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We're about to start our first 12 string.

I imagine adding height to the braces and leaving the top thicker to account for the extra string tension is fairly straight forward, leaving each about 25% above normal six string stats.

What I don't know is how to pitch the top. I foresee the top monopole frequency going way up due to the extra stiffness. The only way I could see getting the same pitch would be to mass load the top, which may possibly be accomplished by the extra weight of the larger bridge and more pins, but I'm not sure if that would be enough. Could be better to let it stay a half step higher.

Can anyone who has target freqs share any insight?

Uvhsam, Allen Carruth, Trevor Gore maybe?

Other insights and observations about 12 strings are welcome too of course...

What gages of strings and what are you tuning it to? What kind of sound are you looking for? Scale length? Box size?
Oh, wait, you want target frequencies...can't help you...

Good questions...

Light strings, 10-47

I'm thinking 24 3/4" or 25", on a J45 based plan.

Looking for a middle of the road sound for now, so I went with lutz over black walnut with ebony fingerboard and bridge.

I should indeed find out what it will be tuned to, hadn't considered that, thanks...

I know you dismiss the scientific approach but I embrace it, simply because of how much better my guitars sounded since I started mucking about with it.

I don't dismiss the "scientific approach", I just don't use it.
Don't build harpy style 12's either, so you wouldn't want my help. Light strings ,10-47 will probably get you to "concert pitch", so I guess I struck out. I know nothing about those...

Fair enough...

The game continues...

What size soundbox? Probably a non-issue on something like a dread or a jumbo, since the large size and high mass will tend to counter the effect of the higher necessary stiffness. On an OM or smaller box, then you might consider reducing the string-height-at-bridge, proportionally reducing the required stiffness, so you can brace it more like a normal guitar.

I'm not entirely sure all the effects the string-height-at-bridge has on sound, but for sure it amplifies the tension change signal of the strings, which is 2x the frequency of the fundamental. So it should make 6 string guitars sound more like 12 strings. But since 12's have actual strings to generate the 2x signal, then the tension change signal should be less important for creating a satisfying sound. At least in theory. I haven't built a 12 string yet. But when I do, I'll probably use 3/8" string-height-at-bridge, or a bit higher.

Here is one I finished about a month ago, which means nothing has imploded!!! its falcate braced, bracing is .350 tall and .200 wide , final freq responses are the main air 108 mhz, main top 186, has a fully compensated nut and saddle for each string, top is Adirondack spruce back and sides cocobola, it has a rigid back, this thing is loud, nut is 1 7/8 wide. nut and saddle are tusq , tuners are gotoh 510

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MK5acoustics.com

I pitch them the same as 6 strings. There's a few permutations to chose from, depending on what type of sound you're looking for.

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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

Well, it'll either be 170 or 180. But with all the extra stiffness from added material height, m not sure how the pitch could stay that low...

Two questions, Ed:

1) How does static deflection vary with stiffness?

2) How does resonant frequency vary with stiffness?

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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

I'm not absolutely sure what static deflection means. A google search was surprisingly unyielding. Lots of things about it, but no actual definition.

Is it correct to consider static deflection as being how much the top will move under string tension while not being excited (strummed)?

Static as I understand it means not moving, but deflection implies movement. But I'm going with static deflection for our purposes to mean how much the top can move under string tension, so...

1) static deflection reduces with increased stiffness (can move as much)

2) resonant frequency goes up with increased stiffness (smaller oscillations, so they happen faster)

That's the crux of my head scratching right now. In order to make the top structurally strong enough to withstand the extra string tension, it has to be made stiffer. If it is made stiffer, the frequency will go up. That's assuming the extra weight of the materials doesn't counterbalance the stiffness. I know it will somewhat, but not likely to the extent that it will be a wash. Which is why shaving braces drops the pitch even though it reduces weight. The stiffness reduction has a greater effect than the weight reduction.

So, we have next up the extra weight of the bridge, and the extra pins bringing the freq down. As well, the extra string tension, but will these combined factors be enough to counterbalance the increase in stiffness?

Static deflection is a short hand way of saying deflection under static loading conditions, which for a guitar is the top deflection (bridge rotation is one way this could be measured) under the string tension.

So let me put the questions a different way:

1) How much does the deflection under string load change if you double the stiffness of the top?

2) How much does the resonant frequency change if you double the stiffness of the top? (Think simple spring/mass system)

(The answers are where you'd expect to find them!)

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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

That's more or less how I understood it, which means...

1) doubling the top stiffness will reduce static deflection by half, I think.

However, all the added string tension will add somewhat to the static deflection, I think, but will it be enough to be a wash?

2) the freq would go way up by doubling the stiffness. Exactly how much, don't know. I'm sure the answers are in the book but written in the math I don't understand.

But I'm sensing that between the extra string tension, the extra pin weight, and the extra bridge weight, it may drop the freq enough to get things more or less in the same ball park as a 6 string...

One way to find out...

Page 1-20, Equ 1.2-14, and Page 1-22 if you prefer it in words:-

Resonant frequency is proportional to the square root of stiffness. So if you double the stiffness, the frequency goes up by a factor of ...

Work out the tension on the string set you want to use and figure out how much extra tension there is over a "normal" six string set and what you'll need to do to the stiffness to keep the deflections under control. Then have a look at this page (fourth guitar down) for some more ideas on reducing string tension/making a loud guitar. The one shown was very well received.

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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

Thanks very much Trevor,

I'll poke my nose into it later when I get the chance...

Does anybody build 12 strings with wider lower braces? IIRC once upon a time 3/8"w x 1/2"h would not have been unusual for the X. Seems like a reasonable idea for increasing strength without excessive stiffness. While they seem to be conflated and used interchangeably, strength and stiffness are not the same, as is obvious to the engineers on this forum no doubt.

Too much stiffness tends to make an already bright sounding instrument too bright.

Okay, I'm new, but increasing the width of the braces has way less impact than increasing the height. So if you want less mass and greater stiffness, then you want height rather than width.

Unless of course I'm completely wrong, cause as I said, I'm new

That's my point. I don't necessarily want greater stiffness, especially on a 12 string. Stiffness biases the sound toward the treble, which 12 strings already have in abundance. The soundboard needs to move, yet not break, hence strength is more important than stiffness - to a degree.

I guess I'm questioning the generally held assumption that less mass and more stiffness is always good. It isn't. We are making instruments not airplanes.

I'm having a hard time understanding the difference between strength and stiffness.

Strength is resistance to breakage or permanent deformation.

Stiffness is how much something bends/deflects when under a load.

It's very unusual to have a guitar break purely under string load. The deflections and cold creep get way too much of a concern long before failure. I run an example in the book regarding the use of spruce, western red cedar and balsa as brace material and examine the stresses on them and when they would fail, which points to why spruce is almost universally used as bracing material.

Going back to my last post in this thread:
the answer is 1.4 (square root of 2). It's possible to design a 12 string with a lot less then twice the tension of a 6 string (although if you string with 12s the tension is almost exactly double) so the resonant frequencies do not rise that much, to the extent that the larger bridge, extra bridge pins etc. add enough mass to get you back in the ball park in terms of the resonant frequencies.

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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

Thanks very much Trevor...