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What about this Back Plate?


David Stiles
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3 hours ago, Don Noon said:

A related question, which I come up against occasionally:  what if the M5 remains high, even as the weight gets lower?  Where would you stop thinning?  For example, the last back I made I stopped at 101g, even though the M5 was 399 Hz.  M2 was 165.  It is not yet fully complete, but early tests indicate it is fine.  I have (for the moment) convinced myself that weight is more important than taptones.

Well, when things get weird, it's hard to make decisions:)

Probably, if the thicknesses allowed it, I would thin it further by lowering the weight down to a limit of 95g (the minimum limit I reached is 93g, but I consider it an exception rather than an ideal weight, and it was a Guarneri model which is smaller than my usual G form).

I actually don't have enough experience with violins with such high back M5 frequencies to make conclusive suggestions, I've never gone above 385Hz (and it has happened to me just once or twice), but some GdGs seem to have their backs around 400Hz so it might work just fine.

I could suggest that you lower the arch from the outside, but I assume this is not feasible, and which I personally probably would not do due to too much work it requires by my finishing standards.;)

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1 hour ago, Davide Sora said:

(the minimum limit I reached is 93g, but I consider it an exception rather than an ideal weight, and it was a Guarneri model which is smaller than my usual G form).

An important and often omitted detail is the model size, where a small form would tend to be lighter with higher taptones.  I have only made 2 of my small model (352 x 205 mm), and ended up 94 - 95g and 381 - 389 Hz for M5.  They worked just fine, and I should note that the client requests were for light weight.

Of the larger models, I have gone to 95g three times.  Two were good, one was pretty awful (M5=392 Hz) until I laminated a large maple plate to the C-bout of the back... I don't know how much it weighed, but it was fairly substantial.  So I think I'd rather not go that light with the larger model back now, regardless of high taptone frequencies.

 

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On 11/8/2021 at 9:22 PM, Anders Buen said:

I havent seen Evans article before, even if I attended that conference. I just looked at the expression for poissons ratio and see that it can’t be correct, as it will lead to poissons ratios around 0,7 or so. None woods do have so high poissons ratios. I think spruce is close to 0,5. It is different in different directions and can be measured by the compressional sound speeds in the different directions. E.g with a «Lucchi meter». 

I am sure there are good material in there. But it is very abstract.

I think that poisson ratio expression is valid for flat plates with a given dimension so both a ring and Xmode appear. It is almost identical to the expression given in McIntyre and Woodhouses paper on measuring orthotropic plate material properties.

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23 hours ago, Don Noon said:

A related question, which I come up against occasionally:  what if the M5 remains high, even as the weight gets lower?  Where would you stop thinning?  For example, the last back I made I stopped at 101g, even though the M5 was 399 Hz.  M2 was 165.  It is not yet fully complete, but early tests indicate it is fine.  I have (for the moment) convinced myself that weight is more important than taptones.

If you thin the plate only in the center area the M5 will quickly drop.  

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1 minute ago, Marty Kasprzyk said:

If you thin the plate only in the center area the M5 will quickly drop.  

If the ultimate goal of violinmaking was to get a specific M5 frequency in a free plate, that would be great.  It's not.

If there was conclusive evidence that a specific M5 frequency was correlated with a good sounding instrument, that would be great.  There isn't any such evidence that I can find in my data or anywhere else (go ahead and prove otherwise).  

In the specific example of my latest violin where the back M5 is a stratospheric 399 Hz, the assembled B mode frequencies are exactly where they should be, and IMO the overall result is well-balanced and very good.  I'd rather have that than a "perfect" back M5 frequency.

 

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