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sound post effect on tone


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10 minutes ago, reguz said:

Dear Don, It's hard to belive that you have now understanding at all that a structure under stress has another behavior a structure without any stress condition. This is no no arm-waving this is simple technical understanding. Contact any structural engineer and you will get an explanation.

Um, you do realize that Don IS an engineer, one that worked for NASA, one that helped develop 'the Mars rover" , you know, like one of our species greatest engineering feats?

and that because of that fact, your statement sounds "comical" ?

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27 minutes ago, reguz said:

Dear Don, It's hard to belive that you have now understanding at all that a structure under stress has another behavior a structure without any stress condition. This is no no arm-waving this is simple technical understanding. Contact any structural engineer and you will get an explanation.

I am gonna have to run to the store, I dont have enough beverages and popcorn for this....

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10 hours ago, jezzupe said:

Um, you do realize that Don IS an engineer, one that worked for NASA, one that helped develop 'the Mars rover" , you know, like one of our species greatest engineering feats?

Yes, however,  in space no one can hear you bow.  :ph34r:

Resonant frequencies shift with tension (and compression as well).  One can see this clearly in the behavior of the strings when they are tuned, but the simultaneous effects on the violin body are less obvious (unless a poorly maintained joint or seam gives way....).   As Don must know this at least as well as anyone else here, I'm confused by part of the disagreement.  :huh:

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For those prepared to be entertained by an argument you'll have to go here.

My response plots show that body resonances don't change noticeably with varying string tension.  While static forces might make some tiny difference, it is swamped by the stiffness of the wood so as to be insignificant.  Anyone with different results is free to show them.  

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3 minutes ago, Don Noon said:

For those prepared to be entertained by an argument you'll have to go here.

My response plots show that body resonances don't change noticeably with varying string tension.  While static forces might make some tiny difference, it is swamped by the stiffness of the wood so as to be insignificant.  Anyone with different results is free to show them.  

Oh gosh, that's easy to refute.  Play the open A string on a properly tuned violin. Then completely detune the G, D, and E strings to zero tension to reduce the static compressive forces on the violin's top.  Now play the open G, D , and E strings and it will sounds lousy.

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10 hours ago, nathan slobodkin said:

Don,

Assuming you are correct about the static force versus vibrating behavior ( which I certainly do) why does the tension of the post affect the sound so much?

It's on my list of things to investigate.

Since I just put in posts with about the same modest force every  time, it is something I have not personally observed.  There is also the difficulty of  varying the tension and having the post in exactly the same spot, which of course means different post lengths... so the experiment is not trivial.  I'd prefer to modify a VSO with a screw-adjusted post, when I get around to it.

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20 hours ago, Don Noon said:

For those prepared to be entertained by an argument you'll have to go here.

My response plots show that body resonances don't change noticeably with varying string tension.  While static forces might make some tiny difference, it is swamped by the stiffness of the wood so as to be insignificant.  Anyone with different results is free to show them.  

 

I found something on Wikipedia about plate theory.  https://en.wikipedia.org/wiki/Plate_theory   I went through it with my hazy eyes and thought it might be relevant to what we care about. In this theory plate of finite thickness is discussed. Vertical deflection as well as in-plane motions are considered here. There is a screen capture of the out come equations.   Just want to point out that the "external stress load" indeed takes a part in the differential equations.  Unfortunately, the chance of finding a closed form solution is unlikely for such messy stuff. It seems that simulations or experiments are the only ways to go any further.  

We all know the stiffness of wood has a major influence on violin resonance modes. However, has anyone ever discussed the stiffness of string on its vibration frequency.  I guess the answer is no.....   Shouldn't that be as strange as what Don has found?

f={\frac  {v}{2L}}={1 \over 2L}{\sqrt  {T \over \mu }}         <<<<<  Where is k, the stiffness of string?    ......

Furthermore, when we strike the violin box and we hear sound.  But what happens if we pluck a lose string?....

As what I recall back from college, when we derive equation for traveling waves, the amplitude is assumed small so that the string is not elongated so that Hook's law is not considered. The only restoring force comes from the applied tension.   That's why we don't see the stiffness of string in vibrating string equation. We neglect the effect of spring force, and we are happy with the results. 

For the case of violin resonance box, the stiffness of the wood alone produced enough restoring force. ( Not like string where  the stiffness alone does nothing.) And according to Don's experiments, the effect of stress from the strings is negligible and "static forces swamped by the stiffness of wood" is a probable explanation. And I have some theory add to it:

Curvature of plates plays an important role in my discussion and the external stress load need not to be small. Assume the deflection is small so that the concave part remains concave and convex part remains convex during the vibration cycle.  And let's assume the stress load is pressure, eg. a squeeze from the outer edges. Applied stress result in a force which tries to make a convex part more convex, and vice versa.  In the upper cycle the plate has a larger convexity then equilibrium, and in the lower cycle the plate has smaller convexity then equilibrium. However, the externally applied pressure tends to increase the convexity all the time. Therefore, half of the time the externally applied stress acts agains the restoring force and half of the time the stress acts with the restoring force.  The time to complete one cycle of vibration is the sum of the time the plate stays in the upper cycle and the time in the lower cycle. The former is prolonged by the applied stress and the latter is shortened by the stress.  We can expect increase and decrease to complete half cycle cancel each other  to some extent.  This would make the mode frequency insensitive to the externally applied stress.

I am not here to prove anything, but just try to find some mechanism to explain Don's experiment results.  

Experimental fact is fact. You can't argue with it, but the video is really fun to watch!  ...      :D 

 

1482954949_2019-09-093_40_48.png.905ee79a1e81bf42d71314f79a75eeb6.png

 

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21 hours ago, Don Noon said:

For those prepared to be entertained by an argument you'll have to go here.

My response plots show that body resonances don't change noticeably with varying string tension.  While static forces might make some tiny difference, it is swamped by the stiffness of the wood so as to be insignificant.  Anyone with different results is free to show them.  

OK, for me, experimental results settle the issue.  In my interpretation, in a traditionally constructed violin, where the belly arching seems to stiffen overall under tension, and the rib-back box resists deformation, the effects of string tension are negligible.  Thanks.

46 minutes ago, Kae said:

 

I found something on Wikipedia about plate theory.  https://en.wikipedia.org/wiki/Plate_theory   I went through it with my hazy eyes and thought it might be relevant to what we care about. In this theory plate of finite thickness is discussed. Vertical deflection as well as in-plane motions are considered here. There is a screen capture of the out come equations.   Just want to point out that the "external stress load" indeed takes a part in the differential equations.  Unfortunately, the chance of finding a closed form solution is unlikely for such messy stuff. It seems that simulations or experiments are the only ways to go any further.  

We all know the stiffness of wood has a major influence on violin resonance modes. However, has anyone ever discussed the stiffness of string on its vibration frequency.  I guess the answer is no.....   Shouldn't that be as strange as what Don has found?

f={\frac  {v}{2L}}={1 \over 2L}{\sqrt  {T \over \mu }}         <<<<<  Where is k, the stiffness of string?    ......

Furthermore, when we strike the violin box and we hear sound.  But what happens if we pluck a lose string?....

As what I recall back from college, when we derive equation for traveling waves, the amplitude is assumed small so that the string is not elongated so that Hook's law is not considered. The only restoring force comes from the applied tension.   That's why we don't see the stiffness of string in vibrating string equation. We neglect the effect of spring force, and we are happy with the results. 

For the case of violin resonance box, the stiffness of the wood alone produced enough restoring force. ( Not like string where  the stiffness alone does nothing.) And according to Don's experiments, the effect of stress from the strings is negligible and "static forces swamped by the stiffness of wood" is a probable explanation. And I have some theory add to it:

Curvature of plates plays an important role in my discussion and the external stress load need not to be small. Assume the deflection is small so that the concave part remains concave and convex part remains convex during the vibration cycle.  And let's assume the stress load is pressure, eg. a squeeze from the outer edges. Applied stress result in a force which tries to make a convex part more convex, and vice versa.  In the upper cycle the plate has a larger convexity then equilibrium, and in the lower cycle the plate has smaller convexity then equilibrium. However, the externally applied pressure tends to increase the convexity all the time. Therefore, half of the time the externally applied stress acts agains the restoring force and half of the time the stress acts with the restoring force.  The time to complete one cycle of vibration is the sum of the time the plate stays in the upper cycle and the time in the lower cycle. The former is prolonged by the applied stress and the latter is shortened by the stress.  We can expect increase and decrease to complete half cycle cancel each other  to some extent.  This would make the mode frequency insensitive to the externally applied stress.

I am not here to prove anything, but just try to find some mechanism to explain Don's experiment results.  

Experimental fact is fact. You can't argue with it, but the video is really fun to watch!  ...      :D 

 

1482954949_2019-09-093_40_48.png.905ee79a1e81bf42d71314f79a75eeb6.png

 

Thanks immensely for bringing this to the table.  :)

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On 9/5/2019 at 8:25 AM, Bill Merkel said:

that means there's not a single correct position for the bridge?

If the top is worn and uneven, then the bridge feet are likely to have only one spot where the fit cleanly (provide they fit well at all).

Other than that, the bridge location  to body is not as super sensitive as the bridge location to the post.  So if the top is smooth, you can learn something about what changes might help.the instrument respond best by moving the bridge.

However, you don't probably want to leave the bridge anywhere but it's best spot.  

Response can be very sensitive to all of: 1) post north south  relation to bridge foot, 2) post relation to bar, 3) bridge foot relation to bar, 4) post east west relation to bridge foot, 5) post relation ship to back's center mass, 6) strings tension, 7) bridge cut, 8) post tension.

But you can't listen or explore any of that unless the post and bridge feet truly fit the top.

So to learn what might improve reaponse, you move what you can without loosing fit too badly, then you adjust or remake things from what you've learned, until everything is working well in a balanced and near as normal position as possible.

Ideally, you then let things settle a while, then reevaluate.  Hopefully your iteration will zero in on a great response, and adjustments will only be subtle past a certain good point.

 

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On 9/7/2019 at 7:45 PM, nathan slobodkin said:

Don,

Assuming you are correct about the static force versus vibrating behavior ( which I certainly do) why does the tension of the post affect the sound so much?

I'm guessing here, but:

The portion of string load bearing down into the top  through the feet depends on multiple factors besides the string tension: angle across bridge, bridge height, and choices of stop length.

However, when its time to do post work, these bridge factors are fairly set, so the overall portion of downbearing is relatively set.

I suspect that the tension carried in the post fairly sensitively effects the transmission and distribution of this bearing into the plates and further into the sides and instrument.

I suspect that this in turn rather strongly alters the admittance/impedance of the system across the frequencies.

Thus we experience strong changes pf response from small changes in post tension.

 

(Strict wild guessing)

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On 9/8/2019 at 5:19 AM, Don Noon said:

My response plots show that body resonances don't change noticeably with varying string tension.  While static forces might make some tiny difference, it is swamped by the stiffness of the wood so as to be insignificant.  Anyone with different results is free to show them.  

I can confirm that this is how it is. Just by tuning the strings enough to hold the bridge the body resonances are pretty much the same as when the strings are tuned. You can get odd changing in B1 modes, but that is most often tail gut frequency disturbing B1+.

If the violin is weakly built you get changes by tension, but that is not happening right away but after awhile when the arches are deformed. 

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9 hours ago, Kae said:

 

 

As what I recall back from college, when we derive equation for traveling waves, the amplitude is assumed small so that the string is not elongated so that Hook's law is not considered. The only restoring force comes from the applied tension.   That's why we don't see the stiffness of string in vibrating string equation. We neglect the effect of spring force, and we are happy with the results. 

 

only because violins don't have frets

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26 minutes ago, JohnCockburn said:

 

only because violins don't have frets

Yes. If you press deep in and close to a fret, the string is elongated by your finger. Hence the tension increases according to Hooks law. And this additional tension results in a higher pitch.  And now the vibrating system between saddle and fret has a higher tension.  Given the increased tension, which is now a fixed value, ( the  string length and mass)  we can derive its equation of motion accordingly.  Unless you pluck really hard, the frequency is given by the formula we see all the time.  

Note that the addition tension is not resulted from the transverse vibration of the string, which we always assume small or you'll get an unstable pitch.  

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14 minutes ago, Jim Bress said:

I believe "Boinking a ruler on your table is all you need to know" is a quote (probably paraphrased) by Rene Morel, and actually says a lot taken in context.  

I was bantering with Merkel (our much esteemed Village Idiot), not stating fact.  Thank you for identifying that as a Morel quote.  I agree that for most of what we do at the bench, the full panoply of modern engineering is gross overkill, but I am grateful to Kae for posting an example of what some of the often referred to, but seldom seen, tools from higher mathematics look like.  IMHO, any rigorous predictive explanation of how violins do what they do must come from this area.  :)

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One thing the off-center sound post does is to restrict the top plate's motion at that point.  This causes the B1-node line(no motion) to divide the top's anti modes (areas of vibration) in to unequal areas.  

Since one anti node plate area moves up while another moves down their sound waves they produce can cancel.  An off center sound post makes these areas unequal so the sounds don't completely cancel so the violin becomes a better sound producer.

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7 hours ago, Bill Merkel said:

Boinking a ruler on your table is all you need to know.

Rene Morel used that metaphor, and Hans Weisshaar used similar metaphors and challenges, trying to get some of us new people to really think, versus just sliding down the easiest and most convenient slope.

I will forever be grateful for teaching like that.

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On 9/8/2019 at 4:19 AM, Don Noon said:

For those prepared to be entertained by an argument you'll have to go here.

My response plots show that body resonances don't change noticeably with varying string tension.  While static forces might make some tiny difference, it is swamped by the stiffness of the wood so as to be insignificant.  Anyone with different results is free to show them.  

Don, very nice statement "anyone with different result is free to show them" Please show your own result befora askling anyone. don't only write!!

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25 minutes ago, reguz said:

Don, very nice statement "anyone with different result is free to show them" Please show your own result befora askling anyone. don't only write!!

Now you're just being ridiculous. Don has described his experiments and outcomes many times here.

If you weren't paying enough attention to already know that, it is nobody's fault but your own.

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