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What does the back do?


Don Noon
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I haven't recorded an instrument without a post, but playing one only sounded anemic and nasal from a tonal standpoint.

Neither have I, since with listening, it seemed to be way too far from a "decent" violin sound to be useful.

 

In a conversation with Norman Pickering, he had the notion that an important function of the back is to give proper support to the soundpost. He went on to say that stiffness is most important at lower frequencies, and the mass is more important at higher frequencies. Kinda makes sense, when you consider that there is more than one kind of rigidity.

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 He went on to say that stiffness is most important at lower frequencies, and the mass is more important at higher frequencies. Kinda makes sense, when you consider that there is more than one kind of rigidity.

 

To be more precise, stiffness is the controlling parameter below the resonant frequency, and mass is the controlling parameter above the resonant frequency... for each individual resonance.  Since there are a gazillion different resonances in a violin, it gets real complicated.  But for the very low note fundamentals (below A0 resonance) just about everything is stiffness-controlled.

 

Actually, I think the real situation is even more obscure and complicated from a technical standpoint.

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I got impatient and removed the back on my cheapo experimental fiddle... 146.8g, measured flex stiffness about double what I normally build.  We'll see what happens to the assembled modes and tone after removing a bunch.

 

I wonder if anybody has ever plotted violin price vs. weight.  I'm guessing it might look like racing bicycles or cars or space probes.

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This may only be related indirectly (or by free association to the phrase "back of the violin") but the issue of how much sound radiates off the back of the violin comes to mind.  There is also the variable of the interaction of the bow with the instrument as well.  Some violins have a great amount of sound behind the instrument, others much less so.  Also the use of different bows can also effect this phenomena on some instruments.

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I recently had 3 5string fiddles tried by 4 very good fiddle players with very surprising results.

fiddle #1 & #2 sitka tops .42SG, #3 engleman .37SG. arching and grads. very near identical. #'s 1&2 GDG 1734 pattern, #3 GDG Cannon pattern.

Each one has a progressively thicker back. #1 4mm. center, #2 $ 4.75mm.center, #3 5.5mm. center

weights from 105gr. to 116gr.

#1 sounds like an old dry fiddle, growly on the G&C, very easy to play

#2 (1piece quartersawn back) much smoother on the bottom end but not as easy to play.

#3 my favorite by far, very smooth bottom end and the sweetest top end I have ever played and very easy to play

#2 has since had the back thinned, and plays more like #1

I will leave to you to guess which one all the players prefered

#1 was the fiddle the 4 players liked. It also had mode 5 in the back 30hz. below the top.

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#1 was the fiddle the 4 players liked. It also had mode 5 in the back 30hz. below the top.

I can see how "fiddle" players might prefer that... an instrument which gives more with less physical work. The flip side is that it might run out of steam ("bottom out"), and not have enough reserve or high end for a "pusher" classical player.

 

A five string might be a very different animal though in terms of what is needed, with somewhere around 20% more string tension.

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A swell happens in a drum with two heads, when you get the tuning just right the lower frequency amplifies and you get a long low end note with slow attack decent sustain and long decay, the ToooOOOOOOOMMMMMMmmmmmmmm..... 

 

To get the swell to happen in a musical manner the lowed head needs to be tuned a little lower than the top, so that the decay lowers pitch.

 

I know a tom and a violin are different animals I was just wondering if the back plate creates longer decay and lower frequencies and complexity in a similar manner to a bottom head, from my take of Jezzupe's explanation it does.

 

Same is true in a tracking room, the decay (reverberations) of the room should lower it's pitch for it to sound musical.

 

Same goes for a violin, viola, cello etc. one want the reverberations to stay out of the way of the new notes being played.

 

Should a back resonate lower than the top for these reasons?

 

Note -  you will not hear it in the samples below through computer speakers, it just does not respond to these frequencies.

Finally got these to play, thanks for those, that was educational, the swell is there, of course the duration is short. To me there is a certain point in it all where the structure in itself gets put in the "back seat" and or it has done it's part, and now the air takes over, the pressure wave and it's ricochet may have something to do with this type of delayed peak then decay... 

 

Think of, or better yet watch a bubble pop in slow motion, the surface tension will collapse away from the point of rupture, as you see the sphere surface loose its shape in a form of catastrophic failure, thus causing oppositional  force of direction within the fluid/shell. At the moment of total collapse you will see the fluid "smack" together with distinct droplets heading left and with equal force some going right {as its a sphere it could be up down} and right after that impact it looks like a "wet firecracker" went off, and or a peak release of energy caused by rapid   depressurization and rapid collapse of surface tension.  So even though it seems like the "pop" or hit of the stick on a head is the main and thus only energy there is a secondary "explosive" moment right after the initial impact {the moment of surface collapse} where the energy of the impact "clashes" or explodes, thus giving that momentary dynamic swell. I think tuning lower would help create this "air" driven peak not so much because the tuning is lower, but because it is lower, there is less tension in the head thus acting like a spongier "trampoline" for the initial blast of air pressure, thus allowing it to "sink" deeper, thus delaying it return or spring back, yet when it does head the other direction it comes back with force, clashing with the trailing initial shockwave, thus combining forces and increasing amplitude.

 

In violins this comes down to elasticity and flexibility mixed with stiffness, To me violin backs are quite complex, need to work with all your other choices and are often asymmetrically graduated in order to get what I want. Guitar similar things happen but in a slightly different way, where a more left right  even gradient tone across the plates is good while try to get the top to have the lowest core tone you can get. So when I carve guitar plates like violins, more symmetry is involved.

 

Here again, just thought experiments

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What does "easy to play" mean?

I have seen many times people writing that the Cannon is hard to play. What does that mean?

 

I wish I had the chance to play it to see what this means but I doubt there will ever be such a chance. 

Notes don't start predictably.

 

This topic has been on my mind as I mess with my ultra-thick cheap fiddle.  The back graduations reminded me of the Cannone, very thick spine, with the thinnest areas at the outer edges of the bouts.  I doubt there is all that much similarity tonally (and probably will never find out), but mine is not easy to play... the strong resonances on the low E string not only make starting the note a bit difficult, but there are also wild variations in power from one note to the next, and it's virtually impossible to compensate for that and get a nice, even dynamic out of the thing.

 

I only have a rough hypothesis of why this might be.  Strong, peaky (i.e. undamped) body resonances are the root cause, and one way to get low damping is for the mode stresses to be stored up in longitudinal wood grain, where the internal damping is much lower than crossgrain.  So it make sense to me that a graduation pattern thick along the centerline and thin at the edges would have lower damping in general, making it louder but at the expense of the peaky resonances that make it harder to play.  Strads appear to be the opposite:  thick bands crossgrain...   = easier to play?

 

Just a working hypothesis currently.

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In the case of a drum, there are two resonant surfaces that can be tuned independently with respect to each other. When the top surface  is struck, this sets it in motion, and it wants to resonate at it's naturally tuned frequency. However, it also creates a pressure front that drives the bottom surface, also creating a natural resonance depending on the tuning of the lower head. These two resonances can buck or boost each other depending on their frequency / harmonic relationship. The air volume within the closed shell will affect this somewhat, but it is a less major factor compared to the actual tuning of the  heads. This buck or boost interference is what allows the heads to resonate in sympathy for a long period, or to quickly cancel each other out.

 

In a violin, because we have now mechanically coupled the two surfaces via the soundpost, the resonant frequencies change and are mechanically dependent on each other to create resonance. It's similar to an algebraic equation: What you change on one side of the equation directly affects the other side of the equation. This is the major reason why I have dismissed free plate tuning as a means to an end; there are just too many variables introduced once the instrument is assembled.  I don't want to open another debate over free plate tuning- this is just my personal opinion.

 

However, this doesn't mean that there isn't an optimal tuning relationship between the top and back, only that they have to be considered as a single resonating unit attached to the ribs, rather than two independent resonating units. This is what makes it a vastly more complex study compared to a resonating drum.

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Sure, free plate tuning doesn't tell much of the whole picture in the end.  But if one does need the plates to have some desired properties (in terms of frequency response, say), what does one do?  Put them together, test, take them apart, regraduate, and repeat?

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... what does one do?  Put them together, test, take them apart, regraduate, and repeat?

 

A lot of good makers I talked to do exactly that... 

experience with taptones, weights, hand flexing, or other in-process tests are only for getting in the approximate zone.

 

I'm sure there are other good makers that do things other ways.

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In a violin, because we have now mechanically coupled the two surfaces via the soundpost, the resonant frequencies change and are mechanically dependent on each other to create resonance. It's similar to an algebraic equation: What you change on one side of the equation directly affects the other side of the equation. This is the major reason why I have dismissed free plate tuning as a means to an end; there are just too many variables introduced once the instrument is assembled.  I don't want to open another debate over free plate tuning- this is just my personal opinion.

 

 

Very nicely put.

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A lot of good makers I talked to do exactly that... 

experience with taptones, weights, hand flexing, or other in-process tests are only for getting in the approximate zone.

 

I'm sure there are other good makers that do things other ways.

 

I've just been informed (through PM) that taking things apart is not necessary as one can thin from the outside.  Currently, I tap and flex.  I should add regraduation into my bag of tricks.

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This topic has been on my mind as I mess with my ultra-thick cheap fiddle.  The back graduations reminded me of the Cannone, very thick spine, with the thinnest areas at the outer edges of the bouts.  I doubt there is all that much similarity tonally (and probably will never find out), but mine is not easy to play... the strong resonances on the low E string not only make starting the note a bit difficult, but there are also wild variations in power from one note to the next, and it's virtually impossible to compensate for that and get a nice, even dynamic out of the thing.

 

I only have a rough hypothesis of why this might be.  Strong, peaky (i.e. undamped) body resonances are the root cause, and one way to get low damping is for the mode stresses to be stored up in longitudinal wood grain, where the internal damping is much lower than crossgrain.  So it make sense to me that a graduation pattern thick along the centerline and thin at the edges would have lower damping in general, making it louder but at the expense of the peaky resonances that make it harder to play.  Strads appear to be the opposite:  thick bands crossgrain...   = easier to play?

 

Just a working hypothesis currently.

I was very fortunate to measure and 'play' a del Gesu of similar thickness and vintage to Il Canonne quite recently. It was supernaturally good in all regards and easily the most responsive fiddle I ever put a bow to. It was not flattering or 'easy to play' for a poor player like me because it was SO responsive that the strings almost seemed to sound before the bow touched them highlighting my poor intonation... but it did make me realize how easy it must have been for the illustrious previous owners to play and compose virtuoso pieces on such an  instrument.

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I was very fortunate to measure and 'play' a del Gesu of similar thickness and vintage to Il Canonne quite recently. It was supernaturally good in all regards and easily the most responsive fiddle I ever put a bow to. It was not flattering or 'easy to play' for a poor player like me because it was SO responsive that the strings almost seemed to sound before the bow touched them highlighting my poor intonation... but it did make me realize how easy it must have been for the illustrious previous owners to play and compose virtuoso pieces on such an  instrument.

 

Lucky you!

 

This is somewhat counterintuitive given its thick top and really thick back.

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I was very fortunate to measure and 'play' a del Gesu of similar thickness and vintage to Il Canonne quite recently. It was supernaturally good in all regards and easily the most responsive fiddle I ever put a bow to. 

 

Did you get to weigh it, as a possible indication of what the wood densities might be?  

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I was very fortunate to measure and 'play' a del Gesu of similar thickness and vintage to Il Canonne quite recently. It was supernaturally good in all regards and easily the most responsive fiddle I ever put a bow to. It was not flattering or 'easy to play' for a poor player like me because it was SO responsive that the strings almost seemed to sound before the bow touched them highlighting my poor intonation... but it did make me realize how easy it must have been for the illustrious previous owners to play and compose virtuoso pieces on such an  instrument.

 

Nice to hear some are still out there in top condition !

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Did you get to weigh it, as a possible indication of what the wood densities might be?  

Hi Don,

I have to admit that I did not take this elementary measurement...it was a hasty trip to a foreign place and I forgot this totally important thing....On the other hand I am just completing a violin based on the architecture of Il Canonne.  My maple was low to mid density but the very olds pruce I chose was.44. My violin is 20 g heavier than the Canonne which has a heavier ebony board and 4 iron nail in the neck. My copy works surprisingly well. The front is 87g with bar! The back is 126g... For me that is very interesting because I am not in favour of trying to normalize violins...on the other hand my thick copy is nothing like the real thing so far....

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