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Do any of you who build 12 string guitars use a lower string-height-at-bridge than on 6 strings? The common knowledge seems to be that the sweet spot for 6 strings is about 1/2"... but what makes it so? Is the static torque on the soundboard tensioning it like a bowed saw, or the dynamic torque of the string tugging longitudinally as it reaches the peaks of its lateral vibration, or some combination of the two?

Since the total string tension is doubled on 12 strings, that doubles the static torque... but the dynamic torque effect should be the same for each individual string. It seems somewhat logical to me, to do a sort of compromise... maybe strings 3/8" above the soundboard, so you get 1.5x the normal torque. Still need heavier bracing, but not a full doubling of the stiffness which would raise the resonant frequencies of the soundboard by a lot. But I've never built a 12 string, much less enough of them to study the effects of string height, so hopefully someone here can shed some light

Good questions.

This should be interesting...

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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

excellent question ...thinking outside the box ...I like it...

Some of your basic assumptions on string tension may be a bit excessive
Most players either use 10's for standard tuning or tune down two steps or more if they are using larger strings.
A set of 10's at standard pitch is 256 lbs wheras a six string set of mediums is 183lbs
So you are really looking at about 40% more rather than 100%

Depending on gauges, tuning and scale length, the tension on a twelve string is not double that of a sixer, more like 50 percent more. From the UMGF String Tension FAQ, most sixers are 165 to 190 pounds, a twelve can be 200 to maybe 300. Remember that most standard 12 string sets are something like 10 to 42 and many players tune down one or two semitones.

Whether or not you put the primaries or octaves closest to the saddle, the farther holes will have much shallower break angle - that may also change the rotational force on the bridge. I've built both pinned and tailpiece 12 strings with pretty normal (1/2 inch) height off the tops with absolutely no problems. The tailpiece instruments are ladder braced and of course have very little rotational torque at the bridge. They are very long scale, use very heavy strings and are typically tuned 4 or more steps down, giving about 300 pounds of tension.

My pinned bridges have a slightly thicker top, larger bridge footprint and bridge plate, third tone bar and are tuned to D - they seem to be holding up just fine.

Not a whole lot of discussion here. I hope my previous comments didn't shut things down.

In my 3 day guitar design courses, looking at how strings work, the static and dynamic forces they exert on sound boards, how they drive the soundboard to produce sound, how to design to take the torque, etc, etc. we look mainly at 6 string classical and SS guitars. Then, as an exercise at the end, I ask the questions you have just asked relating to designing for 12 strings. So I've heard 60 or so views of how to do it already.

~1/2" is still the sweet spot. If you use D'Addario 12s, the string load is almost exactly double - but very few people use 12s (huge sound if you do...). So the string load is quite a bit less than double. You still need the modal frequencies in the usual places, but that sort of comes out in the wash: go a bit stiffer to take the extra torque, adding more bridge mass (12 hole bridge) brings you back to where you'd normally be. No shortage of drive, because you have 12 strings, so the drop in monopole mobility is not an issue. Very few people play them up the neck, so short scales and 12 fret necks work very well. Here's a couple more ideas. (Scroll down the page until you get to the 12 string). There's also a discussion in the current GAL mag.

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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 for the opinions, everyone. It appears the consensus is that by whatever magic of coincidence, 1/2" is the way to go. Great idea with the short scale 12 fret, Trevor.

So now I know what I'm doing whenever I get to building a 12 string. But I did have a secondary reason for asking this question, which is to extrapolate the results onto harp guitars The situation may be different enough that it's not really a valid comparison, though. I think I'll stick with the 3/8" I've had in mind for my current one. Lots of variables... total string tension, frequencies of the strings, string height, soundbox size. Perhaps next time I'll try 1/2" height, but with 18" lower bout width instead of 16", to help support the low frequencies despite the higher stiffness. Depends on how the current one turns out.

there are some other considerations involved when going to a lower string height which you may or may not have taken note of, but neither you nor others have mentioned them.
:
these are not in any order of precedence, but include:

lower string hgt above the top will require a flatter neck/body angle to keep the string/fret clearance. this change, together with the increased string tension, will increase the structural loads which initiate deformation, probably leading to earlier and more frequent neck resets, or perhaps the first twelve to fit in the carryon test rack,

thinner bridge/lower saddle projection and break angle, leading to possible bridge splits and/or tonal degradation.

it seems to me that you are designing to guarantee the occurance of what you are trying to avoid...

The change in top loading caused by changing neck angle over the normal range is negligible. (Less than 0.1%). The change in torque loading of the top caused by reducing the string height above the top at the saddle position is ~8% per 1mm reduction in string height. Much more significant.

Lower saddle projection will reduce the likelihood of bridge splits (depending on where you put the saddle slot) and the break angle depends on where you put the bridge pins, but you don't actually need much break angle (I think Al Carruth determined that anything over ~6 degrees was fine and tone was essentially independent of break angle above that limit). The smaller the break angle the lower the loading on the front of the bridge.


I doubt it...

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au