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What does the back do?


Don Noon
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What if you think of it as a drum, a tom without the bottom head can be louder, easier to tune, sounds less complex, resonances dye faster and the low end just does not get there as it takes time to arrive where it needs to go.

 

If one places two of the same mic top and bottom, same exact distance, flip the phase of the bottom one  and record in separate tracks - one may get a few ideas while playing with faders. 

 

Bill have you done that? Anything interesting?

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What if you think of it as a drum, a tom without the bottom head can be louder, easier to tune, sounds less complex, resonances dye faster and the low end just does not get there as it takes time to arrive where it needs to go.

 

If one places two of the same mic top and bottom, same exact distance, flip the phase of the bottom one  and record in separate tracks - one may get a few ideas while playing with faders. 

 

Bill have you done that? Anything interesting?

I've recorded drums like this, however I usually find I use more from the overheads than the close mic tracks. The problem with the tom analogy is ignoring the shell, that the top and bottom head are only coupled together by air compression, and it's a closed resonator, rather than an open one like a violin.

 

An interesting experiment would be to place a soundpost between the heads, and see how varying tensions and changes in mass alter the overtones. However, because of the transient nature of the attack, it may be again hard to derive anything meaningful.

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 A flat back with a lump?

Yes, a diamond shaped piece about 2"x1" as far as I recall. but he was able to test for the right size on the outside before making it permanent.

 

I think there's a flat backed Gagliano viola in Hamma, but I'm not certain, and I don't have my copy here. I'd like to know how he treated the thickness.

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Right now I'm wasting effort rehabilitating a recently aquired cheap old student-quality fiddle, as most of my other experimental cheapos have been experimented into oblivion.  The top is thinned and will go back on today, with the very thick back intact (5.5 mm center, only very small areas down to 3).  After I see what I have, I intend to thin the back and see what changes.

 

It's together... and holy cr@p, this thing sounds awful.  I realize it has just undergone major surgery, and otherwise hasn't been strung up for probably decades, but still... 

 

Interestingly (especially for the signature mode fanatics) the B1- and B1+ look just fine in frequency and amplitude.  A0 frequency is a bit high at 295 Hz (D instead of the normal C#), but that isn't a big deal.  The big deal (well, actually two) is a couple of very high peaks at 768 and 845 Hz, and severe weakness above 2 kHz.  It's horribly boxy sounding and dull, although I think it has brightened up somewhat in the couple of hours it has been strung up.  

 

I'll give it a few days to settle in before doing anything else to it, but I'm not expecting miracles.  The dullness I suspect is primarily due to lousy spruce, rather than the thick back.

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I use flat backs of uniform thickness for my violas. I used to use a thick patch in the center area too but I have since changed to using high narrow braces there.

I've played a bit with how the back affects the rest of the instrument. In my tests I didn't see much that interested me when the backs were thick and stiff, it seemed that the back only had a strong influence when it was ~4mm thick at the center or less. One exception to this was flat backs. Even a flat back that was braced so that it felt as stiff as the arched back used in the test resulted in very different body modes. I never had the time to investigate exactly how high the arching needed to be before a flat back would start acting like an arched back. Arched backs of different arching heights all seemed to behave similar when they were fairly thick at the center ~5mm which seems to be a reasonable thickness for most violins other than golden period strads.

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It's together... and holy cr@p, this thing sounds awful.  I realize it has just undergone major surgery, and otherwise hasn't been strung up for probably decades, but still... 

 

Interestingly (especially for the signature mode fanatics) the B1- and B1+ look just fine in frequency and amplitude.

Fanatics are the worst kind! Hope you don't encounter anyone on your journey searching for the Strad sound.

These kind of people are irrational and base their believe on, well fanatism. Fortunately I haven't met anyone with this orientation. Most makers accept that violins have these modes within a certain range and they look just fine on a FFT plot.

Good luck with your scroll

Peter

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To me drums are highly dependent on the energy input as well as the duration of the input. Therefore the need to multiply or amplify energy is somewhat negated and the open end design acts like the barrel of a gun and gives the energy/air a directed escape path. Based on the energy output of bowed strings not being anything like a whack from a drum stick I feel the back is a crucial element to amplifying the energy via the Helmholtz resonator that it becomes. With the ability to draw out the duration of the note as well as the energy input from the bow, having a closed box is crucial to sound quality. I think listening to open backed banjos gives one a sense of the more desired in this case percussive sound. It works well with plucking based on the energy duration, as long as the body cavity is not so big that it absorbs too much energy. Percussive sounds do not have the ability to swell based on there short duration and therefore the dynamic range is limited to strike to strike, where as the violin can sound one note infinitely as use dynamic volume swell, something that I feel can not be harnessed without a enclosed box with air holes. An open back fiddle may be ok for pizzicato , but if it were to be used in such a way I have a feeling a 1/2 sized one would sound louder based on the body size not absorbing the somewhat weak energy of a plucked string. Let alone getting into all the structural deformation issues that would eventually come up from prolonged stress loads.

 

In itself that is "one thing the back does" from a structural stand point it adds strength to the entire structure which is crucial to handling prolonged stress loads, being the floor for the pillar that the post is, with no floor you have no post, eventually the top will squash in the middle and the ribs would flair and twist and your fingerboard would keep getting pulled lower and lower. If we have a bassbar in the picture, the treble side with no post would most likely show as asymmetric distortion with it distorting more than the supported bass side.

 

edit; really good drummers can give the "illusion" of swell , for example by playing extremely fast paradiddles where the energy output is increased and decreased, but by all accounts it is an audio illusion based on the individual strikes being so close together that it sounds almost like a continuous note, but of course its not.

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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.

Floor 14".mov

Floor T B 1.mov

Maracatú 20" Drum.mov

Taiko Low Hard.mov

Tom Low.mov

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Interesting is what Craig said. I always finish my back first. 

 

Yes, some people do make the back plate first.

- it doesn't really matter which plate gets MADE first, I'm thinking that perhaps they need to be "mated" in many respects - no matter  what order they're made in.

Two plates - one violin. Is your top plate a 'reflection' of the backplate in any way?

Just curious - I don't think that everyone makes their violins alike.

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I don't think this experiment would produce much in the way of meaningful information. Because the box is now completely open, the natural internal air resonance is gone. This will drastically change the timbre.

You may be right.  I'm just tossing out ideas as the least scientific among us.  

 

Incidentally, as an example of the importance of having an intact "box,"  I had a fine violin which started sounding false. (This was when I was quite young and inexperienced.)  I changed strings several times, before finding that the top had come unglued over a rather large area from the ribs.  Once it was glued the falsity disappeared.   I find it might be instructive that such a small factor can effect the tone so much.   Not all violins seem to be as sensitive to this as mine did.  

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A friend of mine made a violin with a flat back, of even thickness. It sounded like a dog until he added weight to the centre by glueing in  a thick patch of maple. Then it sounded a bit like a violin! 

Maybe it would be interesting to look at flat-back viols.  After all,  they evolved too.  In stead of a lump of maple in the middle, perhaps a board to act like the board that supports the post in a viol.

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I wouldn't say that a large broken glue joint is a "small factor".  It sounds fairly major to me.

 

My cheapo experimental fiddle has improved markedly overnight, with the high end level up by a couple of dB on the response curve, matching the playing impression of it opening up and brightening.  It's still a very long way from good, though... the mid-frequency peaks are still dominating the tone, making it very boxy sounding.  It also shows up as overly strident and near-wolfy on the low position E string.

 

I checked the mode shape of the offending resonances, and it's the same basic pattern as the Titian and Plowden... but with very little movement in the back.  Top vs back radiation measurements also show that the noise is primarily generated by the top, and not much is coming from the heavy back.

 

It will be interesting to see how this all changes when I get around to thinning out the back.  

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Again I'm just a violin play..student!  :) But when reading the OP question an idea came to my mind:

 

Imagine the 'fronton game' (I'm sorry I'm not sure how it's said in english) but essentialy you must hit an elastic rubber ball with a racket, the ball flies off till it reaches the fronton wall which bounces it back.

 

The ball would act as the sound waves

 

The racket (or the racket net) would act as the violin top. It is softer than the wall, imposes direction, stile, angle, etc., to the ball.

 

The wall would act as the back of the violin. It is harder than the racket and has not the same delicacy or accuracy. Its function primarily to bounce the ball back, and different stiffnes and roughness on the wall would directily affect its 'response'.

 

Hitherto my idea :)

 

If you ask how does the violin back do this? that's another question.

 

 

Disclaimer: All information providen on this post has no reliability at all so take care and skip it if necessary  :lol:

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Imagine the 'fronton game' (I'm sorry I'm not sure how it's said in english) but essentialy you must hit an elastic rubber ball with a racket, the ball flies off till it reaches the fronton wall which bounces it back.

 

 

Hand ball.  Racket ball.  And with the "cesta" or basket:  Jai Alai.  All great games, particularly Jai Alai.  Used to go every night during the season in Miami.  I wonder if anyone else remembers Joey, the hero, and Goriena, the goat?

 

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Hand ball.  Racket ball.  And with the "cesta" or basket:  Jai Alai.  All great games, particularly Jai Alai.  Used to go every night during the season in Miami.  I wonder if anyone else remembers Joey, the hero, and Goriena, the goat?

 

 

 

Thank you, racket ball would suit my example :)

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