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Slab cut spruce top - another whacky experiment


Don Noon

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This idea is something of a continuation of finding out more about how crossgrain stiffness (and tangential and shear) affect the workings of a violin.  In the usual quartered top, the tree's radial direction is "crossgrain", the stiffest and most stable way to use the wood.  For slab cut, the tree's tangential direction is now crossgrain, which is much less stiff.

 

Evan Smith mentioned playing an old violin that sounded fabulous, and it had nearly slab-oriented wood.  Actually, that's even less stiff than perfectly slabbed wood... so maybe it might work.  Anyone else encountered slab or waaaay offquarter wood topped violins?

 

I wouldn't do this on a serious violin, so I'm using my trusty Ugly Duckling, whose top has reached the end of its experimental life (the sliced up VMAAI experimental fiddle).  I didn't have a slab-cut piece of spruce on hand, so I had to slice up the thickest Sitka board I have, rotate the pieces 90 degrees, and glue them back together.  Unfortunately, this board was .465 density, so that's another out-of-the-ordinary variable added to the mix to confuse any results.

post-25192-0-12062600-1414279169_thumb.jpg

 

The stained appearance of the glue joints is just oxidation of what was the surface of the board; I only took the lightest cut to flatten the surfaces.  I have had it for about 30 years.

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Looks very cool!  Is that a hole in the top joint in the lower bout?

 

I'm planning to use slab-cut cherry for the back of my first (learning experiment) violin, just because I have a lot of it.  Perhaps the results of your experiment would be relevant (although I've read references to examples of slab-cut backs, so I guess they are not unheard of).

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Interesting thought.   In general slab cut resists bowing more than quartersawn does. For a straight line intended to resist tension, like a neck, flatsawn is generally more stable over time.  So this might hold up to forces better.  As far as structure goes I think it is a great idea.  As far as issues over time goes though, a weak grain caused split, would be much harder to repair.  Instead of a  vertical separation, you would have a layer flaking off.

 

Since cross grain speed of sound is so much slower than the speed of sound traveling with the grain, this might have a few other advantages.

 

Bob

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No comments about the physics, but slab cut tops were in fact used by early (Brescian) instrument makers. On the instruments that I have seen (no violins that I know of) the wood appears to be of fine (mountain) growth. However, surviving examples are extremely rare. I would say that this makes a rather bold (scientific?) statement about the success rate of slab cut spruce.

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While there are potential "down the road" problems with flat sawn wood, it certainly can yield decent sounding instruments. Roger has reminded me that I forgot to mention that all the tops shown are of Redwood, a material that has similar yet different characteristics from Spruce, mostly in that it is much more resistant to bugs, rot and that its structure is much less effected by continuous expansion and contraction based on its ability to maintain its trace fluid content based on that being suspended in its oil. The oil/fluids help the wood structure to remain "wet" and or remain pliable which translates to wood that "holds" together well. 

 

An example of this is to simply observe different softwood species under harsh exterior conditions. Spruce, in large dimension, such as a ship mast, holds together well, but when in thinner smaller dimensions it shows its weakness. Redwood, when compared to Spruce, Fir or Pine maintains its integrity much more, and or the Redwood, barring being grey, will basically look the same months down the road whereas the other species will have split, cracked and or curled when left out to the elements. Cedar has similar characteristics, often in much lower density.

 

I am aware that Redwood is not a traditional Italian tonewood, and therefore maybe a moot point related to Don's experiment, but it does work well flatsawn as a top material if anyone were to want to go down that road.

 

As always it will be interesting to see what Don comes up with.

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There are two measures of success that I don't want to get confused.  One is the sound.  The other is if it holds together.

 

I have no illusions about the likely high failure rate of slab cut spruce tops in a structural sense.  Slab backs, maybe, as maple isn't so wildly varying as spruce in its properties depending on grain direction.  So I would never consider slab spruce tops on anything other than an acoustic experiment... which this is.

 

Normally I try to state the expected result of the experiment before actually performing it.  In this case, it is closer to exploratory surgery just to see what's there.  However, I can at least go into the ideas motivating this test, and perhaps some guesses about the result.

 

The main underlying motivation is from the inability of being able to get a good sense of a violin's final result, based on the measured properties of the wood I start with.  One possible answer is that my construction varies too much to get any consistency... and that may be so.  I just have a feeling something else is going on, something I have not been measuring thus far.  

 

In thinking about the force that a string puts into the body, it's similar in some ways to a periodic impact... i.e. rapid rate of change of force.  Not a nice, smooth sine wave.  Under that kind of driving force, I thought that there might be some compression under the bridge foot, or shear wave transmission in the top that could be important.  These involved wood parameters that I had not been measuring (tangential modulus and shear modulus, respectively).  I had thus far only been measuring longitudinal stiffness and damping, and crossgrain stiffness.  I tried to make some estimates of the bridge foot compression effect, and it's an outside possibility that it matters.

 

Here are some numbers that are relevant; one set for the wood used in this test, and another for some low density Engelmann.  Modulus is in mPa, a measure of absolute stiffness.

 

Measurement                    Sitka        Engelmann

 

Density                               .465            .314

Longitudinal modulus       16000          7300

Radial modulus                  1300            893

Tangential modulus             760            348

 

Just for grins:  another Sitka sample at ~40 degrees off quarter measured 150 mPa... less than 1% of the longitudinal stiffness.

 

Getting back to this test:  by going with the slab orientation, I would double the stiffness under the bridge foot, and likely have far higher shear stiffness as well.  I expect that might do something different (quicker) in the transient response, as well as have less losses in the high frequencies.  Unfortunately, this is really heavy wood, so that might obscure or skew the results.  And the low stiffness across the top grain might also have some effect... perhaps less power around 1 kHz?

 

Right now, I'm approaching a graduation stopping point on the top.  Taptones M2 and M5 are about in my normal range, but the weight, at 74g, is 15 - 20 grams higher than my normal.  It will get a little lighter with scraping, but not much.  I'll use light glue to put the top on, with the idea that I'll likely thin it out some more to see where it goes.

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I do think shear has something to do with it, Particularly related to "feel" or "tightness" related to bow response. I think shear forces and load shifts both the rate of speed and overall force are a very unlooked at area.

 

Related to shear, it is one of the major reasons I keep/use blocks in my guitar rib structure. It add a massive amount of shear stiffness to the rib structure.

 

Controlling or shifting shear load requirements or limits, in certain areas, in my opinion gives one the ability to "drive" or steer more or less vibration to other more desired areas...I want my plates moving, not my ribs, or at least I don't want them robbing my plate movement.

 

I think this is where the confusion in plate tuning has come in, Shaving it here, makes this area "ring" lower, leaving it fat here, makes this area "higher" in pitch. My postulation is that "tone" or a ringing note is a byproduct of shear load shifting along with dimensional thickness flexibility and should be disregarded as the goal parameter to look at. To me the "secret" lies in thicker or thinner areas, along with grain structure creating higher shear loads in that particular area which alters the resulting vibration pattern which gives us the quality of the note we hear when played, the notes we need to be concerned about, not the byproduct notes of tapping.

 

I look at instrument making from a structural view point and that if everything is put together correctly the structural characteristics of both the building and material are what yield the tones and sound we want, not tones of individual areas being built when in an independent state.

 

When I think of shear resistance and how it works in the picture, it is quite vast, but my primary areas of focus are how the rib structure establishes sheer resistance in the entirety of the corpus, along with the neck heel, and how thinner and thicker areas in the plates establish sheer resistance points or areas which will not only bounce up and down, twist and torque, but also the way the shear load works into that, particularly at the bridge foot area as you mention. I think of thicker areas in the free plate as sheer/nodal islands that not only effect stiffness and flexibility but also reaction times based on shear stiffness or weakness. 

 

Similar to thinking of introducing speed into doing shear tests on deck screws. It's one thing if we have an established load setting putting x amount of force down, slowly approaching the snapping point, and it's another when rapidly apply the force all at once. Assuming we are building things that are going to resist breaking by being dimensionally sound, we then want to concentrate on how this shear force works with the radiation of the material and how quickly it will exhaust the moment by moment vibration pattern and thus return on the next momentary bridge cycle.

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One thing I would say, is that by laminating the material, depending on the way he arranged it, he will end up with a more homogeneous piece of material, but the over all cross grain stiffness of this lamination will be much stronger than that of a split back or one piece top of similar dimension and shape.

 

Think of a small 12 cutting board sized piece of soft wood that is say 3/4" thick, when curled/bent, it will eventually have a tendency to crack/split on the weakest grain run, somewhat dictated by how it is fixed and how the pressure is applied , now, applying the same force to a grain alternated lamination, based on the fact the most glues, certainly hide glue, will dry stronger than the wood grain itself this tends to shift the pressure build away from the glue seems and to the grain structures between the wood lamination or glue joints, then, because the pieces are so small, it becomes very much like you trying to take a 1" wide board and snap it in half with your hands, the wider the piece the easier to snap,based on leverage, but eventually they get to narrow for you to snap....so this force goes to the wood grain, but because the pieces are so narrow they will not break, this then sends the force back to, or it will start to bunch again at the glue joint and then eventually you get catastrophic failure, usually in the glue joint.

 

So Don should take that into account in my opinion.

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"So Don should take that into account in my opinion."

 

Or he could just have a little fun with it.  :)

Chris, I wasn't needling Don, or telling him what or how to do anything. Simply going off on a tangent related to the comment about the piece being strips, we're big in tangents here.

 

I have done several post's revealing or showing off the things I do and know, or think I know and Don is one of the few other posters here that does the same. We all have fun when Don, I or anyone takes the time to "put something" together to show us all, regardless of the level of professionalism, experience of the poster or presentation. Besides, Don knows I have a tendency to point out things that all the adults overlooked. :D  

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My take on the lamination:  it acts just like an unlaminated piece for stiffness.  The glue joints are so thin that it makes no difference.  Wave your arms as much as you want, then do a test and see (Rick Hyslop did that test a while back... see http://www.maestronet.com/forum/index.php?/topic/330890-springing-the-bass-bar/?p=639117)

 

About shear: there's shear in the body, which affects the low modes (like CBR, especially), but I'm primarily concerned about shear as it affects waves in the plate travelling along the grain.  

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My take on the lamination:  it acts just like an unlaminated piece for stiffness.  The glue joints are so thin that it makes no difference.  Wave your arms as much as you want, then do a test and see (Rick Hyslop did that test a while back... see http://www.maestronet.com/forum/index.php?/topic/330890-springing-the-bass-bar/?p=639117)

 

About shear: there's shear in the body, which affects the low modes (like CBR, especially), but I'm primarily concerned about shear as it affects waves in the plate travelling along the grain.  

Accounting for the thinness, your right, it wouldn't make much of a different like it would if it were thicker, after thinking about it. Yes I was mixing and mashing the two types of shear. I think flat sawn is prone to different internal shear force than quartered, inherently by the nature of the cut.

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My take on the lamination:  it acts just like an unlaminated piece for stiffness.  The glue joints are so thin that it makes no difference.  Wave your arms as much as you want, then do a test and see (Rick Hyslop did that test a while back... see http://www.maestronet.com/forum/index.php?/topic/330890-springing-the-bass-bar/?p=639117)

 

About shear: there's shear in the body, which affects the low modes (like CBR, especially), but I'm primarily concerned about shear as it affects waves in the plate travelling along the grain.  

 

Don while I did try out the laminate idea in one test I wouldn't say that it proves anything absolutely. My test showed that the laminate was certainly no stronger (perhaps even minutely weaker) than the non-laminate. One problem I have with my test however is that I would have liked to have used a hide glue instead of carpenters glue and I should have waited longer than a few hours to carry out my testing. For what it is worth.

 

My initial feeling when I saw your post was that the plate would be more prone to deformation over time and possibly separation across the grain in cases where it might be most extreme.

 

Nonetheless I am curious to see what you come up with here.

 

Cheers.

 

r.

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I do think shear has something to do with it, Particularly related to "feel" or "tightness" related to bow response. I think shear forces and load shifts both the rate of speed and overall force are a very unlooked at area.

 

Related to shear, it is one of the major reasons I keep/use blocks in my guitar rib structure. It add a massive amount of shear stiffness to the rib structure.

 

Controlling or shifting shear load requirements or limits, in certain areas, in my opinion gives one the ability to "drive" or steer more or less vibration to other more desired areas...I want my plates moving, not my ribs, or at least I don't want them robbing my plate movement.

 

I think this is where the confusion in plate tuning has come in, Shaving it here, makes this area "ring" lower, leaving it fat here, makes this area "higher" in pitch. My postulation is that "tone" or a ringing note is a byproduct of shear load shifting along with dimensional thickness flexibility and should be disregarded as the goal parameter to look at. To me the "secret" lies in thicker or thinner areas, along with grain structure creating higher shear loads in that particular area which alters the resulting vibration pattern which gives us the quality of the note we hear when played, the notes we need to be concerned about, not the byproduct notes of tapping.

 

I look at instrument making from a structural view point and that if everything is put together correctly the structural characteristics of both the building and material are what yield the tones and sound we want, not tones of individual areas being built when in an independent state.

 

When I think of shear resistance and how it works in the picture, it is quite vast, but my primary areas of focus are how the rib structure establishes sheer resistance in the entirety of the corpus, along with the neck heel, and how thinner and thicker areas in the plates establish sheer resistance points or areas which will not only bounce up and down, twist and torque, but also the way the shear load works into that, particularly at the bridge foot area as you mention. I think of thicker areas in the free plate as sheer/nodal islands that not only effect stiffness and flexibility but also reaction times based on shear stiffness or weakness. 

 

Similar to thinking of introducing speed into doing shear tests on deck screws. It's one thing if we have an established load setting putting x amount of force down, slowly approaching the snapping point, and it's another when rapidly apply the force all at once. Assuming we are building things that are going to resist breaking by being dimensionally sound, we then want to concentrate on how this shear force works with the radiation of the material and how quickly it will exhaust the moment by moment vibration pattern and thus return on the next momentary bridge cycle.

There's a lot of meat in here, and you and Don are a couple of the people that I give free pass around my BS or potential BS filter.  Unfortunately, I really don't understand what you guys are talking about.  Jezzupe, Don and others that really get this stuff can you recommend a book or books to help me grasp these concepts.

 

Thanks,

Jim

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The wood's  circular annual rings (cup down) could nearly follow the surface contour of the top in the bridge area if the wood is carefully selected and cut.  I think this would give the maximum cross grain stiffness in that area.

 

On the other hand, at the plate's edges the plate curvature reverses so the ring's would be intersected so the cross grain stiffness would be low along the plate's edges.

 

On the first foot, you might want the wood's annual rings to follow the entire plate's surface contour which suggests that you could glue strips such that the annual rings follow the entire surface contour which would give the greatest stiffness.

 

On the second foot, you could reverse everything to get the lowest possible cross grain stiffness or you could mix everything up to get one side stiff and the other side flexible etc. with lots of different combinations that might have some acoustic effect but then you have to go to fingers.

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The wood's  circular annual rings (cup down) could nearly follow the surface contour of the top in the bridge area if the wood is carefully selected and cut.  I think this would give the maximum cross grain stiffness in that area.

 

This statement hints at the common misconception that the annular rings act like stiffeners, making the ring directions stiff and strong.  Not so, as at the microscopic level, the cell walls do not line up in that direction.  The tangential direction, parallel to the rings, is the least stiff and weakest of the 3 main directions.  Quartered wood maximizes crossgrain stiffness.

 

Although you may have meant that the curved slab maximizes stiffness compared to other slab cuts, which I do agree with.

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The wood's  circular annual rings (cup down) could nearly follow the surface contour of the top in the bridge area if the wood is carefully selected and cut.

If you start delving into some of the local traditional stringed instruments of the past and in some places present, you will find a few where a log is split in half and the instrument back is carved as a bowl out of the log.  Often they pick really hard tough woods like mulberry.  Seems like a lot of work unless there is some advantage to it.  Then again, traditional sound can trump acoustic perfection in this sort of instrument.  The grain would tend to match what you describe.

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There's a lot of meat in here, and you and Don are a couple of the people that I give free pass around my BS or potential BS filter.  Unfortunately, I really don't understand what you guys are talking about.  Jezzupe, Don and others that really get this stuff can you recommend a book or books to help me grasp these concepts.

 

Thanks,

Jim

Here is a very simple vid that explains shear force visually, very basic. As Don pointed out , when we talk about shear, we need to think of it in 2 different way. 1. Shear related to the structure of the instrument itself and 2. Shear related to the internal structure of the wood itself.

 

https://www.youtube.com/watch?v=-Jbdofrbt6Y

 

I truly feel that the "secret" lies somewhere in the structural understanding of what I call desired force and weakness. We want the instrument strong enough to hold together well, yet built just to that point, we want a certain amount, the max amount, of weakness built in. Weakness equals freedom of motion, whearas strength equals limiting motion. Strong enough to hold together, but as weak as it can be to allow for max range of motion without inducing wolf tones or collapsing.

 

Shear forces and how they act internally on the ring growth delineations will effect the 5 main ways of structural motion, bending, compressing, shearing, tension and torsion.

 

So I try to think of what's going on in two/three ways, how are the forces acting on the entire instrument, and then how are the forces acting on individual components, and then finally of the individual components, how is the structure of the wood grain being acted upon, and or how does it's inherent structure based on cut/species of that particular piece effecting things.

 

There are several videos on utube that discuss structural engineering. I am somewhat surprised by the lack of study of structural engineering in instruments and further surprised that plate tuning has dominated so much mental energy in the search for secrets to good tone.

 

I also feel that there is a large benefit of understanding or looking at ALL string family instruments, and just how do forces act on different types of vibration patterns.

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Here is a very simple vid that explains shear force visually, very basic. As Don pointed out , when we talk about shear, we need to think of it in 2 different way. 1. Shear related to the structure of the instrument itself and 2. Shear related to the internal structure of the wood itself.

 

https://www.youtube.com/watch?v=-Jbdofrbt6Y

 

It's amazing how much information a 71 second video with no words can convey.  It also now helps me make sense (more sense) of the bass bar thread that talked about added strength due to lamination.  So to start clawing my way up the learning curve I should look in the direction of structural engineering?  I'm a visual and tactile learner.  I learn from books and/or getting my hands dirty.  I'll start looking for good resource for what I need without getting lost in a rabbit hole.

 

Thanks!

Jim   

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