Violin Forum/Message Board

John Schmidt · Posted: Thu Mar 07, 2013 9:29 pm Post subject:

In using the cycloid program, the part I don't know is where should the lowest point of the trough be? How far from the edge? I am guessing it is not simply the same all around the violin. How about a guide, or some examples? Is the trough 1 cm in from the edge, in the lower bout? This would be very useful info. If you have a good violin, which I don't, you could put a marble in the trough and measure where it sits.

actonern · Super Member Joined: 15 Aug 2007 Posts: 444

You could try this method... Smile

Take the square root of the length of the instrument. Lets say it's 350 mm long. Square root is 18.71.

Divide any plate width point you want the computer to print a cross arch of by this number.

Example: top width 165mm/18.71 = 8.8 mm.(in from the edge)

Mid bout narrowest point 110mm/18.71 = 5.9mm

Bottom bout 205/18.71 = 11mm.

It's not that far off of what can be observed from the Strad posters...

E

jacobyfineviolins · Member Joined: 04 Mar 2013 Posts: 55 Location: Omaha, NE

The low point of the channel should be in the purfling, for arch structure. Measurements of a centimeter in, in the lower bout, might tell you at what distance the arch is level with the edgework, not where the channel is lowest.
Not saying those violins aren't out there: the marble-rollers.
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actonern · Super Member Joined: 15 Aug 2007 Posts: 444

If you go to this page and play the "movie" of a laser scan of an arch... you'll see it about 1/2 way down the page... it looks like the bottom of the trough is inside the purfling to my eye...although pretty much right on it in the middle bouts.

http://darntonviolins.com/violinmaking.php#images

Are you seeing the same thing?

E

Michael Darnton · Posted: Thu Mar 07, 2013 10:10 pm Post subject:

Yes. The German way (ahem, Prier, cough, cough) is to put it on the purfling. On Cremonese violins the purfling is almost aways on the rise of the scoop. There's a method to it, but I've been too lazy to sit down and track out some of the ideas I have--I just steal the low point from STRAD posters and the Biddulph del Gesu books. Me bad.
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kjb · Super Member Joined: 06 Feb 2013 Posts: 385

on the strad posters, they give a height for the long arch and the cross arch as different heights, Is the long arch geometrical center and the cross arch the center between the bouts, and if so it this the acoustical center?

Michael Darnton · Posted: Fri Mar 08, 2013 10:50 am Post subject:

There are a couple of problems with taking templates directly from posters of old violins.

Most violin bodies have a lengthwise sag, a hunched back, from string tension. On the back, this raises the center of the back relative to the ends, and on the top it lowers the center relative to the ends. Therefore, the long arch is not on the same plane as the cross arches, worst at the middle, better as you move towards the ends. That means you can't build on a plane flat ribset using templates taken from no non-planar violin and expect the templates to work. The center ones will always be flatter than they need to be to reach down to level of the edge when the edge is truely level.

Second, there are many other distortions--the post pushes the back out, which draws height FROM the surrounding area, and the body may have twists, too. The post is on one side, not the center, so the distortion is skewed sideways. Tops distort in all sorts of ways, not only because they're softer, but they have various other stresses on them; bar, ends pulling in from neck and tailpiece, post pushing up, bridge pushing down. The ways they can move are very complex.

However, to answer your most direct question last, the reason the middle template is a different place is because template #3 is not in the center of the violin; it's at the narrowest spot, which is about 25mm or so north of the longitudinal center, so it's not at the center of the long arch, where the long arch height is measured, nor is it reliably in the midpoint of the corners or c-bouts.

I'd encourage you not to think in terms of acoustical center. I doubt very much that the original makers had any such concept, given that there was no science of acoustics at the time, and if you start thinking that way you'll be drawing a road on the map that isn't there in reality.
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kjb · Super Member Joined: 06 Feb 2013 Posts: 385

thanks , I had heard the term here and there but never understood what they were talking about.\

Thanks for pointing me in the right direction for the posters, I like the idea of drafting out the shape from what saconni has in his book, and then the cycloids from there. THat program is very nice .

For that program, for the arch heights, is there a basic idea of what height to use for top and back. I remember you suggested not going under 15mm for the top , so I abandoned the one with the shake which would bring me lower.
something like 15.5 for the top and 16 for the back?

Michael Darnton · Posted: Fri Mar 08, 2013 12:52 pm Post subject:

Those are reasonable numbers. An additional half mm on both won't hurt your chances of making a nice violin at all--higher ones are just a bit easier to make work nice.
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jacobyfineviolins · Member Joined: 04 Mar 2013 Posts: 55 Location: Omaha, NE

Michael, do you think the top being higher than the back on old instruments is due to soundpost stretching over time? And do you build the back higher to begin with to try to end up with even arches, or is it simpler; a matter of personal taste?
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Michael Darnton · Posted: Fri Mar 08, 2013 1:38 pm Post subject:

I think they started out the same. Tops have the tendency to go right up, within a couple of years. I think Sam Z wrote an article about that a decade or so ago, where he'd measured his arching heights over time after completion. I like to start the top a little low because for me it seems to sound better. Or not. Well, it doesn't hurt, anyway. :-)
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