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crazy back plate idea...


Mat Roop
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I am not a maker, so maybe this makes no sense but... I'll ask anyway...

While recently repairing a back plate sound post crack made me wonder what would happen if the back plate were to be installed in reverse with the arching concavity outward. that should make for several advantages ( I think)

1- with changes in humidity, the post fit should not change as both plates would expand & contract, not exactly but similarly, keeping the spacing between top & back the same... more significant for the cello or bass

2-pressure on the back plate from the post should be less likely to create a crack

3-sound radiation from the back plate would be to the outside instead of being trapped inside and interfering with the vibrations of the top plate. 

A speaker cone always projects from the concave side... so why not the back plate?? or for that matter why not both plates with the concavity to the outside?

I've been thinking of taking a less valuable violin and making new taller ribs and fitting the back plate in reverse.  Of course I'd have to do some calculation to try to keep the internal volume the same as original... if that matters.

Anybody thought of this before???  Thoughts on what the results might be?

Cheers!... Mat

 

 

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It sounds like a totally crazy idea to me.  In trying to visualize it, it seems to me that that the force of the sound post pushing on the back would tend to spread the ribs apart, as opposed to a normal back where the post force would tend to pull them together.  I have no idea what difference this would make, if any.

Try it and show us a picture.

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3 minutes ago, Brad Dorsey said:

It sounds like a totally crazy idea to me.  In trying to visualize it, it seems to me that that the force of the sound post pushing on the back would tend to spread the ribs apart, as opposed to a normal back where the post force would tend to pull them together.  I have no idea what difference this would make, if any.

Try it and show us a picture.

That's correct.  In a normal violin the "cup down" top plate spreads out (in plane motion) while the back plate "cup up" becomes narrower (like a bimetallic strip).This causes the entire body to bend downward when the soundpost pushes downward from the bridge's rocking motion.  Everything of course is reversed when the bridge rocks the other way.

This body bending vibration generates only a little sound because the body doesn't have any air volume change because the ribs move outward at the top while they move inward at the bottom.  So this energy consumed by the bending the body is wasted by not producing any sound.

The amount of spreading and narrowing of the plates is dependent upon their arch heights. I believe lower arched plates should be more efficient at producing sound than higher ones and this might be the reason why Stradivari and DG began to use lower arched plates.

Making both the top and back plates cup downward eliminates this body bending and should be a more efficient sound producer.

 

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12 minutes ago, Marty Kasprzyk said:

This body bending vibration generates only a little sound because the body doesn't have any air volume change because the ribs move outward at the top while they move inward at the bottom.  So this energy consumed by the bending the body is wasted by not producing any sound.

Is this something you know because you tried it or is it a guess?

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24 minutes ago, Michael Darnton said:

Is this something you know because you tried it or is it a guess?

I try not to guess.  Some things can be mathematically predicted.  Attached is Colin Gough's paper on how the shell of a violin behaves using finite analysis and one of his drawings showing how the plate arching causes the violin body to bend.

Also attached is a graph of the approximate change in width occurs when the top and back plates are pushed downward.  From this I concluded that an instrument with flat plates should be louder than one with arched plates sonmy last 30 or so experimental violins and violas have all been made with flat tops.

Unfortunately none of these have yet been double blind tested in comparison to Strad or DG instruments but VSA judges don't like them.  As Don Noon often points out --anything that sounds different usually isn't liked.  

Arching stiffens a plate which increases the frequencies of its various vibration modes. I'm trying to do similar stiffening by adding various braces to the flat plates to get my instruments to sound similar to conventional ones but also louder.

Colin's bending drawing.jpg

Screen Shot 2022-06-03 at 9.48.05 AM.png

1076234128_JASA2015Shellmodes(1).pdf

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4 hours ago, Michael Darnton said:

So, an educated guess, then.

But I would hate to be the one to put a soundpost in one with the back flipped!

In order to get some sound post tension on the plates it is necessary to have the sound post longer than the distance between the parallel top and back plates.  So you have to pull up on the f hole edge with some sort of hook to enable the sound post to be inserted. 

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2 hours ago, Marty Kasprzyk said:

So you have to pull up on the f hole edge with some sort of hook to enable the sound post to be inserted. 

Or one could just put a good squeeze on the edges of an arched top to increase the distance between the top and back for easier insertion of the soundpost.

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Marty, so am I correct to understand that in a normal violin, when the soundpost moves (vibrates) inward, the top plate spreads pushing the ribs outward, and at the same time the bottom plate moves outward pulling the ribs inward. This suggests that the two plates are working against each other and actually twisting the ribs.

So, with the bottom plate flipped, then the two plates will be spreading in and out at the same time ... a more efficient / powerful system?

I plan on trying this on a cheap markie ... first set up normally then flipping the same back... and see what the difference is. I realize the body volume will be significantly reduced, but ultimately this can be resolved with higher ribs. Maybe I should try it with a small viola and end up with a powerful violin?:P

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10 hours ago, Mat Roop said:

Marty, so am I correct to understand that in a normal violin, when the soundpost moves (vibrates) inward, the top plate spreads pushing the ribs outward, and at the same time the bottom plate moves outward pulling the ribs inward. This suggests that the two plates are working against each other and actually twisting the ribs.

So, with the bottom plate flipped, then the two plates will be spreading in and out at the same time ... a more efficient / powerful system?

I plan on trying this on a cheap markie ... first set up normally then flipping the same back... and see what the difference is. I realize the body volume will be significantly reduced, but ultimately this can be resolved with higher ribs. Maybe I should try it with a small viola and end up with a powerful violin?:P

Flipping the back over is an interesting experiment.

I'm not sure if cup downward for both plates   nn  is better than cup downward for the top and cup upward for the bottom   nu   so this might be a   no-no.

I agree the nu produces some twisting of the ribs but on the other hand nn produces some lengthwise stretching of the ribs.  I don't know which is worse in wasting energy.  The rib portions do produce some useful sound but only at high frequencies because their areas are so small. 

My last instrument is a large 362m  violin which is powerful because of the higher tensions of its longer 343mm strings but it also sounds pretty good with viola strings on it.  I screwed up and should have made it as a small 5 string viola with an added E string.

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Hi Marty, I would love to see you do the experiment professionally... You are so much more skilled and adept at building special  models and likely have some excellent equipment to record the tonal results. Maybe even build it on higher ribs.

As for myself, not being a maker, I will just take a cheap markie set it up and see what it sounds like, and then flip the back, and see what the change is... just for my personal curiosity.

  I will likely need to irreversibly alter the back in the process as the button needs to fit  flush to the base of the neck for gluing ... I might even have to remove some of the recurve so that it will glue securely to the ribs.... not to mention I'll have to strip the varnish for sure.... etc etc

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I don't see that the higher vibration modes would change much, but the lower ones likely would, and structural stability would certainly be worse.  The overall depth of the structure and the middle would be significantly thinner, and much less stiff in bending.  Fingerboard projection I think would wander around.

Speaker cones are cones for stiffness reasons, not so much to get efficient radiation in one direction.  They become too directional if the wavelength gets close to the diameter of the cone... and tweeters (and some midranges) are frequently dome shaped to avoid that problem.  When it comes to violins, I think speaker cones are mostly a misleading concept.

But have fun.  I have certainly done my share of oddball experiments, which have often been educational disasters.

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8 hours ago, violins88 said:

Since the air volume will be drastically decreased, the A0 mode will move much higher, from 270  Hz to maybe 320. Cannot wait for you to confirm this guess. Thanks

Most of the volume is enclosed by the rib structure and only a little is from the back plate's arched shape. I estimate flipping the back over so it is cup down will decrease the total cavity volume by about 10%.  

The A0 frequency is nearly proportional to the volume to the 0.25 power so the frequency increase is only about 2.4%  therefore the A0 would increase only to about 277Hz.  This is still in the range of typical good violins.

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On 6/2/2022 at 10:04 PM, Mat Roop said:

>

I've been thinking of taking a less valuable violin and making new taller ribs and fitting the back plate in reverse.  Of course I'd have to do some calculation to try to keep the internal volume the same as original... if that matters.

>

 

 

Making the ribs higher to duplicate the original cavity volume may also restore some of the violin body's stiffness loss.

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I guess when you have your nose to the grindstone and don't look up, you have a lot to learn!... True in my case.

In my earlier comment I noted that all speakers that I am aware of, are cone shaped with the hollow pointing forward. Well that is not at all true... apparently the better ones costing $25000- 250000 are shaped quite different as in the you tube video. ...and sound far superior to the cone versions, and more like instruments.

Here are a couple of links my audiophile son pointed me to...The first is really interesting reading but way over my pay scale.

https://www.audiosciencereview.com/forum/index.php?threads/omnidirectional-loudspeakers-best-design-available.19024/


https://www.youtube.com/watch?v=JLnSsE6GZAs

Based on the comments in the science review link... I think the experiment I proposed is a lost cause and I will not be sacrificing an historic Markie "Usual" for a test!

Interesting discussion and thanks everyone for their input... Cheers, Mat

 

 

 

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10 hours ago, Mat Roop said:

I guess when you have your nose to the grindstone and don't look up, you have a lot to learn!... True in my case.

In my earlier comment I noted that all speakers that I am aware of, are cone shaped with the hollow pointing forward. Well that is not at all true... apparently the better ones costing $25000- 250000 are shaped quite different as in the you tube video. ...and sound far superior to the cone versions, and more like instruments.

Here are a couple of links my audiophile son pointed me to...The first is really interesting reading but way over my pay scale.

https://www.audiosciencereview.com/forum/index.php?threads/omnidirectional-loudspeakers-best-design-available.19024/


https://www.youtube.com/watch?v=JLnSsE6GZAs

Based on the comments in the science review link... I think the experiment I proposed is a lost cause and I will not be sacrificing an historic Markie "Usual" for a test!

Interesting discussion and thanks everyone for their input... Cheers, Mat

 

 

 

If you are willing to be influenced by audio speaker designs I suggest you do a Google search on "flat panel speakers".  

One take away is that the panel should be very light and stiff.

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There are all kinds of speakers... cones, flat panel, electrostatic, plasma...  but my favorites are these:  the travelling bending wave type on the left, and the pulsating sphere (compression bending leaves?) on the right.  IMO, a violin is none of these, or perhaps a blend of them, but speakers and violins have very different goals in terms of sound and performance.  Trying to "improve" the violin by chasing extreme speaker concepts would make it less like a violin.  However, I'd like to see someone make a plasma violin.

Speakers.jpg.13c162fb73f584ebb2fa787dd0094cce.jpg

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On 6/3/2022 at 8:10 PM, Mat Roop said:

Marty, so am I correct to understand that in a normal violin, when the soundpost moves (vibrates) inward, the top plate spreads pushing the ribs outward, and at the same time the bottom plate moves outward pulling the ribs inward. This suggests that the two plates are working against each other and actually twisting the ribs.

So, with the bottom plate flipped, then the two plates will be spreading in and out at the same time ... a more efficient / powerful system?

I plan on trying this on a cheap markie ... first set up normally then flipping the same back... and see what the difference is. I realize the body volume will be significantly reduced, but ultimately this can be resolved with higher ribs. Maybe I should try it with a small viola and end up with a powerful violin?:P

They move in all of the available modes, depending on the frequencies driving into the instrument.  Gough's work is great to read. Also, look at motion studies and animations on the web.

A violin sets up in 'standing waves' the express the driving signal.   With lower frequencies, larger portions of the plates move in phase.   For higher frequencies, the paterns break up into smaller patches of phase and anti-phase.   

 

The current design does a good job of responding to the whole range of musical input.   As you explore, you don't want to focus on only one of the patterns of instrument movement, because that will reflect only a small range of actual response.

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On 6/7/2022 at 10:16 AM, Don Noon said:

There are all kinds of speakers... cones, flat panel, electrostatic, plasma...  but my favorites are these:  the travelling bending wave type on the left, and the pulsating sphere (compression bending leaves?) on the right.  IMO, a violin is none of these, or perhaps a blend of them, but speakers and violins have very different goals in terms of sound and performance.  Trying to "improve" the violin by chasing extreme speaker concepts would make it less like a violin.  However, I'd like to see someone make a plasma violin.

Speakers.jpg.13c162fb73f584ebb2fa787dd0094cce.jpg

There's some similarities and differences between violins and speakers.  Both use a moving body as light as possible to generate a loud sound without falling apart.  

The violin distorts the bowed string's saw tooth vibration input by having lots of strong resonances and its frequency response curve has lots of peaks and valleys which gives a violin its character.  

A Speaker on the other hand is designed eliminate any resonances in a frequency range so its frequency response curves is as flat as possible.

So a speaker is supposed to give an undistorted reproduction of the distorted string sound of a violin.

 

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22 minutes ago, Marty Kasprzyk said:

There's some similarities and differences between violins and speakers.  Both use a moving body as light as possible to generate a loud sound without falling apart.  

I have tried the "as light as possible" concept with violins, and yes, it can be loud.  But not necessarily what a good player really wants.  There are issues with dynamics and impedance that get abnormal and uncontrollable, and the light plates tend to louden some frequency ranges but not others.

So it's more complicated.

Other than that, I agree with the other things you said.  If you want to hear an undistorted string sound of a violin, there is the electric violin... which sounds very different from the wooden box version.

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