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About Johnmasters

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  • Birthday 05/08/1944

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    Physics of violins
    Finite element analysis for eigenmodes, stresses, whatnot

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  1. to make a colored, dyed, resin which could be made into a varnish.
  2. I have been given to understand from a source which I cannot remember that originally a set of instruments had TWO violas. A large one and a small one (perhaps l6 inches or so).
  3. I worked with the Michelman varnish starting in 1965. It is not stable; after two or three decades it continues to oxidize and eventually has sever "alligatoring" making small islands about 3-4mm in size. The problem is that it is a cold-mix varnish without the pre-polimerization that comes with a cooked varnish. The only reason it became popular was because it provided a way to make a colored varnish. In fact, most makers seem to be attracted to whatever varnish has a color. As for me, I like glaze coats that are not pigments, and which are very thin. Also the supporting varnish is transparent and also very thin. Michael Molnar will attest to the fact that one can make a water-borne varnish that contains silex to hold a glaze and is also only about one ten-thousandth of an inch thick. Still, it is rugged. A completed finish on a violin is no more than 3 thousandths of an inch... you can convert that to mm or microns as you wish.
  4. How difficult do you want to make this ? Yes, curvature is by definition one divided by the radius of curvature. And whatever you want to represent can be done in polar coordinates which you can understand if you google it. You can make a radius of curvature a function of the angle of rotation. It is not necessary to do all of this fudging around with shapes. You can define a curve in polar coordinates and find some simple graphing program to give the image.
  5. Both limed rosin and limed linseed oil are old and traditional (cheap) components of varnishes in the first half of the 20th century. The calcium rosinate is less acidic than plain rosin and is also tougher.
  6. It seems to be a repair where the neckblock came loose and the worker did not want to take the top off. Perhaps a pragmatic repair. Before the "pins" likely there were screws and a strip of wood to clamp a gluing. After the glue was secure, the screws were removed and the pins put in. I have not done this myself, but I have done similar things when repairing school instruments; many of them and the school had finite resources. They cannot afford Rene Morel for all of their work.
  7. You can buy various types of "stand oil." Also there is "sun-thickened" oil. I find the latter dries more quickly. It would be nice to standardize your ingrediantes The art folks have worked on these things for decades, if not for centuries.
  8. An FEA program which considers the top a simple shell with no extra thickness at the end blocks shows a definite depression at the two ends. I use all cross-directions for my curtate cycloids, as you seem to indicate that you do according to your templates. I programed my machine to program all curtate cycloides, and this does not have a recurve at either end. It is a very gradual change, so it does not look "fat." The results have seemed to work well. I still of course leave the extra wood for the block gluing, but this not a physical constraint on anything importantn IMHO. If you wish to replay, do so. Most people do not reply to my remarks, but I make them anyway. I have also done considerable study of stress-strain deformaions on decent models of plates. I use many more points than you see in the simplistic models from Schlescie. I have done both single shells and thin plates as I have 1000 nodes abailable. A plate takes about 380 points per surface. (I use the student version of ABAQUS) And most people have not done such calculations. Most use CNC with models drawn in other ways, not the way I have done. (My longitudinal arch is not a curtate cycloid.. I devised another curve which is analytic and it can be adjusted as to shape analytically.)
  9. Yes this is a great invention from the automotive field. Shim under valve stem.
  10. Perhaps damp wood is more flabby, and the stiffnesses involved have decreased. Keep in mind that a resonant frequency of a mode is of the form of square root of (k/m) where k is some kind of stiffness, and m is an effective mass involved with the resonance. (I don't mean resonance of the entire violin, I am thinking of the resonance of a particular mode.) Mode frequencies and move downward with decreasing stiffness, and perhaps even more important, damping could increase. Damping is a difficult property to measure and there are many ways to model it. You might compare a bassbar blank dry with one soaked in water.. That would certainly suggest something to you.
  11. Perhaps there is more than one protein; one hardens (makes bubbles) upon exposure to air, whereas the other does not. I know that heat changes at least one protein. Oh, I see that Mampara has already pointed out such a thing.
  12. You need to also consider the reflections of the wave from the boundary. Intuition is useless unless you actually can solve the problem. At least, I can see that there are two distinct resonators here; a somewhat simlar situation is involved with those Carribean drums with the hollows beat into them. I suspect a good model and proper solution would be quite difficult. Don't even try to think of an explanation.
  13. Ha Ha !! That is a new piece of information for me. Most of my violas were sold to advancing students, and I never had complaints. So this is new to me... I would explain how certain things were compromises, such as broader bodies for a given length. I tried to keep the string length down a bit from standard sizes by enlarging the lower bouts a very small amount. It seemed to give good results.
  14. Well, don't forget that violnists are notoriously neurotic. Violists, not so much. Do you make such adjustments for local densities in the wood? Does that help, or is that just a loose concept?" I don't think that I have ever seen a top or back that varies significantly in density over the size of a violin or viola; at least decent wood seems pretty uniform. If it was NOT uniform, I would not see good justification for leaving it slightly thicker in a small area. As I said before, an effective stiffess for mode 5 gives a good adjustment for a given piece of wood. There is another measurment that I believe corresponds to Don's radiation ratio in a near-finished plate. That would be the frequency of mode 5 divided by the mass. For a spruce top, I invariably get a superior result if this is 5 or more. Much below 5, and the sound is less alive and perhaps a bit muted.