bkwood

Top Arching Principles

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I am building my 4th violin. I've made an effort to carve the tops of the first 3 very similarly to each other, from similar wood, for consistancy in tonal result. I followed arching templets matching a Stradavarius style. I am arching my 4th fiddle using templets more matching a Guarnerius style this time, again using similar wood to the first 3. I've heard that some luthiers prefer to do their arching more intuitavely, by eye, not using templets.

I know a good mandolin maker who works by eye, and stresses to me that the thinnest part of the plate must be around the edges so the whole top can move there, like a speaker cone. And there are several other factors like how far in before the arching starts to rise , how steeply, etc. to consider. If you carve your tops by eye did you first practice methodically with different archings before taking off on your own. If so, all things being equal, what are the basic principles to be followed?

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    I don't think the speaker cone analogy is the correct one to use. I have seen grad patterns where the edges are strong ~ 3.2 mm and the thinnest parts are in the central upper and lower bouts.  Do a search  for grad patterns and look for general commonalities.   

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Thanks. I have seen graduation patterns like you say, but it seems to vary quite a lot although most all graduation patters show the thickest part where the sound post goes and in the highest part of the arch.

I am most interested in freehanders rules of thumb to the arching itself.

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I think the most important thing is to look carefully at the real thing, i.e. good examples of Strad or Guarneri or any other instruments that work well.  Templates are OK, but they are necessarily limited in number, and can't show everything.  For example, the way Strad treats the arching and F-hole wings in the center bout area won't show up on the usual templates, and you need to see the real thing in order to get that right.  That example might not be a big influence on tone, but other similar transitions might.  

Also, I find it easier to think of the plate shape as a whole, and how the convex/concave areas move and blend, while templates are more of a paint-by-numbers method.  One more thing... I might want to use different arching heights, depending on the wood or the sound I want to get, and then templates from a different arch height might not be so helpful.

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1 hour ago, Don Noon said:

I might want to use different arching heights, depending on the wood or the sound I want to get, and then templates from a different arch height might not be so helpful.

That's what I would like to understand. How do you know a particular piece of wood wants a higher arch, for instance?

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1 hour ago, bkwood said:

That's what I would like to understand. How do you know a particular piece of wood wants a higher arch, for instance?

As an example : with denser wood an higher arching allow you tu make it thinner and so lighter, while with a lower arching the same plate may result more (to much?) heavy.

However, I believe that the choices on arching heigth and shape also follow a more irrational way, also based on the aesthetics (high arched violins are so voluptuous....) and personal idealization of tone that you try and wish to get, that you associate with the violins you have seen and heard played.

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17 hours ago, bkwood said:

That's what I would like to understand. How do you know a particular piece of wood wants a higher arch, for instance?

 

15 hours ago, Davide Sora said:

I believe that the choices on arching heigth and shape also follow a ... personal idealization of tone that you try and wish to get...

In my minimal experience, a lower arch tends to have more brute power and a less refined sound.  It seems as though lower density wood should be used for the lower arching, to keep the bending stiffness up without getting too heavy... but I haven't experimented too much with that to really say.

edit:  It has been my impression that lower density wood (with "usual" arching) tends to be similar in behavior to low arching, i.e. louder but less refined... so I might want to do the opposite of what I stated above to get a desired tone, in spite of abnormal taptones and perhaps signature modes.

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I wonder why more people haven't experimented with bent tops for violins (like what's done for the gamba family -- rough bend the shape from several staves of wood, join them together and refine the aching from there). I've seen some makers create bent tops for violas out of 3 staves, but haven't seen anyone do this on a violin. I tried this once for a treble gamba -- which is not all that much bigger than a violin. It is an interesting workflow. Theory is bent tops are stronger at a given thickness and can therefore can be made thinner overall. One is limited to a barrel shape however on the arching profile. 

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29 minutes ago, Urban Luthier said:

I wonder why more people haven't experimented with bent tops for violins (like what's done for the gamba family 

Because it's more work, and so far those who use bent tops haven't consistently proven to be any better sound-wise.

34 minutes ago, Urban Luthier said:

Theory is bent tops are stronger at a given thickness and can therefore can be made thinner overall. One is limited to a barrel shape however on the arching profile. 

You could also say that low density wood can be made lighter for any given taptone.  And that is true.  However, the acoustic results are the combination of many different variables and not just the ones you are looking at, and maximizing something is not necessarily the way to achieve a balanced tone.  And it's all in the balance.

A barrel shape is not necessarily a bad thing, and there are ways around it.  I have seen one maker use a central strip bent for the long arch, and side pieces for the rest.  The results weren't anything extraordinary, but it is something I might consider experimenting with... perhaps not bent, but using different wood types for different areas of the top.

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If you believe that wood with a high radiation ratio R  (or c/p where c is the speed of sound in the wood and p is the wood's density,  or E^1.5/p^0.5 where E is the elastic modulus) is good then it should be also be good to use bent tops where the longitudinal grain isn't cut.

Since bent tops were not historically used it then follows that a high radiation ratio isn't helpful.

Since cutting the grain lowers the elastic modulus E, the speed of sound c, and the radiation ratio this suggests that it would be good to use bad wood and bad to use good wood. 

Maybe we're barking up the wrong trees.

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Marty,

I can interpret the math another way:  

Since bent tops were not historically used, then it follows that using a higher radiation ratio than theirs might end up sounding different.  

Since cutting the grain lowers the radiation ratio, this suggests that using bad wood might benefit from bent tops, and good wood might benefit from carving normally.

Still, all these arguments only address a part of the picture; bending may do other things, such as changing how the stiffness varies across the arching, damping, creep resistance, and who knows what else.  It's probably not a big deal, though, as I have heard bent tops that sounded fine, and carved tops that sound fine too.  No big difference that I could tell.

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10 hours ago, Don Noon said:

 

In my minimal experience, a lower arch tends to have more brute power and a less refined sound.  It seems as though lower density wood should be used for the lower arching, to keep the bending stiffness up without getting too heavy... but I haven't experimented too much with that to really say.

edit:  It has been my impression that lower density wood (with "usual" arching) tends to be similar in behavior to low arching, i.e. louder but less refined... so I might want to do the opposite of what I stated above to get a desired tone, in spite of abnormal taptones and perhaps signature modes.

 

I am glad you corrected this because this is what I experience: Higher arching for lower density.

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I think one of the fastest ways to "loose it" is by allowing for bad ideas to enter ones perspective of the build, I think a quick way to loose perspective is by focusing on one "area" or function of the entirety of the instrument. I feel all parts are significant with their interactions, and the first starting place is the material. Knowledgeable people know that the factual reality is that because material characteristics are a massive variable that the material itself will dictate final outcomes to a large extent and that each aspect of a build should be taken on a case by case basis and that things that need to happen or not will reveal themselves as things progress.

Arching shapes while extremely important are unfortunately for lack of better terms a guess, there is no way to carve the same material 10 different ways, shape wise, to determine what one was the best of the ten, and because of this fact, there is no way to determine if that shape would have been better than the other and at the same time we could say that just because "it" worked for this one instrument does not mean that a replication of those shapes, weights or thicknesses will work on the next.

I look at this instrument building stuff as a combination of a few different things and that these things must unify in order for "it" to be natural. One is on a spiritual level, where you are using your intuition, imagination and precognition to generate desire for forward motion of the project, the other is the "real world tactile" "things we can do" while building, mostly related to bending, flexing and tapping and the final is the advanced, unsolved understanding of the physics and mathematical formula that most likely includes a combination of classic physics,quantum string theory as it applies to the structural engineering and perhaps the more esoteric "ancient" knowledge of Fibonacci ratios, Pythagorean math and fractals of the individual parts.

Related to the mandolin makers advice, I would say that this is not bad advice as it, like all other advice is rooted in physics. Harnessing or controlling energy dissipation is a very important part, but only one part, and a part that's role is undefined. A violin is a combination of many things that interact in very complex ways, it is not a speaker cone, but sometimes in various modes of oscillation it is, just like sometimes it's a trampoline, sometimes its a spring, sometimes it's a billow, sometimes its a house in a wind storm, sometimes it's a beam with lots of weight on it, sometimes it's a static suspension bridge, sometimes it's dynamic, but most of the time it's all these things and more at the same time.

Remember, regardless of what carrot you have been directed to go after, even if they tell you there is a benchmark for tone, there is not. Go forth trying to build things to the best of your ability using your intuition and the wisdom you gain along the way. Regardless of if you or any of us achieve monetary success, know that tonal success generally will come by simply sticking with it and building a body of work. I would urge you to keep in mind the sound you like and the way you would like your instruments to sound, and that in time you will develop your sound, I would always encourage you to use yourself as a benchmark instead of the masters. Try to emulate them, but develop yourself and sound in your own image.

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On 1/8/2017 at 11:41 PM, Davide Sora said:

As an example : with denser wood an higher arching allow you tu make it thinner and so lighter, while with a lower arching the same plate may result more (to much?) heavy.

However, I believe that the choices on arching heigth and shape also follow a more irrational way, also based on the aesthetics (high arched violins are so voluptuous....) and personal idealization of tone that you try and wish to get, that you associate with the violins you have seen and heard played.

The raching shape of the belly becomes compressed by the load on the end blocks. If the arching is high we need thicker arching structure since the arc becomes forces outward . This is buckling. When we lower the arching we need higher compression forces in order producing structural deflection= outwaed buckling. So we become able reducing the thickness in relation to the higher arc shape. In order undertsanding these complex conditions we must study the load on a column and how the end points are fixed or pined. The belly has support by the sound post and the structure from the end block to end sound post thus is a given lenght of the column. Pinned at the sound post and fixed at the end block. A similar conditiopn arised by making slits as we see the F-holes and the structure at the upper F-hole eyes and the minimal cross section on the instrument at that location produces a second point so the upper part arc shape is equala long the lower and the buckling conditions become equal. The structure between the two points the soundpost and the second point must be a straight shape in order to allow a downward deformation of the structure between the F-holes. If there was a arc shape we have a chord line and the chord line than must lenghten in order allowing a downward deformation of the structure. This is the reason why we see flat structure alöong the lenght axis of the arching shape.

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On 1/9/2017 at 1:51 PM, Urban Luthier said:

I wonder why more people haven't experimented with bent tops for violins (like what's done for the gamba family -- rough bend the shape from several staves of wood, join them together and refine the aching from there). I've seen some makers create bent tops for violas out of 3 staves, but haven't seen anyone do this on a violin. 

Charles Francois Gand did it , but the instruments he made this way are not as good as the normal ones.

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