that note, and some of the fretted octaves of that note. With the body sizes being a MOL common factor (that is; the air volume) why is that 3.90" [edited] sound hole size become a "standard"?


Guitars I measured (sound hole with calipers, frequency with REW analyzer):

• J-45
• Mexican Martin
• Yamaha Dread
• 3 "Borum-built" dreads (of course the same hole size, specified by my plan/ drawings)

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Measure Twice,

Karl Borum

My 1974 D18 and 1980 D12-28 both have 4 inch sound holes which I thought was standard. There are also the various large sound hole D-28's, usually 4.25, credited to Tony Rice. There are some discussion at UMGF.

I haven't bothered to measure the main air resonance, I mostly build OM's and use 3-7/8.

4.9 seems like a pretty big sound hole, and would raise the main air frequency considerably. How confident are you in you REW measurements?

I guess I could take a spectrum (I have Alan Lambert's software) of the D18. Its tricky to get in a form that is easy to post someplace like this (a jpg or pdf) but I certainly mail one to Karl. I assume you are just measuring the response to a tap on the bridge

EDITED in OP: 3.9" - thank you for catching that, 4.9" would be a monster.
Very confident in REW measurements- I shelled out $100 for a mic with a calibration file and validated REW frequency measurement with an independent tone generator application.

I also did a repeatability test, performing 3 different acquisitions on the same guitar and overlaying the graphs. The frequency capture was virtually exactly the same, frequency wise


Yes, tapping on bridge, 32 samples, measuring mic distance, mindful of mic placement directed at the bridge, quiet area, striving to make all the measurement criteria equal across samplings.

Other plots builders have plotted of D-28 styles have same 83hz peaks, yet some "book" references are 96-100 ish hz. Cant wrap my head around the relatively large variation between plots online and values referenced in books (p2-16: ... [i]Volume 1: Design; Trevor Gore) et. al.

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Measure Twice,

Karl Borum

Don’t forget depth of body plays a role as well…

OK, I just took a bunch of samples of my old D18 (and yes the sound hole is 4 inches. I pulled the strings off, tapped the bridge with a little "hammer" made with an eraser stuck on the end of a wire probe (its my standard). Alan's software. Guitar on its back on a towl.


Dominate peak is 176 hz (F3), othe peaks at 218 and 228. Very consistent. Did two taps on its side, one was 197 hz, the other 176.

I had always heard that dreadnaughts have an air resonance right around G3, this is a little lower but certainly lower than the OM's I build (G# or A).

Hope that helps, I could always try to save this to a file if you want to see the whole spectrum. Let me know

The numbers are only correct measured with the strings on and up to pitch, an ideally in playing position.

I know the graph is rather inclusive, so to add clarity, the question is concerning the main air resonance frequency- T(1,1)1 - the first peak, and its relationship with a dreadnaught's sound hole diameter. The two things that significantly contribute to that frequency are the volume of the box, and the diameter of the sound hole. As the volume of the box is fixed, the question relates to the diameter of the soundhole, and why is 3.9" so "universal".

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Measure Twice,

Karl Borum

In that graph, is your main top resonance 155.7hz, and your back at 196hz?

Just to confirm - you’re asking how the sound hole size influences the frequency, correct?

I’m going to ask a dumb question here because I only understand this stuff enough to populate the spreadsheet and thickness my tops, but….the 4” sound hole comes before anyone was tapping anything with bouncy balls on bamboo skewers.

In a parallel timeline where everyone universally loves a D shaped guitar with a 2” sound hole the formulas and target frequencies would be in an effort to recreate that config.

Right? I mean, the whole thing here is working backwards from guitars that are universally accepted as great guitars and the math is derived from them not the other way around. So 4” or 3.9” is universal not for anything to do with frequency analysis, material properties, or the formulas in Gore’s book but because of decisions made by luthiers and instrument companies.

Again, all the math is way above my pay grade and as usual I’m probably looking at it way too simply.


Sent from my iPhone using Tapatalk Pro

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That is correct, 191hz back, although I had not considered the back active.
Top resonance 155.7

Side note, I probably removed too much stiffness from my braces.

Main pain is the air resonance too close to Open E 6th string
AIR Resonance 83.5
E note: 82.4

Can of worms I've opened here with that graph.... and I'm exposing my poor knowledge of brace carving and top stiffness to the group.

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Measure Twice,

Karl Borum

My question was really just why is the dreadnaught hole 3.9" in diameter.

The question was motivated by playing the guitar up the neck and the 12 fret, 6th string, volume and sustain just died. I also notice my tuner needle wanders on the bass E string. I believe it is because the air resonance is too close to the 6th string open E. I've discovered several dreads I have do that, not just my builds.

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Measure Twice,

Karl Borum

Can of worms I've opened here with that graph.... and I'm exposing my poor knowledge of brace carving and top stiffness to the group.[/quote]

Asking questions and making mistakes is how we learn -

Thanks for asking - I’m learning something.

M

No surprise your air resonance is low compared to what it ‘should’ be in the books. Your top and back are very low too. For a dread the top ‘should’ be about 170 and the back 215. If those numbers were present, it’s likely the 96-100 would be there as well. 15hz discrepancy in the top is a lot. 20hz on the back is a lot.

I would measure your factory made guitar again, I would very very much doubt them to be in the 155 top range.

Anyway, to fix the guitar with the confused E string, punch in a sound port to raise the frequency of the air chamber to decouple it from the main top resonance.

On a side note…while the measuring apparatuses have come a long way, the fundamental understanding that the top, back, and air chamber have fundamental resonant frequencies has long been known, and it has long been known that having the top and air chamber couple is a bad thing. The size of the sound hole has been worked out by trial and error over time to accommodate most ‘averages’.