Structural Question


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2 hours ago, Bill Merkel said:

On the practical level this nose dived to,, I have this crazy theory.  If you want to make good instruments, simply copy, unless you think you're going to better the archetype, which is by definition impossible.  You don't know more about what's desirable in an instrument than centuries of archetypal musical artists either.  In fact you probably came to violin world late in life, know next to nothing about music, and don't take it nearly as seriously as you think you do.  So use your remarkably finely honed skills to make copies.

I try to copy process not specific instruments but the rest of that fits!  :D   I have a theory that the back functions to some degree as an amplifier since rubbing the edge of a block of wood the sound seems to come just from that one point but on a hollowed out plate seems to radiate from the whole surface.   

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Because they built with repeatable structured geometry, and varied things by varying the geometry recipes, their 'trials and errors' accumulated into a constructive evolution of the design.

 

200 yrs N Italy bowed string making in very traditional repetition with only slight variations and slow evolving design

>>>> Andrea Amati and violins

150 yrs more of Cremona making in very traditional repetition with only slight variations and slow evolving design

>>>>>> Strad and Del Gesu

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3 hours ago, Shunyata said:

The idea of mass affecting the vibration makes sense, although I suspect the added stiffness would far more impact than the added mass.

At the risk of post hoc rationalization, a stiffer back would tend to keep vibration in the top plate (as would added mass)

You would guess those things. But does that make them true.

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As a general observation, combined bending and shear forces in the top plate would be higher at the bridge and near the ribs, and lesser everywhere else. So the tendency is to make these areas thicker to lower the stresses and avoid possible fracture or creep failure.

How thick is thick enough? That would take a sophisticated finite element stress analysis to determine. Luckily, we have a few hundred years of "common practice" to draw on so computer modelling seems less compelling.

But if someone has a few grand  in their bank account and are willing to part with it, I'd  be happy to do the computer analysis. >grin<

The roll of plate thickness in the back plate is more mysterious to me. I am inclined to think of the entire back as, ideally, a vibration node point.  One wants the back plate thick enough to reflect most of the vibrational energy into the air cavity and top plate. The player might enjoy the sound coming out of the back plate, as well as the orchestra members sitting behind the performer, but it seems like an overall waste of energy for the audience.

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My point is that under the right circumstances, repetitive trial and error will evolve a solution that's actually very hard to beat.

When google turn on its AI to learn chess, they gave it the rules.  Through massive repetition it evolved it's approach (learned) and in just hours time became the strongest chess playing entity the world has ever seen.

It seems, that Northern Italy since circe 1350, and Cremona especially from circa 1550 some how had the right cultural circumstances that their designs for string instruments developed by a kind of 'evolved' rooted in trail and error, and supported by the nature of their traditions.  The same cutural evolution of very conserving traditions seems likewise to have evolved their cheese, cured meats, and wines very well also.

So, as Don said early in the thread, the back mass 'is what works'.

It's not reasonable to believe that one of us is going to correct or improve the design that evolved in old Cremona by removing that extra back mass or by making its max point at the center of the plate because 'we don't see why it is the way it is'.

Nor are we likely to improve the total overall balance or success of the instruments by tweaking one isolated physics parameter.

It's not too different from think to oneself, 'Ah, I see, a knight can attack two pieces at once, but only one can escape!', and believing that one single simplistic insight will enable you to beat Google's chess A.I.

 

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1 hour ago, David Beard said:

My point is that under the right circumstances, repetitive trial and error will evolve a solution that's actually very hard to beat.

And yet, there are differences.  Strad and Guarneri came up with distinctively different ideas, as did other makers.  I think of it as different settings on an acoustic processor with lots of knobs.  Copying (assuming you know how ALL the knobs are set, which you really can't) would only get into one setting.  My goal has always been to understand as much as possible about what all the knobs do, and so be able to tailor sound and playing characteristics to a particular desired result.  A lofty goal... but I need something to keep me entertained.

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Just my opinion :

It not unusual to see soundpost cracks on backs.

Then to avoid this a logical improvement is to increae the thickness in the the middle area.

The middle is a better choice than the sound post area  for résonnance reason - it is the sames for all tables, drums,  woofer. : the mass must be symetrically to have a good resonance.

then the common graduation combine stiffness and resonance compromise.

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4 hours ago, Don Noon said:

And yet, there are differences.  Strad and Guarneri came up with distinctively different ideas, as did other makers.  I think of it as different settings on an acoustic processor with lots of knobs.  Copying (assuming you know how ALL the knobs are set, which you really can't) would only get into one setting.  My goal has always been to understand as much as possible about what all the knobs do, and so be able to tailor sound and playing characteristics to a particular desired result.  A lofty goal... but I need something to keep me entertained.

Well....

For me, I believe one of the benefits of digging deeply into their historical geometey choices, and both into how the choices were consistent and how they varied, is you actually get to see specifically which knobs they moved and which they held steady in their traditional making.

This approach skips the issues of why, but is quite interesting. 

Also, I believe the way the geometry relates some things and not others, and protects some things and not others, is all reflective what they learned over generations of trial and evaluation. It shows plainly which knobs they collective thought were the ones to turn.   And, the geometry also suggests how they viewed the instruments separated into components.

 

For your goal to tie such knobs to sounds: you could just try turning some of their chosen knobs and see what happens.

 

 

[Yikes that needed editing. I shouldn't have posted in such a rushed moment. Sorry.]

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Structurally it seems to be only logic to give the soundost some support. If this portion bulges out too quickly (what would happen if we would think of an even and thin thickness on the back) the soundpost looses somehow 'contact'. 

However I think it is more important to look on how the central mass is connected to the borders. The whole mass and stiffness in the center doesn't have any effect if the thickness in the C bouts (close to the ribs) isn't sufficient. Or in othe terms: if the central mass is not connected in cross grain direction to the ribs it doesnt have much effect.

Just my experience based opinion on this matter.

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