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the function of arching shape


reguz

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No need to apologise. I haven't followed a lot of this closely. As a matter of interest I set out to make ribs, particularly the centre bout ones, as thick as I could. I was aiming for at least 1.5 mm, only managing about 1.2-1.3 mm for the last set. Probably because of the old rib material I'm using or its density. I tried varying the thickness from thick at the centre to thin at the blocks but it all got a bit difficult.

I see Evan Smith in another post was saying how he was bending ribs as thick as 2 mm. So I assume and agree that doubling rib thickness, at least at the centre bout, could be beneficial.

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1 hour ago, Dennis J said:

I see Evan Smith in another post was saying how he was bending ribs as thick as 2 mm. So I assume and agree that doubling rib thickness, at least at the centre bout, could be beneficial.

Could be beneficial,,,,, or not. It depends on the particular violin and the manner of the rest of it's construction.

I don't believe that I stated that I bent them at 2mm, but I have glued together two 1 mm ribs to make them a total of 2 mm thick.

And yes I have doubled ribs in the center bouts only, and I feel that it has been beneficial. I glue one rib on the mold, then glue another one right on top of it, then finish it all as nothing ever happened. You would never know till you pulled the plate off.

I have also added extra c-bout ribs on several completed instruments that were a few years old so I could observe the results clearly. You have to remove the top plate, the linings, slot the corner blocks to slide the rib ends in place then replace the linings. You would never know till the plates were removed that it had been done. It didn't change the basic character of the sound, just added more clarity and focus, and stronger. On some violins that would be too much, all things have to be taken into consideration.

As I have observed that removing wood from the centerbout ribs quickly leans toward softness and maybe mushiness, keeping the area strong gives more room for flexibility in other places that produce sound.

Of course as always, I could just be full of it, Time answers all questions.

 

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11 hours ago, Dr. Mark said:

Something like this...?

 

bridgeTemp2.png.3c7c17a92ca938c31953cf7575bfee24.png

On the violin back the sound post is at the lowest point of the curve. Thus were you see people on the bridge above. They load the curve downward and the cuirve of the bridge will become forced becoming flat between the people and the end of the bridge were it is fitted. On the violin the circumstances are the oposite this means the end points of the curve thus the location of the end block become pulled upward/inward whilethe sound post do not move. Coming to an understanding of this behavior stydy the vector diagram. The force of the string = the lenght of the vector. All ather vector forces can be measured by the lenght of the vector. Hope you can follow the vector explaination AFE is your bridge. As you can see the curves AF and FE thus become more curves not as it is on the bridge. When the curves change shape they become loaded with energy. All depending on thickness graduation. When the instrument becomes dynamic by bow action on the strings they become dynamic and use the energy like a spring. This is what happens on the back. On the top plate we find eqaula circumstances. As you can see curves AG and GE become buckled outward.

Hope this explenation give you something to consider2016vectorforces.thumb.jpg.5ca07ca0205b3796b4db0a459774b0dc.jpg

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6 hours ago, reguz said:

Coming to an understanding of this behavior stydy the vector diagram. The force of the string = the lenght of the vector. All ather vector forces can be measured by the lenght of the vector. Hope you can follow the vector explaination AFE is your bridge.

Well, yes, but you know how everyone wants to do things their own way.  For example, you have an arrow indicating the vector string tension at the end block, and arrows representing the direction of the non-orthogonal components of the force parallelograms e.g. of the string tension, so ok I like to use orthogonal coordinates.  One thing, however - the reactive resistance of the back to deformation transmitted through the sound post also contributes to P2.  'Buckling' and 'bending', according to Webster's, either mean the same thing or the former indicates collapse.  I guess I also would be wary of over-generalizing from a very idealized diagram.  There are things it shows you and things it doesn't.  Just my preferences...

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18 hours ago, David Burgess said:

Yes, but it wasn't done in 1,000,000 increments, was it?

I think Colins point was to be able to track the free plate modes (violin body with very weak ribs) gradually into the assembled violin range and somwhat beyond (2-3 times normal stiffness, or so). I think his mode tracing curves are rather smooth. I guess he must have done quite a few runs of that model. Not 10^6, likely more than 10 and less than a 100. The rib model is parametric I think. Just one number to change and then re-run. Chat with his wife for a while, save the run data and redo.

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14 hours ago, David Burgess said:

I like experiments which involve only a single variable, when that is possible. You don't?

That's one of the great advantages of computer modeling--one change can be made at a time.

But it has three serious disadvantages--you have to be really smart and be wealthy enough to afford the computer programs and be retired to have enough time to use them.

 

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55 minutes ago, Dr. Mark said:

Well, yes, but you know how everyone wants to do things their own way.  For example, you have an arrow indicating the vector string tension at the end block, and arrows representing the direction of the non-orthogonal components of the force parallelograms e.g. of the string tension, so ok I like to use orthogonal coordinates.  One thing, however - the reactive resistance of the back to deformation transmitted through the sound post also contributes to P2.  'Buckling' and 'bending', according to Webster's, either mean the same thing or the former indicates collapse.  I guess I also would be wary of over-generalizing from a very idealized diagram.  There are things it shows you and things it doesn't.  Just my preferences...

Equibrilium is always present at the sound post. You may make any vector diagram but the result will be the same. The instrument do not move in any direction but structure on the instrument may do. I say may because it depends on how the structure IS shaped and how it is GRADUATED in thickness.

Hopefully violin makers do undestand this. As we were able reading earlier mentioned persons did  not.

 

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1 hour ago, reguz said:

Equibrilium is always present at the sound post. You may make any vector diagram but the result will be the same. The instrument do not move in any direction but structure on the instrument may do.

Uh, no on both counts. You seem to have become blinded to other and better explanations, from having forever placed blind trust in your own, which appear to be at about an "introduction to physics 101" level.

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2 hours ago, reguz said:

Equibrilium is always present at the sound post. You may make any vector diagram but the result will be the same. The instrument do not move in any direction but structure on the instrument may do. I say may because it depends on how the structure IS shaped and how it is GRADUATED in thickness.

I don't see a response to my question about the sound post pressure, so if that's because your response is over my head I'll have to ask you to put it in simpler terms.  I'm under the impression that there's equilibrium in your building example as well, but if you're between the building and the soil under it you're going to be crushed.  I don't recommend that you run that experiment.

In the same way, if you support the belly of a violin from inside using its back as a brace you're going to feel some pressure as the belly deforms in response to the vertical string load.  The back will deform a bit as you push back while these forces equilibrate, but you'll still have to push to stay upright.  The center of mass of the instrument doesn't move at all which is why I like to use it as the origin of coordinates.  Once you and the top and bottom plates stop moving you've reached equilibrium.  If you walk away now the back plate will return to it's original position and the top plate will depress further.

I guess you could avoid all of that - apply the full string load and then fit the sound post.  I don't think that's common practice and I'd only ask that you not use an expensive violin to experiment with.

...and compression is better than depression I guess, except pills or a good party can help with the latter.  I don't know what helps with compression except maybe stretching, which is what we need to do to learn new things, or hanging, which is what we should try to avoid doing to ourselves...

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3 hours ago, David Burgess said:

Aren't desktop PCs these days bunches faster and more powerful than the computers used in the spacecraft which first landed people on the moon?

Yes.  So are  laptops, as well as cellphones.  Touring the computer room in Houston with an NSF group in 1969 was an awesome experience, at the time, now it seems like "stone knives and bearskins".  Anyone else here remember what a stack of punch cards look like?  :lol:

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4 minutes ago, Violadamore said:

Yes.  So are  laptops, as well as cellphones.  Touring the computer room in Houston with an NSF group in 1969 was an awesome experience, at the time, now it seems like "stone knives and bearskins".  Anyone else here remember what a stack of punch cards look like?  :lol:

yes, I remember those, I'm that old.  :(

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24 minutes ago, Violadamore said:

Some of your household appliances are "smarter" than the AGC was.  :)

I've been resisting "smart" and internet-reliant home appliances, but it's getting pretty hard to find anything else these days.

Getting back to the punch-card days: There were only enough punches available on a card to represent a limited number of characters, so the middle name of one my sisters, which is Elisabeth, was always printed as "Elisab". :D

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54 minutes ago, David Burgess said:

I've been resisting "smart" and internet-reliant home appliances, but it's getting pretty hard to find anything else these days.

Getting back to the punch-card days: There were only enough punches available on a card to represent a limited number of characters, so the middle name of one my sisters, which is Elisabeth, was always printed as "Elisab". :D

I sent one as a valentine which said "Do not fold, spindle, or mutilate". 

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49 minutes ago, David Burgess said:

I've been resisting "smart" and internet-reliant home appliances, but it's getting pretty hard to find anything else these days.

Getting back to the punch-card days: There were only enough punches available on a card to represent a limited number of characters, so the middle name of one my sisters, which is Elisabeth, was always printed as "Elisab". :D

I sent one to my girl friend which said "do not fold, bend or spindle".

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3 hours ago, Violadamore said:

Anyone else here remember what a stack of punch cards look like? 

Not to mention the fun at 2AM in the computer room so you could get your run back quickly... and then discover that one character was mistyped and scrapped the run.

6 hours ago, David Burgess said:

Aren't desktop PCs these days bunches faster and more powerful than the computers used in the spacecraft which first landed people on the moon?

Depends how quickly the astronauts could sling the beads back and forth on the abacus.

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I remember being on the production floor of the city's main newspaper which was an early acquisition by Rupert Murdoch. Computerised photoset was being used at that time. The old, massive main-frame computer which had controlled typesetting was still there. I asked someone where the computer was and he said that I was leaning on it. It was about a metre high and square. In those days it was a bit hard to get your head around although desk-top Apple Macs used by printing shops were fairly common.

 

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