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Densities

 
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Dave Chandler
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PostPosted: Tue Feb 17, 2015 6:19 pm    Post subject: Densities Reply with quote

Two questions -- Question 1 -- If you have made a top from a piece with .38 density and back and ribs from a .60 piece, and you're happy with the result, is this a reliable indicator for making a future fiddle from similar density woods and similar arching/graduation patterns? I've never had the opportunity to make two violins from the same set of logs.

And "B" -- is there a workable ration between top and back, i.e. a top of .35 works best with a back of .55 or anything similar?

Sorry to sound so naive, but now that I'm getting technique down the more subtle aspects of making are dawning.

And while talking densities, I'm presuming the easiest technique is to slowly lower a piece of wood into a big 5 gallon bucket and mark the point where it starts to float, then divide submerged dimension by overall dimension? Of course, not so slowly it gets waterlogged before you get your measurement.
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Dave in the Blue Ridge
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Michael Darnton
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PostPosted: Tue Feb 17, 2015 11:57 pm    Post subject: Reply with quote

I've made violins with all sorts of wood, randomly, and the only ones that have been total failures have been those made with very low density tops. When I was regularly using very high density wood, I worked to weight to bring the weights down to normal (100-120gms for a back, 65-70gms for a top) but now I don't even do that, and have made some pretty heavy ones that worked just fine.

My conclusion along the way is that arching is critical: if you get that right, it builds the right kind of flexibility into the structure that weight can't take away. However if the arch is wrong, and not flexible in the right coordinated way, the only way you can bail out is to remove wood until the violin moves, in spite of the way it's made.

If you're making heavy violins and they work, you're fine, though the trick is to be able to do it consistently: any mistakes, and they won't work. People who have to make their violins thin to work don't really know what they're doing, in my opinion--they're just using a cheap dodge to make a turkey sing.

What I found with light wood was that it moved responsively, but there was no way to build complexity into the sound--it just dumped everything it had, all at once rather than portioning it out according to how the player was playing.
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Dave Chandler
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PostPosted: Sat Feb 28, 2015 10:57 am    Post subject: Densities Reply with quote

I've found this article, which poses the same questions I have posed. After examining sg for a large number of old cremonese violins, he comes up with a ratio of 150% back to front densities. But when you look at the individual instruments, they're all over the place. Of 20 Strads, the tops range from .34 to .46 and ratios top to back that are 1.1 to 1.8. I think the answer is .... no relationship. So, as Michael Darnton suggests, other things are more important.

http://www.trioviolinproject.com/plate-densities/
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Michael Darnton
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PostPosted: Sat Feb 28, 2015 2:18 pm    Post subject: Reply with quote

Interesting chart. Permit me to notice something. Let's take their "Standard Density Average" for Stads, 1.48, and look for all of the instruments within plus or minus 10 points of that--that's a 21 range out of 148, or something around 14%, a pretty wide range for someone who might profess to pay attention to this factor in making. How many instruments of the 20 fall within that? Six. That means that only six out of twenty Strads--30%--fell within 14% of their declared ideal.

Are you impressed that this ideal figure is meaningful? I'm not. As Dave says--it's all over the map.
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Dave Chandler
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PostPosted: Sun Mar 01, 2015 2:05 pm    Post subject: Densities Reply with quote

So what use is Specific Gravity to us? I've heard other makers state that for less dense wood, they make the plates thicker and vice versa. Other makers go for a standard weight whether thick or thin. I think they're making the same general adjustment for plates with wood of different specific gravity. So whtat is a rule of thumb for making these adjustments?

KEEPING IN MIND THAT I'M A RELATIVE NOVICE -- Since we know that there is a strong relationship between density and stiffness (and hardness, and overall strength in a general sense), we can have a starting point in considering what changes we might make in graduating thicknesses for different densities of wood. If we construct a spruce top of .38 density spruce with a successful outcome, then if we use wood from a log that is .35 in our next instrument we might consider an increase of thickness by about 11% (.38/.35) for this lower SG/less dense wood to get a (somewhat) relatively stiff top. So instead of 2.5mm in the lower bouts, you might go with 2.7mm.

This is probably too simple, isn't it?
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Michael Darnton
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PostPosted: Wed Mar 04, 2015 1:00 am    Post subject: Reply with quote

The problem with rules like this is that they assume that there is one correct answer; that there's a perfect thickness for every density of wood. But players are different, and need different things.

I think you could do just about anything you wanted regarding density vs grads, and find a player who felt that you had done the perfect thing. . . and many others who thought what you'd done was exactly wrong.
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Dave Chandler
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PostPosted: Wed Mar 04, 2015 6:52 pm    Post subject: Densities Reply with quote

Michael Darnton wrote:
I think you could do just about anything you wanted regarding density vs grads, and find a player who felt that you had done the perfect thing. . . and many others who thought what you'd done was exactly wrong.


Thanks Michael. Just seemed intuitive to me that there is a relationship between specific gravity and graduations. Your response implies you wouldn't consider the SG in determining graduations.

Once again, I'll ask the question -- why do we give a flip about specific gravity anyway? Do you consider SG when thinking about arching?
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Michael Darnton
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PostPosted: Wed Mar 04, 2015 8:37 pm    Post subject: Reply with quote

I don't think about weight/grads any more except in one context: how hard of a pusher will be playing this violin. If it's likely to go to someone with a heavy hand, I make it thick; to a light hand, I make it thinner. Regardless of the thickness I choose, the easy response is always there at the low-pressure end of things. I no longer depend on thin plates to make my violins "work", because they do work, regardless.

I keep saying this, and am surprised how little impression it makes: if you get the arching exactly right, many of the other problems of violin making totally disappear. In my summer violin making classes I have seen people make huge leaps going from making mediocre-sounding violns to really good ones in the space of one violin, simply by following directions. Nevertheless, many people seem to be resolutely driven to "do their own thing" and never try what I suggest even once.
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Dave Chandler
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PostPosted: Thu Mar 05, 2015 11:06 am    Post subject: Densities Reply with quote

There we go! That makes sense.
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