Jump to content
Maestronet Forums

How to further thicknessing this back plate for violin


pt3
 Share

Recommended Posts

I am now thicknessing the back plate and meet with a problem as follows:

Lower bout thichness has already reached 2 mm, upper bout 1.8 mm, and central part 4.5 mm.

using software Audacity, I have measured that Mode 5 frequency = 379 Hz, Mode 2 freq. = 157 Hz.

My goal frequency for this back is 357Hz. so 20 Hz must be lowered, and I want that Mode 2 frequency to be kept as high as possible. What should I do to further this thicknessing process? The upper and lower bouts seem too thin already. and thin the central and intermediate parts will further lower M2 frequency.

Link to comment
Share on other sites

I am now thicknessing the back plate and meet with a problem as follows:

Lower bout thichness has already reached 2 mm, upper bout 1.8 mm, and central part 4.5 mm.

using software Audacity, I have measured that Mode 5 frequency = 379 Hz, Mode 2 freq. = 157 Hz.

My goal frequency for this back is 357Hz. so 20 Hz must be lowered, and I want that Mode 2 frequency to be kept as high as possible. What should I do to further this thicknessing process? The upper and lower bouts seem too thin already. and thin the central and intermediate parts will further lower M2 frequency.

What is the weight of the back ?

Link to comment
Share on other sites

Those thicknesses look thin enough, unless you have some REALLY dense, stiff stuff and your plate weight is still over 100g.

In my opinion, trying to thickness plates to a particular set of taptone frequencies is not a good idea. You already are concerned about the thickness... and that should be a clue that the taptone numbers are taking you someplace you probably shouldn't be going. I would suggest assembling it the way it is, and if things are radically too stiff in the white, open it up and take out some more wood. Or off the outside, if you prefer. Too thick is easy to correct; too thin, not so much.

Link to comment
Share on other sites

i suggest you take your advice from someone who tunes their plates, not someone that doesnt think it matters, what little i know about tuning, lowering the pitch is done by thinning in the middle, raising the pitch by thinning the ends and sides, youve already raised the pitch by overly thinning the top and bottom, so your left with leave it or thin the middle, but im no expert on this as all my experience is with a totally different type of tuning, not concerned with free plate resonances

Link to comment
Share on other sites

The weight of this back plate is now 114.8 g. so further thicknessing to reduce the weight seems to be necessary, but I hope not to reduce M2 frequency (reducing the thickness of the central region of the plate will lower M2 frequency far more than M5 frequency).

Link to comment
Share on other sites

As Don said, the wood must be quite dense to be at 114g with such thin graduations. there has already been few threads about the final weight of plates (back and front) and I don't think anyone suggested that the back plate had to be under 114g. I was actually looking at the graduations of the Ole Bull back plate, and with an average of 3.5 mm in the bouts and 6mm in the center, unless it was made of spruce or balsa, the weight must have been way over 100g.

Link to comment
Share on other sites

M. Hutchins recommended that Mode M2 frequency should be tuned to a frequency = 1/2 M5 frequency (the so-called octave tuning). In a recent paper it is said that Mode 2 frequency can be constantly negative with regard to octave (in case the transverse sound velocity is lower) during the thicknessing. however, it must be no lower than 25 Hz below its octave upon completion.

In my case, 357/2 = 178 Hz, 178 - 25 = 153 Hz

the measured M2 freq. is now 157 Hz.

I worry about how to keep M2 frequeny not less than 153 Hz, when thining the central region of the back plate and at the same time attaining my goal frequency of M5, 357 Hz.

Link to comment
Share on other sites

i suggest you take your advice from someone who tunes their plates, not someone that doesnt think it matters, what little i know about tuning, lowering the pitch is done by thinning in the middle, raising the pitch by thinning the ends and sides, youve already raised the pitch by overly thinning the top and bottom, so your left with leave it or thin the middle, but im no expert on this as all my experience is with a totally different type of tuning, not concerned with free plate resonances

I partially agree with Lyndon:

Try thinning along the C bout sides but don't thin the ends.

Marty

Link to comment
Share on other sites

i suggest you take your advice from someone who tunes their plates, not someone that doesnt think it matters

why is the m2 so important when the frequency is lower than the lowest note on a violin, which is about 200hz??

I would suggest taking your advice from someone with a reputation for building great-sounding instruments. While I do not claim to be such a person (yet), I would also take into consideration whether Hutchins' instruments are sought-after acoustic wonders. I'm thinking not. I would also look at some Guarneri examples, such as Vieuxtemps or Cannone, where plate weights and taptones would certainly be far from any normal prescription.

I wouldn't go so far as to say that taptones don't matter... they do indicate something about plate stiffness, in a way that Anders surely can (and has) shown to influence the frequencies of some of the lower modes of the assembled instrument. But focusing so intently on these free-plate taptones without regard for arching, wood properties, and what tonal result you are trying to achieve... that's ignoring things that probably matter even more. I'll bet I could build a fiddle with perfect taptones that sounds absolutely awful :lol: .

I don't want to start yet another brawl about taptones, but here's the specifics of a few of my fiddles for reference. I take the data, but do not "tune" to any set of numbers (obviously). I also have sound clips of them in the thread of the sound of modern instruments , if you want to hear them.

Back plate data:

#5 121.3g, M2=175, M5=351 very dense European maple, very low arching

#6 (snakefiddle) 100g, M2=171, M5=315 Dense bigleaf, not very stiff

#8 98g M2=179, M5=379 very low density bigleaf

#9 97g M2=171, M5=372 same low-density bigleaf as #8

Lyndon - even the M1 free-plate mode, at ~80Hz, indicates something about the free-plate stiffness, which will show up (however slightly) as some aspect of stiffness in the final instrument. The mode itself does not survive assembly, but it is an indicator of a property that might survive.

pt3 - just to repeat: I'd put it together and see what you get, rather than thinning any further. You can always take out wood later.

Link to comment
Share on other sites

My goal frequency for this back is 357Hz. so 20 Hz must be lowered, and I want that Mode 2 frequency to be kept as high as possible. What should I do to further this thicknessing process? The upper and lower bouts seem too thin already. and thin the central and intermediate parts will further lower M2 frequency.

You will find that when you glue the back onto the ribs the M2 and M5 frequencies will be much lower. I would just use it as it is and keep records for future reference. If you like the sound you can try to repeat your thicknessing. How flexible is it when you twist with your hands?

But if you must follow you plan to reduce M5 without reducing M2 then just draw the nodal lines of both modes on the back. It will look like a Venn diagram then thinning on the inside of the nodal lines lowers the frequency and thinning outside the nodal lines raises the frequency a bit. Anders posted a picture about a month or two ago showing this. In your case thinning outside of the M2 nodal lines around where the f-holes would be if a back had f-holes.

Link to comment
Share on other sites

I would suggest taking your advice from someone with a reputation for building great-sounding instruments. While I do not claim to be such a person (yet), I would also take into consideration whether Hutchins' instruments are sought-after acoustic wonders. I'm thinking not. I would also look at some Guarneri examples, such as Vieuxtemps or Cannone, where plate weights and taptones would certainly be far from any normal prescription.

I wouldn't go so far as to say that taptones don't matter... they do indicate something about plate stiffness, in a way that Anders surely can (and has) shown to influence the frequencies of some of the lower modes of the assembled instrument. But focusing so intently on these free-plate taptones without regard for arching, wood properties, and what tonal result you are trying to achieve... that's ignoring things that probably matter even more. I'll bet I could build a fiddle with perfect taptones that sounds absolutely awful :lol: .

I'll largely agree with what Don has said.

However, tap tones, combined with weight and tonal results from a series of experimental violins, may be able to factor in some things like archings and wood properties.

Some makers, claiming various kinds of free-plate tap tone schemes, produce pretty good and seemingly consistent sound outcomes. I don't always understand these schemes, or think they make sense, but will need to allow that the "thought model", whether technically accurate or not, may be able to produce good outcomes.

I'm still largely a "proof is in the pudding" guy. When lacking those results, I'll default to what seems logical, but that's a third choice. The second choice is attempting to combine the two in a meaningful way.

Link to comment
Share on other sites

I'll largely agree with what Don has said.

However, tap tones, combined with weight and tonal results from a series of experimental violins, may be able to factor in some things like archings and wood properties.

That is something I have been trying to do. My personal database is ~10 violins (more like 18 if you count the reworks), and I definitely am paying close attention to taptones, weight, and other stiffness measures of the top, where most of the action seems to happen. This thread has been primarily about the back, where I don't find as much influence on the sound (I'm NOT saying there's none).

So, yes, taptones can be a useful tool, when combined with other information and experience. Taptones alone are about as useful as driving directions... without taking into account where you're starting from and where you want to go.

Link to comment
Share on other sites

I partially agree with Lyndon:

Try thinning along the C bout sides but don't thin the ends.

Marty

i was saying thinning along the c bout sides and/or the top and the bottom should raise the tap tone, but not the stiffness which actually goes down while the tap tone goes up, tap tone is not an accurate indicator of stiffness, unless all the samples are graduated equally, then a lower tap tone would probably indicate lower stiffness

Link to comment
Share on other sites

You will find that when you glue the back onto the ribs the M2 and M5 frequencies will be much lower. I would just use it as it is and keep records for future reference. If you like the sound you can try to repeat your thicknessing. How flexible is it when you twist with your hands?

Hi Catnip,

I’ve measured the frequencies of violin backs and then measured the frequencies with the ribs glued on and I found consistently Mode 5 goes down but Mode 2 goes up.

Here are a couple of examples from two violins.

Mode 2: 168 changes to 202

Mode 5: 374 changes to 356

Mode 2: 180 changes to 201

Mode 5: 363 changes to 361

With these frequencies numbers, I'm just measuring things as I make and writing down the data- so I’m not drawing any conclusions here. I just thought I’d share it.

Link to comment
Share on other sites

Hi Guy,

Thanks for sharing. I used to use an audio sweep generator,an amp and tea leaves to measure the modes but now I find it more convenient just to use audacity and its FF transform. It shows all the modes at once and depending on how you hold the plates and where you strike it you can emphasize one mode over the other.

I, too have no conclusions.

Link to comment
Share on other sites

I’ve measured the frequencies of violin backs and then measured the frequencies with the ribs glued on and I found consistently Mode 5 goes down but Mode 2 goes up.

Very interesting Guy, Thanks for putting this up. I was curious on what happens to the modes when ribs are attached. I'm also interested in how these are predictors of resonances of the fully assembled violin.

Do you measure Q in any way? Resonances are one thing, but I suspect that a strong resonance will have a different effect then a weak one, and I'd like to characterize this.

If you subscribe to plate tuning for specific modes, Jonathan Rowe has a couple diagrams on his site on adjusting modes somewhat independently. Based on the CAS journals. I’ve tired it with some success.

Stephen

Link to comment
Share on other sites

Very interesting Guy, Thanks for putting this up. I was curious on what happens to the modes when ribs are attached. I'm also interested in how these are predictors of resonances of the fully assembled violin.

Do you measure Q in any way? Resonances are one thing, but I suspect that a strong resonance will have a different effect then a weak one, and I'd like to characterize this.

Stephen

Hi Stephen,

I don’t measure Q- how are you measuring this?

Guy

Link to comment
Share on other sites

Hi Stephen,

I don’t measure Q- how are you measuring this?

Guy

The Quality Factor can be estimated by dividing the resonant frequency by the bandwidth at the 1/2 power point, which in a plot of power in decibels at 3db below the peak. So effectively the frequency of the mode divided by horizontal thickness of the resonant frequency 3 db lower then the very peak on the FFT plot. I haven't made a test of how consistent my measurements are yet.

I'm curious if this is how makers are measuring Quality Factor (given that the peak is dependent on the source of energy - how hard one might be tapping, it may cause small variations in measured Q). Also is this the quantity that some makers are referring to when they talk of damping... From what I've seen here there is no consensus on whether allot or a little damping is better. I guess that might depend where it is in the response of a plate and how that relates to the spectrum of the completed violin.

Cheers,

Stephen

Link to comment
Share on other sites

The Quality Factor can be estimated by dividing the resonant frequency by the bandwidth at the 1/2 power point, which in a plot of power in decibels at 3db below the peak. So effectively the frequency of the mode divided by horizontal thickness of the resonant frequency 3 db lower then the very peak on the FFT plot. I haven't made a test of how consistent my measurements are yet.

Cheers,

Stephen

Thanks Stephen. I'll have to check that out when I have some time.

Guy

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

  • Recently Browsing   0 members

    No registered users viewing this page.



×
×
  • Create New...