Structural Question


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The simple answer is "because it works best that way".  As to WHY it works, that's more debatable.  My hypothesis is that:

  • The center thickness adds stiffness so that some of the lower modes get more functional area and power.
  • The central mass supports the soundpost, giving more resistance to the feel of the bow.
  • The central mass keeps high-frequency energy more in the top plate, where it is radiated more efficiently.
  • Structurally, the thick area helps prevent soundpost cracks and distortion.

Debate away.  Those are my ideas, and I'm sticking with them.  For now. :)

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I believe that mass also increases the capcity to build up lower frequency energy in strong playing.

The physical resonances in an musical instrument cycle energy between elastic flex of a matter and kinetic motion of a mass.  More such mass in a resonance means more capacity to hold and accumulate energy in the kinetic phase of the resonance.

But, for the back that extra mass will only help energy capacity for frequencies that move that mass as a whole, or just in a few simple parts.

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I have questions about that also.  When did it first appear in instruments, is it in the earliest Amatis?  Has anyone tried making a back plate the same thickness all over and what does that sound like vs. one that has a thick center area? 

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2 hours ago, MikeC said:

...Has anyone tried making a back plate the same thickness all over...? 

I'm sure people have done that.

2 hours ago, MikeC said:

...what does that sound like vs. one that has a thick center area? 

Not as well, apparently.  If it sounded better, most violins would be made that way.

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5 hours ago, baroquecello said:

My question would be: is this the same in violins, violas, cellos and basses, or is this different for the larger instruments?

I don't know about cellos and basses, but violas seem to work nicely with graduation patterns similar to violins.  However, I also have NOT tried anything radically different on violas... and I haven't experimented that much with violin backs either.  The near-universality of the thick center on backs has been off-putting for ideas about messing with it... not the same with tops, where there's often "reverse graduation".

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The orig. question was regarding structural issues, not tone, which is where most of the comments seem to be going.

Why wouldn't one, 4 or 500 years ago, simply start out making the highest point, the center, the thickest, and the lowest point, the bottom of the channel, the thinnest? Unless there is some engineering concept that would be discovered later on that indicated a different pattern, why do it otherwise? It seems that time has borne out that it works, and perhaps the other experiments, if they existed, were either regraduated to what works or simply didn't survive since they didn't sound or were structurally unsound.

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1 hour ago, duane88 said:

The orig. question was regarding structural issues, not tone, which is where most of the comments seem to be going.

Why wouldn't one, 4 or 500 years ago, simply start out making the highest point, the center, the thickest, and the lowest point, the bottom of the channel, the thinnest? Unless there is some engineering concept that would be discovered later on that indicated a different pattern, why do it otherwise? It seems that time has borne out that it works, and perhaps the other experiments, if they existed, were either regraduated to what works or simply didn't survive since they didn't sound or were structurally unsound.

I believe your assumption of a structural motive is wrong.  I believe it had a tonal purpose from the beginning. And that purpose is better served if the mass is stimulated by the sound post from a position away from the maximum of the mass.  And this pushes its location north of the bridge.

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27 minutes ago, David Beard said:

I believe your assumption of a structural motive is wrong.  I believe it had a tonal purpose from the beginning. And that purpose is better served if the mass is stimulated by the sound post from a position away from the maximum of the mass.  And this pushes its location north of the bridge.

I agree (in theory, since I don't think anybody really knows). Driving a mass with a little flexibility between the driver and the mass can allow some resonances to occur which otherwise would not exist, or would be constrained.

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19 hours ago, Shunyata said:

thick in the ... center of the plate ... unintuitive,

Since a scientific explanation isn't forthcoming, to me it IS intuitive.  Picture flicking a rubber band between your fingers.  Then add weight to the middle of it, like tying on a hardware nut.  The vibrations are more powerful than before and there's more excursion.   Maybe totally wrong, but intuitive....

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https://lpsa.swarthmore.edu/Analogs/ElectricalMechanicalAnalogs.html

 

Capacitance and mass are conterparts in an analogy between electrical and mechanical vibration systems.

A resonance driven at a fixed frequency will take in what energy it can, and at the same time will bleed off whatever energy it must given the system.

There will be no build up of volume/intensity/energy in the resonance unless for a while energy is coming into the resonance faster than it leaves.   So, when a resonance builds up, there is implied a delay of time between energy entering and leaving, there is a short duration storage of energy in the resonance.  No time delay of energy flow and storage, no build up of resonance.

Both the easy and degree of flex available in the elastic material of the mode, and the amount of mass moved in the flexing limit how much energy can readily be stored in the resonance.

This is why the extra mass in the classical backs changes how and how much the effected resonance can build up (store energy).

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There's always trial-and-error.  I tried adding a 10g lump of clay to the center of the back on one of my violins, and it seemed a lot better to me.  Maybe I should make my backs thicker.  (Edit:  I checked my notes, and the back on this particular violin was 3.5 in the middle.  One of my old regraduated beaters.)

Doing a quick test saves a lot of arm-waving and gets right to what matters.

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2 hours ago, Don Noon said:

There's always trial-and-error.  I tried adding a 10g lump of clay to the center of the back on one of my violins, and it seemed a lot better to me.  Maybe I should make my backs thicker.

Doing a quick test saves a lot of arm-waving and gets right to what matters.

I have been wondering if it's the added mass that is important or the extra stiffness of thicker wood that matters.  Just adding a weight to increase mass would not increase stiffness so that's one way to test the idea.  

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1 hour ago, MikeC said:

I have been wondering if it's the added mass that is important or the extra stiffness of thicker wood that matters.  Just adding a weight to increase mass would not increase stiffness so that's one way to test the idea.  

I have recently migrated to the idea that mass matters a LOT, a change from my old aerospace idea that higher stiffness/weight should be the main thing.  The right mass in the right place is what I'm looking for lately... but starting with good stiffness/weight.

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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.

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33 minutes 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.

Yes and I also suspect that any "improvements" are likely to emerge over the long term from makers who try to copy and do it very well, and accidentally and unknowingly deviate from an exact copy in subtle ways that co-incidentally improve rather than (more likely) degrade the sound, rather than from deliberate changes to the design.

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3 hours ago, Bruce Carlson said:

The thicker area is there in the earliest Amati instruments.

 

Which makes me wonder just from historical curiosity was it an original idea of the Amatis or is it found in earlier precursors to the violin.  

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