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Soundpost Tension


John Masters
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No doubt about it.

 

Very many makers and repairmen both, I believe, think close enough is OK.

Which is sort of the crux. Close is OK - sort of, but  it must be VERY close.

 

(gimme the painstaking work - it's a job I love doing...)

Becker-close is something I found out about when he saw that I had moved that post.  He had not seen that  violin for years.   I had moved it what I considered a very small amount.  Less than 1mm away from the bridge.

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Wouldn't it be nice to a have a piezio pad the size and shape of the post end,that could direct actual preasures and even live time played data? that would be cool. short of haveing Jerry or some other show how ,I wonder how variable any given setter/settie is ?

  So in all could it be a decent guess to say the tighter post because of the preload would support higher end harmonics and looser low end?

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Wouldn't it be nice to a have a piezio pad the size and shape of the post end,that could direct actual preasures and even live time played data? that would be cool. short of haveing Jerry or some other show how ,I wonder how variable any given setter/settie is ?

  So in all could it be a decent guess to say the tighter post because of the preload would support higher end harmonics and looser low end?

Hard to say...  There is some relation of static loads to sound.  I once took a Hopf violin and drilled & tapped an 8-32 hole in the back where the post would go.  I put a small maple shoe with a divit  behind the bridge foot on top.  The screw had a point turned on the end.  I made the "post"  snuggish and strung it up.  I had the screw slot as a reference and tried playing it with different tensions.  When strung up,  a little more tension improved the violin.  Too much slowly made it thin and shrill.  However, retreating from way too tight to too loose,  the tone improved slowly until suddenly it became very dead rather quickly.  The point of this sudden change in tendency was near where the top started to be compressed.

 

I think this is a test that anyone could do with a junker.  Just rough the thicknesses down if they are really thick and do it.

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That's pretty ingenious, John! 

 

Now, if one were to combine your adjustable post with the piezo pad that James suggested, they could generate some data points.

 

Mac

You would get lots of answers.  But what are the questions?  I played with FEA (finite element analysis) models for a couple of years and learned a lot of qualitative things....  The FEA was not detailed enough to get precisely accurate predictions.  However,  one can definitely see trends.  The basic behavior of static distortions can be seen. 

 

I stopped doing it when I ran out of questions...... As Doug Adams reported in Hitchhikers Guide to the Galaxy,  Deep Thought calculated the answer to "life, the universe, everything" as being 42.  This astonished people, but then Deep Thought said that he was afraid that they did not actually know exactly what the question was.   I think it is a very fun book,  and Adams certainly seems to have been a bit of a philosopher.

 

I think that Carl Becker Jr. asked a very good question:  "How much will the body bulge out if compressed without a post?"  

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Yes and no. If I'm about done with the archings, and they're symmetrical, I leave them. If they're slightly asymmetrical in a way which will counter creep over time, I leave them. If they're asymmetrical in the direction of creep, I do some tweaking. Both top and back.

 

Can't say that the distortion on the back really matters though. All old instrument have it, including the best sounding ones. But according to Rene Morel, when it gets bad enough that the post position is the lowest part (on the inside), the instrument won't sound good anymore and the back arching needs to be corrected.

Wouldn't you say that this was the situation where the post was most rigidly attached to the back?  Any ideas? 

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Given that the soundpost is usually positionned relative to the bridge, is it ok to move the bridge 0.5mm north, south east or west to see the effect moving the soundpost would have if moved the same way (or rather a symmetrical way?) So for minimal move, is it possible to simply move the bridge?

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Just a comment-

Years ago when I was still making, all violas, without a doubt there would be a beautiful F# wolf.  In frustration, I made   4-legged sound posts (X),

3-legged sound post (Y), up or down, bent 2-legged sound posts, with maybe some damping of the wolf and not much change in sound as I recall.

fred

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I guess I was more curious about the back. I was wondering if it would flatten over time if giving enough pressure at the heal. Or at least counteract the tendency to bulge at the soundpost area.

Haven't noticed flattening of the back, but it's probably because the back is much stiffer than the top.

Bulging at the soundpost area (treble side) seems to mostly borrow arch from the bass side. In other words, the treble side bulges, and the bass side flattens. That's based on making cross-arching templates for a number of instruments, and comparing them with the instrument over periods of many years.

 

Becker-close is something I found out about when he saw that I had moved that post.  He had not seen that  violin for years.   I had moved it what I considered a very small amount.  Less than 1mm away from the bridge.

Carl Jr. had an amazing eye for detail, and an amazing memory!

 

That's pretty ingenious, John! 

 

John Masters has come up with some cool things. I don't always understand what he's saying, but when I do, it's usually at least been valuable food for thought.

 

Wouldn't you say that this was the situation where the post was most rigidly attached to the back?  Any ideas? 

Yes, I'd guess that that when the conventional post position becomes the lowest point of the back arch (from the inside), with adjoining areas flattened, it would make the post support more rigid statically.

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Given that the soundpost is usually positionned relative to the bridge, is it ok to move the bridge 0.5mm north, south east or west to see the effect moving the soundpost would have if moved the same way (or rather a symmetrical way?) So for minimal move, is it possible to simply move the bridge?

I think that works quite well for newer instruments. On older instruments, where the bridge is sitting in a ditch, you've got bridge foot contact variables throwing things off when you move the bridge even a little bit.

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Becker-close is something I found out about when he saw that I had moved that post.  He had not seen that  violin for years.   I had moved it what I considered a very small amount.  Less than 1mm away from the bridge.

I mentioned to Mrs. Becker that I hadn't had the sound post adjusted the way most people do, but I hadn't felt any necessity.  She told me they don't move the way Carl put them in.  She also told me, "Don't let anyone touch it."  I haven't!  :)

 

Incidentally, there's a pencil outline of a former bridge location.  It's not close to where Carl set it.  Probably 2 or 3 mm away.

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Haven't noticed flattening of the back, but it's probably because the back is much stiffer than the top.

Bulging at the soundpost area treble side) seems to mostly borrow arch from the bass side. In other words, the treble side bulges, and the bass side flattens. That's based on making cross-arching templates for a number of instruments, and comparing them with the instrument over periods of many years.

 

This is bourne out with my FEA experiments.  I had a model of an entire body with struts to represent a neck and 4  points for a nut.  It had a bassbar.  removing the post caused a sag at the right foor of the bridge as expected.  It has been a while,  and I had a lot of models.  I can't really answer any questions about it unless I get back into it.  I recall that the bridge pushed the bar down and the compression did not compensate completely.  I wish I had numbers,  but they are temporarily lost.  In other words,  I don't know how far up the bassbar side would have been pushed up with no bridge and just the end compression.

 

Carl Jr. had an amazing eye for detail, and an amazing memory!  Yes, he did.  and he felt that the post pre-stressing was important.

 

John Masters has come up with some cool things. I don't always understand what he's saying, but when I do, it's usually at least been valuable food for thought.    Please ask about it.  The Becker experiment seems too important to ignore.

 

Yes, I'd guess that that when the conventional post position becomes the lowest point of the back arch, with adjoining areas flattened, it would make the post support more rigid statically.

 

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Wouldn't you say that this was the situation where the post was most rigidly attached to the back? Any ideas?

For the lowest body modes don't we need stretching/bending (especially longitudinal) in both plates. I assume when the back has stretched so much statically (tent shaped) that there just isn't that much going on anymore?

Thank you John for your fabulous thoughts on this topic.

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I assume when the back has stretched so much statically (tent shaped) that there just isn't that going on anymore.

 

That sounds about right.  If the back is a straight-sided cone, with the soundpost at the apex, that's as statically stiff as you can get (at the soundpost, that is).  Additionally, I think just about all of the normal modes will have a nodal point at the apex... in other words, the back won't generate any sound.  While this is an extreme example, I think that if the shape tends toward that direction, it will acoustically tend toward that behavior.

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I can remind you and the others that Carl Becker did something to decide what size shim to put on the end of the post at the back contact end. 

 

A guess with no evidence:  He had a jig which held the violin rigidly.  Perhaps at the end blocks.  Also a dial guage for precision measurements which would measure at exactly the points on the outside of the violin where the post contacted.  Then he could measure the distances with and without the post.  Of course it is necessary that the jig hold the dial gauges or caliper in a fixed location.  Both back and top move,  so maybe two dial gauges.  That is how I would do it.

 

And you could do this and see what happens:  Pull strings up to tension with a lightly-fitted post.  Then again with no post.  The back will come up and the top will go up a bit more.  I think he fit a just-snug post and did the two measurements.  He winds up putting about a .4mm shim on the post. Perhaps the difference of the measurements was .8mm and he split the difference.  Or some other measurement which he took a fraction of.

 

He specifically told me that the shim was "What the violin needed."   You are Roger Hargrave,   what do you think ?  If Becker did it,  it must be something clever and useful.

Couldn't you just measure the distance between the plates on the inside instead of two calipers on the outside? Perhaps one of those special sound post inside calipers people use to measure a post the first time. You would need to know where to measure but I know the Beckers had a method for this.

My experience has been that position is the most important criteria for sound post fitting. Becker's admonition that once they had found the correct placement there was no need to ever move it would seem to support that observation.

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curious1, on 12 Dec 2013 - 5:41 PM, said:

For the lowest body modes don't we need stretching/bending (especially longitudinal) in both plates. I assume when the back has stretched so much statically (tent shaped) that there just isn't that going on anymore?

Thank you John for your fabulous thoughts on this topic.

It depends on whether the wood is still in an elastic condition, whether it has stretched beyond its elastic limit. I will explain.

I used to have a summer job for the Ohio department of Highways in their testing lab (for materials used in road construction.)

We would take a piece of reinforcing steel and stretch it in an hydrolic machine until it broke. The force and stretching was proportional up to a "knee" where it would suddenly stretch much faster and then break. (A graph of force vs length would be a straight line up to the "knee" whereupon the line would bend over to have a smaller slope.)

I personally don't think anything in a violin is pushed to any elastic limits. Jim Woodhouse thinks this is true also. If you preload a spring, is it "stiffer" or will it still stretch in the same way? Within limits, the force vs stretch is linear (proportional). I don't think we need to leave the linear region to discuss violins, but I may be wrong... after all, players criticize to the nth degree.

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Couldn't you just measure the distance between the plates on the inside instead of two calipers on the outside? Perhaps one of those special sound post inside calipers people use to measure a post the first time. You would need to know where to measure but I know the Beckers had a method for this.

My experience has been that position is the most important criteria for sound post fitting. Becker's admonition that once they had found the correct placement there was no need to ever move it would seem to support that observation.

Certainly,  I only guessed at a method that I would find reliable.  It could have been as simple as that for the Beckers.   Placement and pretension are independent. 

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That sounds about right.  If the back is a straight-sided cone, with the soundpost at the apex, that's as statically stiff as you can get (at the soundpost, that is).  Additionally, I think just about all of the normal modes will have a nodal point at the apex... in other words, the back won't generate any sound.  While this is an extreme example, I think that if the shape tends toward that direction, it will acoustically tend toward that behavior.

A node may be near the post.  But it is not an absolute node with zero motion any more than the point where strings meet the bridge. It is only question of how near the motion is to nill,   With FEA,  I did not find nodes at the post because the back still can move with the top.  How stiff would be described perhaps as an impedance.  Yes,  high impedance for stiffly-held posts,  but not infinite impedanece.

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Certainly,  I only guessed at a method that I would find reliable.  It could have been as simple as that for the Beckers.   Placement and pretension are independent.

Yes, I agree, placement and tension are independent.

Some time ago there was another discussion of sound post fitting. At that time I mentioned those sound posts called "perfect post". Two pivoting nylon pads (like on a clamp) with a tube connecting them. You would trim the tube to the correct length and the pads would take of the "fit". I had imagined these would be useful in a shop because you could have sound post in .1 mm increments that didn't need fitting so you could very quickly find by trial and error what was the correct length at least tonally. Then you fit a real post.

Your/Becker solution is much more elegant.

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Yes,  I said twice in this thread and a in a couple threads before.... Carl Jr. himself told me, "What the violin needs."  It was obviously something measured. 

 

Mr. Jacoby:  Yes,  you can see .4mm,  but where do you get that number?  Carl Becker seems to have calculated it from an actual measurement. 

 

I see that there is a lot of resistance to this idea.  It makes perfect sense to me,  any preloads seem to affect sound.  So what should they be set to be ?

My post was misinterpreted so I'll try this again.  Of course I noticed that you said Carl told you, etc.  Have you contacted any of the surviving Beckers since you started this thread to ask them what they thought Carl was doing with the shims and how?   It was probably discussed with them.  The whole family are working luthiers.  If it's not some deep dark proprietary shop secret they might just tell you outright, and no one need speculate further :)

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My post was misinterpreted so I'll try this again.  Of course I noticed that you said Carl told you, etc.  Have you contacted any of the surviving Beckers since you started this thread to ask them what they thought Carl was doing with the shims and how?   It was probably discussed with them.  The whole family are working luthiers.  If it's not some deep dark proprietary shop secret they might just tell you outright, and no one need speculate further :)

No,  I have not tried to contact them myself.  Since he could easily fit a post and decide any amount it had to be slid to make it tight, he would not need a shim.  I could see no other reason for a shim than that he did NOT slide it into place from the center outward....  Or at least not with an older violin that had already been prestressed.  I trust my perhaps more than I should.  I could imagine no other reason for the shim.  Maybe it WAS a quick way to modify an existing post.  But I like to think of it as more.  Whether Becker did it or not,  it would still be an interesting thing to try.

 

Curious1:  I thought of the adjustable post myself at one point.  Maybe a thin-walled stainless tube with a post and shims inside.  And a ball-socket made better than I can use a lathe.  Or a turnbuckle with left and right handed screws,  like a chinrest clamp.  If you can make one,  that is easy to use and make,  you could make a lot of money perhaps.  Or even some small wood things like feeler gauges to insert one at a time.  Or steel feeler gauge slips themselves.  I doubt the mass of small ones would disturb things much.  You might even involve some kind of jack to allow the feeler gauge pieces to be slipped in.  You invent it, and show us what seems the simplest way to do it.

 

But Carl did not play.  I expected that he would do something which  dealt only with the structure and that he did not have to judge sound.  As his wife indicated,  he had a method,  and don't touch it.

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No, I have not tried to contact them myself. Since he could easily fit a post and decide any amount it had to be slid to make it tight, he would not need a shim. I could see no other reason for a shim than that he did NOT slide it into place from the center outward.... Or at least not with an older violin that had already been prestressed. I trust my perhaps more than I should. I could imagine no other reason for the shim. Maybe it WAS a quick way to modify an existing post. But I like to think of it as more. Whether Becker did it or not, it would still be an interesting thing to try.

Curious1: I thought of the adjustable post myself at one point. Maybe a thin-walled stainless tube with a post and shims inside. And a ball-socket made better than I can use a lathe. Or a turnbuckle with left and right handed screws, like a chinrest clamp. If you can make one, that is easy to use and make, you could make a lot of money perhaps. Or even some small wood things like feeler gauges to insert one at a time. Or steel feeler gauge slips themselves. I doubt the mass of small ones would disturb things much. You might even involve some kind of jack to allow the feeler gauge pieces to be slipped in. You invent it, and show us what seems the simplest way to do it.

But Carl did not play. I expected that he would do something which dealt only with the structure and that he did not have to judge sound. As his wife indicated, he had a method, and don't touch it.

I like his/you method.

Perhaps a hollow carbon fiber rod with aluminum pads that are flat on one side and domed on the other (I guess that's a fairly crude ball and socket). You would only need to have the tube cut in .1 mm increments. The pads could be held in place with rubber bands. That would be very light, stiff and adjustable. Here's a very rough sketch.

post-53756-0-67346100-1386900672_thumb.jpg

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Hi everyone! This is Paul Becker and it's my first post. I just read through this whole forum, and I thought I might as well chime in so I can describe my father’s method for judging soundpost length.

 

First, I have to say that "Why, as much as it needs" sounds exactly like the kind of thing my dad would have said when he didn't have a measurement in mind. 

 

For him, the most important thing to fitting a soundpost is its position. On a Strad-model Becker, he would place the soundpost 38.5mm in from the lining, equally in from the top and back, regardless of the arching height. He would measure this with pieces of wood, of which he had a set in .10mm increments. There were, of course, variations within a mm of this measurement.

 

As for the length, he had a drawer of old soundposts arranged according to length and, using trial and error, would try different ones until he found one that felt right.  So, it was about how things felt and sounded, rather than relying on precise measurements. 

 

Once he determined the appropriate length, he would then take the next largest soundpost (maybe .3mm taller), place it in front of where he wanted to fit the soundpost. He used it to hold the top and back apart .3 mm so that he could fit the soundpost with clearance. That way, he could give the soundpost a perfect pivot without tension. Once that was done, he would remove the second soundpost and VOILA—perfect sound!

 

Once a post is fitted in the proper location and length, the instrument will eventually change. As described by my father, the back would “give” and create the need for a longer soundpost. The shim material he used was .3mm thick for a violin. For cellos he put two shims on for .6mm. My grandpa (Carl G.) purchased a stack of .3mm thick plain maple, which we still use today.

 

I put a shim on by adding a drop of tightbond glue to the end of the soundpost and, while applying the maple veneer shim, I press it against my hot plate for about 30 seconds. In all, it takes me less than 5 minutes to glue a shim on a soundpost. But then, of course, it has to be finished. The end result is a perfect .3mm addition to the soundpost so it fits exactly as intended, in the same place as designed.

 

So, unfortunately, he did not have a sophisticated or elaborate measuring jig. It was simply trial & error, mixed in with a lot of experience. Dad admitted, in the end, that it was intuition. 

 

 

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