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How Good is Our Theory?


Michael_Molnar

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In my tests the shape of the arching has been the most important variable, so far. I've played with a lot of different things and changing the arching has been the only thing that has resulted in huge changes in how the violin produces sound. In comparison, the other things like graduations and wood selection have been relatively small effects.

I'm not disagreeing with you but in my experience dramatically changing the sound of the violin is very non linear. Very very small changes can sometimes have an extraordinary change in the sound. OTOH large changes seem to have no apparent effect on the sound. Go figure?

Oded

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Very interesting. I have heard others say similarly.

So what happens to the sound of a master Cremonese instrument when it has been vacuum packaged into a new shape?

Look, everything changes the sound of a violin and pressing it out will change it. A good restorer will try to return the instrument to it's original shape.

I think that wood may have an intrinsic 'memory'so that even as it's shape is distorted there are stresses and tensions in the wood that make it retain it's original behavior.

I can't prove this or demonstrate it either. This falls into the religion category of violin making. Any coreligionists out there,those that worship the same deity? :rolleyes:

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I suspect that can only be achieved by someone who can string up a violin (most likely in the white), hear what's wrong with it, and either improve it or chuck it in the bin (my personal favourite for "the secret of Stradivari"). This seems to be what Oded is suggesting, and I agree with him whole-heartedly - in fact the only great modern violins I have played were made in this way (with a bit of tap-tone gimcrackery in the early stages).

But Oded is using a form of tap tuning in the final stages. ;)

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So what happens to the sound of a master Cremonese instrument when it has been vacuum packaged into a new shape?

I don't know, I don't work on them.

At one point I tried reshaping the top from a Chinese violin to track the effects. At the time, my measurement system wasn't good enough so I need to repeat that test some day.

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i wouldnt call that documentation, as you literally "proved" almost all the great italian violins have defective arching, william, more like your first high school science project, where you learn that whatever your hypothesis is, you end up finding a way to prove it, true or not.....

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I think that wood may have an intrinsic 'memory'so that even as it's shape is distorted there are stresses and tensions in the wood that make it retain it's original behavior.

I can't prove this or demonstrate it either. This falls into the religion category of violin making. Any coreligionists out there,those that worship the same deity? :rolleyes:

Me not. You're saying that the sound is in the WOOD. I say that the sound is in the AIR. I guess I'm worshipping a different deity then.

However. In an attempt to meet halfways: Perhaps the so called creep goes in the direction of "most harmonic". I.e. the creep will go in the direction where the vibrations work the best? That means that the arch will deform in a certain way depending on how the violin is played. I.e. depending on the player.

*Like* this thread. cool.gif

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Perhaps the so called creep goes in the direction of "most harmonic". I.e. the creep will go in the direction where the vibrations work the best?

Nope... creep will go in the direction that the static forces push it. Vibration patterns will depend on the shape, but if it sounds better after creeping, it's just coincidence.

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Me not. You're saying that the sound is in the WOOD. I say that the sound is in the AIR. I guess I'm worshipping a different deity then.

So I guess when you go to a symphony concert you believe that all the sound comes from the man in the middle waving his arms in the air..... :D

Nope... creep will go in the direction that the static forces push it. Vibration patterns will depend on the shape, but if it sounds better after creeping, it's just coincidence.

I find it difficult to believe that vibrations which generate a certain amount of heat have no effect on material (wood) deformation.

John Masters mentioned that he thought that a catenary arch will deform into a curtate cycloid shape. Have I got that right or not?

Oded

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I find it difficult to believe that vibrations which generate a certain amount of heat have no effect on material (wood) deformation.

John Masters mentioned that he thought that a catenary arch will deform into a curtate cycloid shape. Have I got that right or not?

There are two separate things going on, static and vibration stress. Vibration, being cyclic, will not have any net force to make wood deform in any particular direction. It might act to increase the rate of deformation... if there is static stress applied. The creep will go in the direction dictated by the static force. John Masters was concerned about the static stress deformation.

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Well I must admit, the thing I love about the violin and this group of minds is... Here we have a gathering of fairly intellectual guys, with many various backgrounds, and, an intellectual guy, Mike M. asked for "science only". And after only X amount of pages, our group of learned men, with the mighty force of "science" behind them, have quickly turned it right back to where it always goes, speculation,conjecture,hypothesis,scamboogery and over all guessing. I love it, only the power of the violin can bring the mighty to their knees, guessing, chasing feathers in the wind.

I agree with Don lots, not because of who he is or what he says. Just because I know he kicks ass and builds a lot and has the methodogy to test. That, to me is the best way to find out stuff. You know, the rocket science thing don't hurt, but it will not send his violins into space.

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I think you brag to much to be taken seriously, Mr Stross. I have a very firm ground under my comments. Arching stiffens the plates and supresses the vibrations because some of the energy goes into in-plane stretching. Such effects are used in the automotive industry to suppress certain frequency regions or modal problems. The theory is very simple. A plate shape that looks like the vibration will supress it.

Look up Claes Fredös article: http://www.afconsult.com/upload/konsulttjanster/LjudoVibrationer/pdf/SAE_2005-01-2342_Fredo_Hedlund.pdf

What you say about arching can't be true. At the Oberlin Acoustics workshop and other settings, we are blessed with a central specialist from the aviation industry sharing state of the art modeling tool results on the violin. Arching is one of the factors, but not the determining one. Experience talks against you too.

Dear Anders,

Fredos' concept seems pretty solid to me from the tests he did.

The violin's arched plate shape might look like the what the vibration mode shape would be if the plate was flat. I think the only mode shape that would look like that is the AO mode where the top and back plates balloon in an out.

This suggests that the plate's arch is suppressing the AO vibration. I'm guessing that the AO amplitude is reduced as the arch is made higher. Have you ever seen this trend in your data mining?

Thanks, Marty

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The violin's arched plate shape might look like the what the vibration mode shape would be if the plate was flat. I think the only mode shape that would look like that is the AO mode where the top and back plates balloon in an out.

This suggests that the plate's arch is suppressing the AO vibration. I'm guessing that the AO amplitude is reduced as the arch is made higher. Have you ever seen this trend in your data mining?

Yes, a weak tendency for that. There also seem to be a systematic difference between violins and Hardanger fiddles. The latter has some 3dB extra level in average, possibly due to a flatter chross arch between the f-holes.

Strads do have weaker A0 than del Gesus, in spite of being thinner in general. The arching height is a bit higher for Strads, so at least in theory the flatter arch of the del Gesus might contribute to the somewhat stronger low frequency response of these. They also have slightly higher A0 frequencies which may also be a factor.

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Nope... creep will go in the direction that the static forces push it. Vibration patterns will depend on the shape, but if it sounds better after creeping, it's just coincidence.

I was also concerned about the shape of minimal stress. In other words, a shape that would creep the least. For a material with no creep, any displacement under stress would shift in some amount that gave back curtate cycloids and the longitudinal arch one of the same form but with a simple modification (depending on the function giving the shape of the long arch.) Easier to describe: The deformed shape under static loads would look the "most" like the unloaded shape.

I find it difficult to believe that vibrations which generate a certain amount of heat have no effect on material (wood) deformation.

John Masters mentioned that he thought that a catenary arch will deform into a curtate cycloid shape. Have I got that right or not?

Oded

This was in regard to the inside-first carvers. The center of a catenary (or the extremely close parabola) would fit well to a curtate cycloid inside the inflection. The graph shows deep curves for this. A shallow curve would have a longer region of blending a catenary to a CC. That is because the inflection is more inboard.

But it might be very natural to cut catenaries IF one knew the endpoints on the flat surface... or carved the long arch first. Then just make a nice blend to the edge of the linings.

post-6797-0-29538200-1325440125_thumb.jpg

No gradual deformation would start an infelction. That is my main difference with your quote, Oded. I still wonder about the role of the inflection in a creeping or deforming plate under static loads.

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If you like the Saconni model of a thin plate of nearly constant thickness, one could make a very thin (1 mm?) top and then load it with the static loads. Then see how it bulges under the loads. Find the shape that puffs up the most uniformly.

This would be simple to do. It work best with uniform thickness wood (because stiffness is not proportional to thickness)

In fact, I have not done it out of shear laziness. I would do it ifirst with no bar. And maybe no f-holes.

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In the "for what it's worth" department. John, you asked at one point about measuring deflections and their connection to sound. I have no empirical data but one of my habits in deciding whether to buy a new instrument for my shop is to press the area just above the treble F hole. I like to see it move just a bit. My point is that I believe that deflection at this location of an instrument may give useful information about it's acoustical potential.

Oded

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Only as long the edge can rotate.

I think the edge is more flexible to rotation than the plate is to creeping of the inflection.

In the "for what it's worth" department. John, you asked at one point about measuring deflections and their connection to sound. I have no empirical data but one of my habits in deciding whether to buy a new instrument for my shop is to press the area just above the treble F hole. I like to see it move just a bit. My point is that I believe that deflection at this location of an instrument may give useful information about it's acoustical potential.

Oded

The curvature is a lot there. Maybe you could push anywhere, but other places would have more flex because of the flatter curvature. It would be hard to judge without a dial guage.

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In the "for what it's worth" department. John, you asked at one point about measuring deflections and their connection to sound. I have no empirical data but one of my habits in deciding whether to buy a new instrument for my shop is to press the area just above the treble F hole. I like to see it move just a bit. My point is that I believe that deflection at this location of an instrument may give useful information about it's acoustical potential.

Oded

I would have guessed there would be more flex halfway between the f and the upper edge, i.e. farther up? Or is this where wolf notes come from... when there is excessive flex here? What about the bass side?

Thanks.

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Yes, I've thought about how to measure the deflection. It shouldn't be too difficult to do. What get more complicated is finding the locus where the bending takes place.

I wonder if the nodal lines, which often emerge at the top and bottom of the F holes,are significant in this measurement. I'm thinking of your measuring stiffness using mode 5 (ring mode) ?

I think the edge is more flexible to rotation than the plate is to creeping of the inflection.

The curvature is a lot there. Maybe you could push anywhere, but other places would have more flex because of the flatter curvature. It would be hard to judge without a dial guage.

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