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Why arching shape?


reguz

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My key point is that wood is much stronger lengthways against compression, than against the pressure from the bridge. Therefore, with a flat arch, the bridge presses downwards. Once the outside of the belly is concave, pressure from the bridge creates a tensile stress along the grain. With a high arch, provided the shape of the arch is strong to resist buckling, the bridge force helps counter the string tension. Buckling is essentially a bending force either side of the bridge (compared to the bending across the whole plate in the back). This means that this part of the plate is shorter, but the same thickness. Thus this part of the plate is stronger.unless an arch is high, but flat in the centre, buckling is not a problem. 

Another issue is the cross arch shape, as this affects the total strength as well. 

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Nigel Harris has written concerning the arch shape of both the top and back plates of the violin in "Controlling the arching shape of violin plates to achieve tonal optimization and repeatability". I do not have the web address but if you search his name and violin making you will find him. He has written a number of articles on the subject which I found very helpful to understanding how the violin works and it should possibly be made. He points out that there are two systems producing the vibrations in the violin body. First the rocking motion of the violin bridge with its transverse vibration system and second what he calls a longitudinal vibration system which I think is more how the guitar works. I had to read these articles a few times to take it all in. Two other articles were ON ARCHING SHAPE AND VIOLIN TONE #1 AND #2. I recommend reading these if you are thinking about the violin arch shape of the top and back. 

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On ‎3‎/‎16‎/‎2019 at 3:49 PM, reguz said:

Sorry Guys, Convex is always outward, Thus to higher altitude. Concave is bowl shape

I was responding to Marty's joke. He likes to joke sometimes. It stops his brain from exploding with calculations debunking spurious hypotheses.

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On 3/18/2019 at 12:27 PM, Anthony Panke said:

Not to change the subject, but how can one predict/controll sound quality with the arching?

are there any rules of thumb or is it too complicated to ever fully understand?

One may be able to control sound quality and possibly be able to predict sound quality over time by keeping the following in mind while working wood.  These are the words of one of the residing fiddle forgers here at Maestronet.

.......for more power and less complexity take away some of the curvature and make the center more of a flattish plateau - for a sweeter, more complex tone with less power take the recurve further into the plate [long archwise] and add a more curved profile from the trough to the peak.  Doing so adds complexity to tone at the expense of power.

All I've found out with using the above so far is that I get a more nicer looking outer plate arching profile going the sweeter/complex route.  Makes me slow down a little bit too while working wood.  I haven't noticed any power or complexity issues though as compared to using, for example, the Murtov 6th's templates, which could be called the median between a flattish plateau made fiddle and the more condensed arching scheme for less power/ more beauty.

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22 hours ago, uncle duke said:

......for more power and less complexity take away some of the curvature and make the center more of a flattish plateau - for a sweeter, more complex tone with less power take the recurve further into the plate [long archwise] and add a more curved profile from the trough to the peak.  Doing so adds complexity to tone at the expense of power.

I've noticed that for some reason people always force those two things, sweetness and power, to be at odds with each other.  In all those crazy old books Amati and Stainer are sweet, and Stradivari and Guarneri are powerful. I don't think power and sweetness coexisting is particularly rare.

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6 hours ago, Bill Merkel said:

I've noticed that for some reason people always force those two things, sweetness and power, to be at odds with each other.  In all those crazy old books Amati and Stainer are sweet, and Stradivari and Guarneri are powerful. I don't think power and sweetness coexisting is particularly rare.

I'm not yet a maker (working on it), but I've set up, repaired, and sold violins through the music store I owned. I had two "Thomas Perrys" go through my shop (one a likely Perry, the other definitely from his shop). The rap on Perry violins is "good for orchestras, not soloists," because of the sweetness vs. power thing. A Perry, they say, is sweet but not powerful.

Maybe so. It's true that both these violins were sweet-sounding, with the D and A strings the sweetest, and that neither violin was particularly loud.

But as Bill says, power (do we mean something like SPL for a given bow pressure, or a tonality that "cuts through the mix," or what) and sweetness (whatever that means) aren't mutually exclusive at all. While the Perrys weren't particularly loud, they nevertheless commanded attention when played. Maybe that's power, too?

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On 3/18/2019 at 6:27 PM, Anthony Panke said:

Not to change the subject, but how can one predict/controll sound quality with the arching?

are there any rules of thumb or is it too complicated to ever fully understand?

Anthony. Start thinking of how you can produce a specific frequency on a string. Simple, you need a specific tension on the string otherwiase it will not vibrate with the frequency you like to produce. The violin body has may crved shaped on the two bouts, upper and lower bout. When string load becomes applied the arc shapes become stress on the belly by buckling forces and on the back by bending forces. Depending on the stress condition, based on arching shape and plate thickness graduation, you will have a specific stress condition that may function for many frequency. Some time there is a less good stress condition that make the not allow the structure vibrating with good amplitude. What can we do? Yes we can easy increase the stress condition on the belly bout shaps withoput touching them. How is that possible? Well this problem is based on interaction betwween the bally and back bout shapes stress conditions. By polishing on the varnish of the back bout area on one of the two bout areas or onan instrument in the white by using a scraper we can remoce som structure. This reduces the stiffness of theback bout area and a graeter moment of forece will bend the violin structure and the belly buckling force increases. That increasing contion will produce new other stress condition on the bout area and the responce of the frequencvy will change. Since we have to deal with two such structural condition we can adjust the behavior of interaction so an more optimal condition becomes produced. I have five years of experience improving sound quality wich thus is based on the interaction between the bout shapes of the upper and lower bouts. Kind regards.

 

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12 minutes ago, FiddleDoug said:

Ten pages and over 230 post, and still nothing conclusive, or even any kind of consensus?

On the contrary, we now know conclusively (as if we didn't before) who holds what opinions very strongly and likes to post a lot about them.

But as far as advancing knowledge or changing anyone's mind... not so much.

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Anthony, You got it. A larger area is easier to contol, The violin thus is harder to improve of the Viola and Cello.

Yes, you need an arch shape that on the belly buckles and on the back bend. Both will bulge. When you give structural quality on the arc's in the vector diagram and consider the quaity of wood structure and thickness graduation you will come to an understanding how interaction problem must be considered. When the left side is less in width the graduation must be made less in order to be equal sensitive the right side. Just sensitivity of the structure answering on a string attack predict the quality of the dynamic state that may arise. In order to make the thickness graduation less in order achieving a better respond need good contol of the arching shape. I have found that the framework that arise with STLs is very important for this possiblity.  STLs are easy to controll while arching shape on bout area is very difficult. 0,1 of mm influence the stress condition that arise on arching surface locally and over the complete bout area. Kind regards

 

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On 3/21/2019 at 3:38 AM, Bill Merkel said:

I've noticed that for some reason people always force those two things, sweetness and power, to be at odds with each other.  In all those crazy old books Amati and Stainer are sweet, and Stradivari and Guarneri are powerful. I don't think power and sweetness coexisting is particularly rare.

Do keep in mind that what was written was written for a beginning hobbiest violin maker, not the seasoned pro who doesn't or shouldn't need such advice.  Back then I wrote everything down if I thought it would help me work wood better.  I'm working on getting the sweetness out of a completed build though I may be hindering myself with cheap bridge wood stock and low dollar strings - I'll learn one day.  

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On 2/10/2019 at 6:47 PM, jezzupe said:

Um, I assume you live in a house....the floor is plywood, yes? if so the plywood is supported by joists, if not, what do you think will happen when you put that grand piano in the middle of the room?

Yes Jezzupe, the load of the piano will produce a moment of force in downward direction and when the plywood is not thick enough it will crack. On the violin we have a load almost on the sound post and and still there is no movemet downward to measure. What forces do produre equilibrium Jezzeppe? On the floor with the piano the walls are supporting remain on the same level. On the violin the sound post remain on the same level. Try to undrstand that. No force is cominhg out under the sound post producing a downward force. You may explain the contrarry if you can!!!

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On 2/10/2019 at 11:09 AM, reguz said:

Explain for us why you must support in order producing downward movement.

After about the 4th page, I couldn't be bothered to read any further.

What are you trying to prove or disprove? If you think you know better, get to the point an re-educate everyone here. If not, why waste everyone's time with circular questions?

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2 hours ago, Bill Yacey said:

After about the 4th page, I couldn't be bothered to read any further.

What are you trying to prove or disprove? If you think you know better, get to the point an re-educate everyone here. If not, why waste everyone's time with circular questions?

The real point here is that it’s not about the violin. It’s about being able to get attention. If I have the biggest pile of garbage in the world, well, people will notice me. That is the point. And look.. I just add added to the pile. What did I get out of it? Just sayin.

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On 2/10/2019 at 12:29 PM, reguz said:

WHY????

 

The Romans used arched forms to support downward loads.   Everybody knows that.  The differential equations for a curved shell are more complicated than for a flat plate.  They are fundamentally of a different sort.  For example,  curved shells introduce a new variable,  stretching of the wood in first order.  (Flat plates will have stretching for large displacements,  but not to first order.)  The new equations involve two more orders of differentiation because of the new variable of stretching.  

Because you say you are an engineer,  I expect that you know about wave equations on different types of surfaces.  A curved surface is fundamentally different from a flat one.

I expect that  Carl Stross will back me up here.

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On 2/13/2019 at 10:21 AM, reguz said:

I'm not sure about all this I'm still searching for a acceptable explanation but this condition means that some tome the sound post is the center of rotation in the dfynamical state of the instrument while playing hteh E and A string some very special condition come up. So when you say I agree at lower frequency the sound post is stiff ( with stiff I mean do not move) The connection of the top of the sound post on the belly is soft and many makers reinforce for material compression.

There is a node of sorts.  But it depends on the mode of vibration.  A given violin has a complete (and orthogonal) set of normal modes.  This is true of any such bounded surface.  Some will involve portions of the back going along with the top.

You can say nothing simple about any of this.  It is not simple.  It is analogous to quantum mechanics because in both cases one gets eigenfunctions with their eignevalues (frequencies of vibration.)   If you cannot understand the nature of these normal modes,  you should not talk so "qualitatively" about ANYTHING.   

As for the rest of you,  violin physics is not a mystery,  it is just very complicated.  Tap tones are the few lower eigenstates of the vibrating instrument.  To solve the problem,  one needs finite-element-analysis because of the strange and complicated shapes involved in a violin.  Numerical analysis,  in other words,  is the only way to find anything about what is going on.

And one more thing,  each normal mode will have characteristic damping.  There are many kinds and  models for damping, and this would make a numerical analysis much more involved.   

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