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2 hours ago, Don Noon said:

1)  With reduced crossgrain stiffnes and normal longitudinal stiffness, slabcut will be more ANisotropic.  MDF is isotropic, the same stiffness both ways... compared to maple, it's very low longitudinally, but about the same crossgrain.

2)  60% loss of modulus is a huge number, especially if it is longitudinal where the cellulose chains are mostly oriented.  I think aging affects the softer components first, and that would show up in crossgrain stiffness before much happens longitudinally.

I guess you are right on the slab-cut wood maple data. Your memory is probably fresher than mine on that. I have been working on OSB strand plates lately and have the responsibility within our company for the plate data we use in our building acoustic prediction models for sound insulation and impact noise.

But you did not answer my real question, though. The effect of slab-cut backs on the spectrum.

The E module influences the flat plate bending stiffness under a root sign, so a 60% change of the E gives some 77% of the original bending stiffness. Almost within natural variation, if i do another of my "shoot from the hip" assessments.

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15 minutes ago, Anders Buen said:

But you did not answer my real question, though. The effect of slab-cut backs on the spectrum.

Perhaps because I have never used a slab back, and therefore have no real answer.

Just theoretically, with the slightly lower crossgrain stiffness, mode frequencies might tend to be lower... unless you made the back thicker to compensate.  There would still be a different cross/longitudinal stiffness ratio, therefore different mode shapes, even if the frequencies were the same, and I have no idea what that might do.

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5 minutes ago, Don Noon said:

Perhaps because I have never used a slab back, and therefore have no real answer.

Just theoretically, with the slightly lower crossgrain stiffness, mode frequencies might tend to be lower... unless you made the back thicker to compensate.  There would still be a different cross/longitudinal stiffness ratio, therefore different mode shapes, even if the frequencies were the same, and I have no idea what that might do.

Ok, thanks. Your slab cut back competition instrument was a viola then?

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23 minutes ago, Anders Buen said:

>

The E module influences the flat plate bending stiffness under a root sign, so a 60% change of the E gives some 77% of the original bending stiffness. Almost within natural variation, if i do another of my "shoot from the hip" assessments.

Bending stiffness is proportional  to E not E^1/2.  

I'm getting stiff in my old age and I should do more bending stretches.

 

 

 

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I never made a slab back on anything, ever.  The one slab experiment was a slab TOP on a violin... actually this same VSO testbed that I have the MDF back on now (although I have put on several different tops since then).

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6 minutes ago, Don Noon said:

I never made a slab back on anything, ever.  The one slab experiment was a slab TOP on a violin... actually this same VSO testbed that I have the MDF back on now (although I have put on several different tops since then).

Ok, I must have misinterpreted. 

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6 minutes ago, Marty Kasprzyk said:

Bending stiffness is proportional  to E not E^1/2.  

I'm getting stiff in my old age and I should do more bending stretches.

 

 

 

Yes, I just saw that in the book too and was about to correct myself. The compressional wave speed has the E-modulus under a root sign. The bending stiffness for a flat plate goes as the thickness cubed. 

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The MDF plate data given in our resource for building material and "how to" information are classfied in different densities and hardnesses from 500 kg/m3 to 900 kg/m3 with the highest E-modues for the densest plates. There are softer plates too down to 250 kg/m3 used in floors or as wind shield with some asphalt in them. 

All of these have in general higher E-modules, when dry, than Maple crossgrain in average. I look at Haines data then which is a small data set (n = 22) with a few Yugoslavian data points.

Average along grain: 11152 +/- 2635 [MPa], Across grain: 2029 +/-548 [MPa] G-module: 1950 +/- 926 [MPa]

Yugoslavian subset: 10080 +/- 1644 [MPa] Across grain: 2180 +/- 253 G-module: 1848 +/- 811 [MPa]

One standard deviation variation, 68% confidence interval. If you want 95% double the sigma.

Not even the lightest plates have such a low E-modules. [N/mm^2/1000] = [MPa]

I also enclose a graph showing the effect moisture content (MC %) has on the MDF plates . I think we may see very similar effects for natural wood. 

image.png.e4a36d5765e81e2dbc3a256d046edbdd.pngimage.png.73ac647b4b3e0d1966cedb18f2306457.png

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54 minutes ago, Anders Buen said:

The MDF plate data given in our resource for building material and "how to" information are classfied in different densities and hardnesses from 500 kg/m3 to 900 kg/m3 with the highest E-modues for the densest plates. There are softer plates too down to 250 kg/m3 used in floors or as wind shield with some asphalt in them. 

All of these have in general higher E-modules, when dry, than Maple crossgrain in average. I look at Haines data then which is a small data set (n = 22) with a few Yugoslavian data points.

Average along grain: 11152 +/- 2635 [MPa], Across grain: 2029 +/-548 [MPa] G-module: 1950 +/- 926 [MPa]

Yugoslavian subset: 10080 +/- 1644 [MPa] Across grain: 2180 +/- 253 G-module: 1848 +/- 811 [MPa]

One standard deviation variation, 68% confidence interval. If you want 95% double the sigma.

Not even the lightest plates have such a low E-modules. [N/mm^2/1000] = [MPa]

I also enclose a graph showing the effect moisture content (MC %) has on the MDF plates . I think we may see very similar effects for natural wood. 

image.png.e4a36d5765e81e2dbc3a256d046edbdd.pngimage.png.73ac647b4b3e0d1966cedb18f2306457.png

I suspect maple is used for the back plate because it is a poorer sound producer than spruce so that more sound will come off the top plate than the back plate. 

MDF is not as good a sound producer as maple thus it should be an even better back plate material.

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5 minutes ago, Marty Kasprzyk said:

I suspect maple is used for the back plate because it is a poorer sound producer than spruce so that more sound will come off the top plate than the back plate. 

I suspect figured maple is used 'cuz it looks cool, and the longitudinal/crossgrain stiffness is way different from spruce.  If it was the same, I think you'd tend to get one honkin' huge B mode instead of the split B1+ and B1-.

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19 minutes ago, Anders Buen said:

Maybe if the MDF is "varnished" with thin superglue, it might stiffen up a little again. The eyes will hurt a little from the fumes.

How about  putting the thin superglue just on certain plate areas to stiffen it just there.  Maybe you could kill wolf notes that way.  I recall you mentioning putting dimples in metal plates to stiffen them in areas where the vibration was too high.

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1 hour ago, Anders Buen said:

Superglue on bridges can do wonders. :-)

What about superglue on soundpost?:)

But perhaps it is better not to make such suggestions, I'm sure someone could take us seriously!:lol:

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42 minutes ago, Davide Sora said:

What about superglue on soundpost?:)

But perhaps it is better not to make such suggestions, I'm sure someone could take us seriously!:lol:

I use "clear" Gorilla glue.  It has a much longer setting time which gives ample time to make sure everything is correctly positioned.  I also use it for gluing my bridges down.  However I do use super glue for hardening the string bridge's notches.

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2 hours ago, Don Noon said:

I suspect figured maple is used 'cuz it looks cool, and the longitudinal/crossgrain stiffness is way different from spruce.  If it was the same, I think you'd tend to get one honkin' huge B mode instead of the split B1+ and B1-.

The big difference speed of sound of spruce and maple, does this help the overall sound?

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MDF, huh? Don't talk too loud about it, or IKEA might catch wind of it and enter the instrument business :ph34r:

 

I wonder how that stuff stands up to varnish. My intuition tells me it would just die upon exposure to any sort of liquid.

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10 hours ago, Deo Lawson said:

MDF, huh? Don't talk too loud about it, or IKEA might catch wind of it and enter the instrument business :ph34r:

 

I wonder how that stuff stands up to varnish. My intuition tells me it would just die upon exposure to any sort of liquid.

I use water based Minwax polycrylic clear gloss finish on my MDF inside molds.

Long also I also used it for finish on my spruce top plates and maple back plates for my violins and violas.  Unlike other varnish- ground systems it did not change the tap tone resonance frequencies at all after it was applied.  Apparently it was adding stiffness and mass in exactly the same ratio as the base wood had to start with so it had no effect.

I thought this was great benefit until I realized that I was increasing the plate's impedance by adding stiffness and mass--the resonance frequencies were staying the same but their amplitudes were decreasing thereby reducing the sound output.

So if you are making MDF plates be careful to use only a light coat of polycrylic finish.

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On 5/4/2021 at 11:46 PM, Andreas Preuss said:

Just for clarification on this experiment:

You are replacing a back plate made of normal wood with a back plate made of MDF, right?

Arching is supposedly approximately the same?

What I have seen on my super light violin project is that changing back plates does not change awfully lot in terms of sound. What is more interesting is how playability and and perception of sound under the ear does change. Softer back plates make the sound more diffuse. At the same time lower resonances get stronger.

All this looks to me that a ‘good sound’ (whatever a maker defines as such) is based on how the top is designed as a membrane. All the rest is a kind of modifying those properties. When making the back a kind of weaker the whole setting of the string angle needs to be readjusted (re calibrated) to make it function to its best again. In that case I would most likely lower the string angle.

i am not too much experienced with reading sound graphs, but there my prediction is that you might end up with a graph of less sharp peaks in the  higher registers. 

In term of wood treatment I am not quite sure how valid this experiment is, because MDF as a material lacks one important factor because stiffness in vertical and horizontal direction is the same. Natural wood is always different in both directions. Wood treatment changes the stiffness in both directions, but never to the degree that they become equal. My experiments with steaming wood (not boiling in water) made maple measurably lighter and tap tones of the uncut planks got a bit higher presumably due to weight loss. On spruce this method didn’t show a significant change.
 

I was always wondering how internal stress in natural wood affects the sound. I am inclined to think that treatments like ponding wood in water relieve stress. There was one scientific paper which compared violins and the violin made with ponded wood showed different all over sound characteristics. (I would need to look it up again, I think it was a paper where Terry Borman participated as a maker) 

Usually my predictions are just the opposite what you come up with. :D

Very good prognostication. In fact, kudos. However, playability and sound under the ear determines acceptance by the player.

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20 hours ago, Don Noon said:

I never made a slab back on anything, ever.  The one slab experiment was a slab TOP on a violin... actually this same VSO testbed that I have the MDF back on now (although I have put on several different tops since then).

Great experiment, Don.  
Why didn’t you ever try a slab back? Just asking. I have one that is gorgeous, but the violin did not pass muster with an expert player. According to your work, I will look closer at the top - when I have time.

 

 

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21 minutes ago, Michael_Molnar said:

Why didn’t you ever try a slab back?

I have seen too many slab-back violins with cracks.  Wood is weakest in terms of crack prevention in the tangential direction, so I think I will continue to avoid using slab-cut backs.

I have made backs with maple that was cut very far off-quarter... similar to slab in terms of crossarch stiffness, but more crack-resistant, I think.  And I saw a Sam Z. violin at VSA with a high off-quarter back that sounded fantastic.  Spruce is another story, with high off-quarter being as stiff as a wet noodle due to the rectangular cell structure.

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On 5/6/2021 at 4:36 PM, Don Noon said:

Because, as I have mentioned many times, exact mode frequencies are not important, so I don't care... except in a theoretical sense in a test, to check my understanding of the physics.

Yes I know, soloists are morons! They wouldn't notice if the free A string and C or C# stands out. Probably would say - look a dog, when they hear a wolf.

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24 minutes ago, Andreas Preuss said:

The neck should have dropped by now maybe 2-3 mm? 

Unfortunately I didn't measure the projection before or after replacing the back, so I don't know for sure.  It "seems" like the neck dropped a bit after the switch, and maybe a bit more over time.  Definitely nowhere near 2-3mm creep since it was strung up.

However, I took this opportunity to liberally brush water on the unvarnished, non-waterproof MDF back, and surprisingly there was no measurable change to the fingerboard height.  I will try to think up some more destructive tests to perform on this fiddle before pitching it in the trash.

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