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How does a violin reproduce overtones? - Theorizing a model


Andreas Preuss

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19 hours ago, Don Noon said:

I think in complicated terms... because it is.

7 hours ago, Andreas Preuss said:

Can’t be too complicated. If I really believe one thing it is that makers in Cremona in the 18th century and before didn’t think in complicated terms. Their results speak for themselves. We just need to get rid of the ‘data approach’.

If you want to go into "theorizing a model" for overtones, the actual physics/acoustics is extremely complicated.  If you want to make a violin that has good overtones, you try stuff.  Then abandon what doesn't work and repeat the stuff that works... less complicated.

I do both... the first one for thinking up stuff to try, and the second one for the yea or nay on the theory.

I think data can have a place as a tool to help understand results... but much less so to judge results, and can be misleading if used rigidly as construction rules.

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45 minutes ago, Don Noon said:

If you want to go into "theorizing a model" for overtones, the actual physics/acoustics is extremely complicated.  If you want to make a violin that has good overtones, you try stuff.  Then abandon what doesn't work and repeat

Good approach. 
 

But still, I don’t think you need to understand the details of the ‘overtone chaos’ in detail.

So far my model is vertical strength versus lateral (cross grain direction) strength governs the overtones. Lengthwise the body of the violin doesn’t have much room to move or vibrate freely, because it is ‘clamped’ by the string tension. Horizontally m mm, however, there are no other forces than structural stability to control the vibrations. Overtones or very high frequencies just need enough resistance or ridigity. If there is anywhere a weak point in the structure, it can’t function any more.

The paper back experiment showed that forces on the back beam right to the c bouts. Another experiment with placing the loose top on the rib garland and stringing the violin up (without sound post) showed that forces push the top down on the c bouts, while upper and lower bout flanks are curling up. So all forces connect on the c.bouts. Therefore the cross stiffness in that area is what matters most. 
 

The weak point is the cross grain stiffness of the top. Anything what helps to strengthen it makes the overtones stronger. Arching, linings, breast width between f holes and to a certain degree the bass bar connecting cross arch points along one line making them less flexible. 
 

The ‘complicated’ thing about those elements is that none of them alone can bring the desired result and each of them has to contribute a bit to it. 
 
The only thing one shouldn’t do is try to get this with additional thickness. In any case it doesn’t make sense in my view to manipulate the overtones in the areas where you detect them. Somehow it is similar like a buzz. You detect in one place but the cause for it is almost never in the same spot. 

i usually look at the entire ‘mountain’ of overtones and which peak is the center of it. I think the location of the center peak is also important. That’s something I am still chewing on. But once I understand it I am pretty much done with everything. 

I might go for re-bending the top into a higher arching, and I am still thinking of the enlarged  top block. 

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8 hours ago, Andreas Preuss said:

Interesting!

could you give an example for one of those engineering ‘rule of thumb’?

Of course.

The chain curve from which catenary arches are generated. It's simple to apply, it is perfect if the material has constant density and all these while the maths intricacies required to find the curve and prove from first priniciples it does what's believed to do  are very difficult.

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6 hours ago, jezzupe said:

Maybe? I'm not so sure, I do think that perhaps intensity would be the determining factor? again I wish when they did the 3d imaging that they also had some consensus on some gawd awful violins so as to compare them to the Titan.

I'm sure. That's how FFT works. Those "patterns" are missing important details. But this is not a subject I like to elaborate. And anyway, not much can be gathered from an FFT. It's simply the wrong tool for the problem.

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2 hours ago, Carl Stross said:

I'm sure. That's how FFT works. Those "patterns" are missing important details. But this is not a subject I like to elaborate. And anyway, not much can be gathered from an FFT. It's simply the wrong tool for the problem.

Problem? Carl the difference between an ordeal and an adventure is ones attitude :D, but well I agree about the fft

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On 8/11/2021 at 2:08 AM, jezzupe said:

this is a more sciency kinda way of describing what I was trying to describe.....yes I agree 100% imo graduations along with arching "benchmarks" create the ability to "control" particularly those very close highs where "action" is happening. This starts to bump into "wolfs" a bit and good cancellation vs bad, or constructive or destructive forces. 

And or if we can locate specific regions that we want to minimize out of phase motion counter acting negatively motions we do want, that it is simply a matter a fattening that area* to prevent as much motion as compared to one next to it, assuming at this point the arching is "done" and your working with what you have established as a benchmark

So, what I've been doing is trying to see if I can create "braile maps" of really good violins and compare them to bad ones, this is very early stage stuff with limited pool of instruments, but to the best of my knowledge I have not read much on anyone trying to approach it from my angle.

I think in simple terms I'd like to know if all very good violins have similar "patterns" of activity when examined with the hand during playing in order to create "vibration maps"  and that if bad ones are also similar. all with the intent of trying to see if there is not some way to "feel" if there are areas that would be better thinner or thicker in order to establish some method of repeatability as well as teachabilty so other could do the same 

I do think of it like trying to establish a form of braille for violin making that allows a builder to

1. know where a good violin should be moving/ vibrating the most, as well as know the dead zones, for all played frequencies 

2. to use the simple yet highly effective sense of touch to be the tool to determine this {anyone can do it} and it can be extremely accurate 

3. with the hope that not only can amplitude be maximized , or a high ceiling can be installed, and volume can be called if needed but perhaps more importantly , hoping advanced understanding of a "vibration map" can help  one "blend" all the regions together cohesively with some level of deterministic control. 

I think we'd all love our own 3d map maker so we could "look" at our progress, I "think" that "feeling" it {literally} may not only be a cheaper {free} way of doing a very similar thing that may even be more accurate and detailed region to region.

It's early stage stuff, but there seems to be something to it.

*there is no simple, by thickening one area to counteract an "issue" may create another one somewhere else

 

It is certainly true that we don’t trust our senses so much any more and learning to use them improves our skills.

However, I am wondering how you approach in detail with your Braille map? 
 

First, just technically, do you play the violin and try at the same time to feel the vibrations or do you need a second person to play it?
 

Do you select important frequencies or do you feel the vibrations and look at the note (frequency)? (Supposedly you don’t map every possible note on every string…)

Then, let’s say you determine that there is not enough vibration in a certain area, what can you do about it? Do you scrape from the outside or do you pay attention to it on the next violin? 

Or, do you leave some critical areas of the plates too thick by purpose for calibration? (That’s what I would probably do)
 

Do you think, all the ‘tonal problems’ are in thickness and arching of top and back? (My view on this has changed pretty radically.) 

And most important, what is the feedback from professional violinists? 

(Just curious)

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Here I made with audacity a graph from a live stereo recording I have on a CD. Its a violin solo piece and the performer is using a golden period Strad. 

So even with the mic at some distance I think we can see what matters.  The strongest peak is around 2700Hz and we have peaks slowly ascending to it on one side and slowly descending from it on the other side covering easily one octave. 887135804_Stradivariinliverecording.thumb.png.f7566e69718820c0d2dabd60d5609317.png

The violin simply sounds brilliant. 

(Picking this recording was just a lucky guess, I think I am going to dig out more live recordings to get a better picture.)

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13 hours ago, Andreas Preuss said:

It is certainly true that we don’t trust our senses so much any more and learning to use them improves our skills.

However, I am wondering how you approach in detail with your Braille map? 
 

First, just technically, do you play the violin and try at the same time to feel the vibrations or do you need a second person to play it?
 

Do you select important frequencies or do you feel the vibrations and look at the note (frequency)? (Supposedly you don’t map every possible note on every string…)

Then, let’s say you determine that there is not enough vibration in a certain area, what can you do about it? Do you scrape from the outside or do you pay attention to it on the next violin? 

Or, do you leave some critical areas of the plates too thick by purpose for calibration? (That’s what I would probably do)
 

Do you think, all the ‘tonal problems’ are in thickness and arching of top and back? (My view on this has changed pretty radically.) 

And most important, what is the feedback from professional violinists? 

(Just curious)

As I mentioned this is very early stage, potentially "palm reading hoonanny", and it's just something that I've been "messing with" for less than 2 months, it may get ditched it may expand, I don't know yet....

I think tonal problems are primarily psychological defects disguising themselves as highly trained individuals with acute hearing :D and that the realm of tone that we are really talking about, that last little % that separates super amazing from really good has much more to do with materials and age than minutia within the building process.

I start to think to myself , what if I could make every instrument "that" instrument every time, and I think how boring that would be, I think there are many types of good, tone wise, and I think that frankly, I'm pretty happy with the sound I get from my instruments and I think that unlike many of you, I not really too hung up on it, but I do like to "theorize" with the lot of you, cause well frankly I can't stand/loathe tv or movies and I prefer to spend my time thinking about things in abstract ways then mindlessly getting hypnotized and brainwashed.

I think you have lots of interesting thoughts and experiments and well, I dunno, all you can do is build them and find out.

Also, as mentioned, one can only play open string themselves to "get an idea" of what your trying to do, to do it more completely you do need another person to play while you feel the instrument. 
 

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8 hours ago, Andreas Preuss said:

Here I made with audacity a graph from a live stereo recording ...  a golden period Strad. 

Can you say what specific Strad it is?  I note a very strong CBR response, and a sharp dropoff just above 4 kHz.  Very similar to the golden period Strad I am most familiar with... but maybe he made them all sound about the same in his golden period. 

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2 hours ago, jezzupe said:

As I mentioned this is very early stage, potentially "palm reading hoonanny", and it's just something that I've been "messing with" for less than 2 months, it may get ditched it may expand, I don't know yet....

I think tonal problems are primarily psychological defects disguising themselves as highly trained individuals with acute hearing :D and that the realm of tone that we are really talking about, that last little % that separates super amazing from really good has much more to do with materials and age than minutia within the building process.

I start to think to myself , what if I could make every instrument "that" instrument every time, and I think how boring that would be, I think there are many types of good, tone wise, and I think that frankly, I'm pretty happy with the sound I get from my instruments and I think that unlike many of you, I not really too hung up on it, but I do like to "theorize" with the lot of you, cause well frankly I can't stand/loathe tv or movies and I prefer to spend my time thinking about things in abstract ways then mindlessly getting hypnotized and brainwashed.

I think you have lots of interesting thoughts and experiments and well, I dunno, all you can do is build them and find out.

Also, as mentioned, one can only play open string themselves to "get an idea" of what your trying to do, to do it more completely you do need another person to play while you feel the instrument. 
 

I see. 
 

I myself was often wondering if  we can’t use tapping on the finished instrument in a better way. 
 

i have thrown myself into experiments because I wanted to get information first hand by hearing myself. On the way questions come up which I put here for discussion. And I am not afraid to ask stupid questions. 
 

Good luck with the Braille map method. In any case all time spend here is certainly better used than watching tv or lingering somewhere on the internet. 

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19 hours ago, Andreas Preuss said:

Here I made with audacity a graph from a live stereo recording I have on a CD. Its a violin solo piece and the performer is using a golden period Strad. 

So even with the mic at some distance I think we can see what matters.  The strongest peak is around 2700Hz and we have peaks slowly ascending to it on one side and slowly descending from it on the other side covering easily one octave. 887135804_Stradivariinliverecording.thumb.png.f7566e69718820c0d2dabd60d5609317.png

The violin simply sounds brilliant. 

(Picking this recording was just a lucky guess, I think I am going to dig out more live recordings to get a better picture.)

The high frequency fall-off above about 3000Hz  is desirable and most other orchestra instruments are designed to do this too.

Perhaps another topic heading should be : "How does a violin not produce high frequency overtones?"

 

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12 hours ago, Marty Kasprzyk said:

Perhaps another topic heading should be : "How does a violin not produce high frequency overtones?"

Well, the answer is flimsy ribs. My first idea on the new concept violin was to safe as much weight as possible on all parts except top and back thinking that they are the important sound producing parts. They are the sound producing parts but need to be backed up by the ribs, which are almost functioning like a sound amplifier. The sound with 0.2mm ribs and minimized linings was completely dull because there were literally no overtones. It was actually so bad that I wouldn’t show it to any violinist. 
 

There is IMO a huge field to explore in rib construction. After all the many experiments, I have the vague feeling that all stiffness we can possibly add to the rib garland helps to make the top and back thinner for a louder and overtone-rich sound. 

When I try to visualize in my head a neck which wobbles to easily around because it is not good enough anchored in the upper part of the rib garland, I think this must have a negative effect on the sound volume. If we would use rubber for the top block I guess this must dampen the sound pretty much. 

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It is interesting to see the process of experimentation leading to theorizing about why the results are what they are.  I have often come across writings by well-regarded, experienced makers who will expound upon something they think works well... and then go into an explanation of why it works that makes no sense whatsoever in terms of physics/acoustics.

I'm not saying anything about the current thread... I can't tell anything for sure one way or the other.

But a incorrect explanation doesn't matter, in the end.  Something that WORKS is what matters.  

14 hours ago, Marty Kasprzyk said:

Perhaps another topic heading should be : "How does a violin not produce high frequency overtones?"

Definitely a topic worthy of investigation.  The obvious player, bridge rocking frequency, I have tested... but in test and theory, is is a slow rolloff at best, or about -6dB/octave for a 1st order lowpass filter... far less than what I actually measure.  At the moment, I haven't looked all that deeply into higher order filters and how a structure might perform higher order filtering... but it is a very intriguing idea.

This is a response plot of one of my violins, showing a steep dropoff corresponding to about a 4th order (-24 dB/octave) filter.

rolloff.jpg.99d45fdb5751865524bea3bda949c8f3.jpg

 

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2 hours ago, Andreas Preuss said:

Well, the answer is flimsy ribs.

Your Audacity plot is essentially log(displacement) vs log(frequency). The approximately straight-line roll off at high frequencies is predicted by relatively simple damped harmonic oscillator models, which predict a linear relationship between the logs of these two measures.

Moreover, wood is one of those materials that has frequency dependent damping: the higher the frequency, the higher the damping. A closer inspection of response charts shows a roll off that accelerates as the frequency increases, consistent with frequency dependent damping.

The highest root frequency played on the violin is around E7. That is ~2600Hz. Any measured response above that is driven by weaker overtones.  It should not come as a surprise that the inherent damping properties of wood would dominate at frequencies much higher than this.

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56 minutes ago, Don Noon said:

But a incorrect explanation doesn't matter, in the end.  Something that WORKS is what matters.  

Well said. 
 

But usually we try to theorize something before we try something and sometimes it works sometimes it doesn’t. And the really frustrating things start if it worked on the first fiddle but can’t be repeated on the second. 
 

Nevertheless, violin making has changed dramatically over the past 30 years in terms how makers try to get good results. What used to be just a list of measurements from violin making school plus a self made idea of graduation maps went to much more acoustic oriented making procedures.

To know what works one has to admit ones own failures in the process. And it is always a mistake to talk oneself into ‘improvements’ which are not there. (For this sound graphs are pretty good and very stubborn if we try to really change something massively) 
 

In the end ‘what works’ boils down to maybe a handful of procedures and ‘recipes’ plus the experience how to fine tune everything to each other. 
 

What works?

Make  the top as thin and light as possible and adjust the rest to it.

As simple as it sounds it contains all the problems we are facing to make a really good sounding instrument. 

In the end each of us has some abstract ideas about how the sound comes out and why it is good or bad. It is probably best to describe procedures and their acoustic results without trying to analyze why this happens. 

 

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41 minutes ago, ctanzio said:

Your Audacity plot is essentially log(displacement) vs log(frequency). The approximately straight-line roll off at high frequencies is predicted by relatively simple damped harmonic oscillator models, which predict a linear relationship between the logs of these two measures.

Moreover, wood is one of those materials that has frequency dependent damping: the higher the frequency, the higher the damping. A closer inspection of response charts shows a roll off that accelerates as the frequency increases, consistent with frequency dependent damping.

The highest root frequency played on the violin is around E7. That is ~2600Hz. Any measured response above that is driven by weaker overtones.  It should not come as a surprise that the inherent damping properties of wood would dominate at frequencies much higher than this.

I suppose your answer was directed to Dons graph? (I probably understood only half of it.)
 

It looks like the well educated answer to the dampening effect over 2600 Hz, but I have to say that I got the impression if resonance peaks go down too rapidly in the region of 3-4khz this isn’t good either. 

If there is anything I’d still like to know it is how to get higher peaks between 2khz and 4khz. 
 

Otherwise I am interested in a textured sound. But more important things first.

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58 minutes ago, Andreas Preuss said:

Nevertheless, violin making has changed dramatically over the past 30 years in terms how makers try to get good results. What used to be just a list of measurements from violin making school plus a self made idea of graduation maps went to much more acoustic oriented making procedures.

I haven't seen any convincing evidence that the modern methods are responsible for any improvements in the last 30 years.  There are those who would argue that the modern methods have been the cause of decline over 300 years... but let's not go there.  Personally, I view a lot of the modern methods as more of a distraction.

58 minutes ago, Andreas Preuss said:

What works?

Make  the top as thin and light as possible and adjust the rest to it.

The "as possible" can mean many things.  I can (and have) made some tops very thin and light, no problem with structural collapse (immediately), and I wouldn't say they worked all that well (unless you want to play bluegrass with a microphone and like the "tubby" sound).  Probably could be salvaged to some degree with a massive bass bar, but I think that "thin" can be taken too far and still be "possible".

Balance in all things.

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11 minutes ago, Don Noon said:

I haven't seen any convincing evidence that the modern methods are responsible for any improvements in the last 30 years.  There are those who would argue that the modern methods have been the cause of decline over 300 years... but let's not go there.  Personally, I view a lot of the modern methods as more of a distraction.

The "as possible" can mean many things.  I can (and have) made some tops very thin and light, no problem with structural collapse (immediately), and I wouldn't say they worked all that well (unless you want to play bluegrass with a microphone and like the "tubby" sound).  Probably could be salvaged to some degree with a massive bass bar, but I think that "thin" can be taken too far and still be "possible".

Balance in all things.

Yes things can go from a shell of wood to paper flapping in the wind pretty quick, I would say a more accurate statement would be that in certain select areas we may want to go "as thin as possible" or certainly "thin" 

Andreas have you made any guitars? I can't really explain why, but if you make guitars it seems to help with violins,your suggested approach, if I gather it correctly is that you want a wafer thin wood shell supported by bracing in select structural areas to support the shell and prevent structural failure, thus allowing for unfettered plate excitation in the areas unsupported by bracing? sounds like a guitar to me. I like to think that my building techniques for guitars, which basically are Strads/Amati methods for violins when combine with the right materials makes for some of the best sounding guitars I've ever heard and I don't say that with bias as the maker, I say it as a surprised guitar player who has a hard time believing such a "obvious" way of building isn't really done by anyone but me.

And so perhaps there is some merit in your ideas, and you must build them to find out.

Have you heard Doug Martins Balsa violins? the structure I believe is similar to what you are describing, some of them sound quite good.

He used to post here, but have not seen him in many years 
 

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15 hours ago, jezzupe said:

Yes things can go from a shell of wood to paper flapping in the wind pretty quick, I would say a more accurate statement would be that in certain select areas we may want to go "as thin as possible" or certainly "thin" 

Andreas have you made any guitars? I can't really explain why, but if you make guitars it seems to help with violins,your suggested approach, if I gather it correctly is that you want a wafer thin wood shell supported by bracing in select structural areas to support the shell and prevent structural failure, thus allowing for unfettered plate excitation in the areas unsupported by bracing? sounds like a guitar to me. I like to think that my building techniques for guitars, which basically are Strads/Amati methods for violins when combine with the right materials makes for some of the best sounding guitars I've ever heard and I don't say that with bias as the maker, I say it as a surprised guitar player who has a hard time believing such a "obvious" way of building isn't really done by anyone but me.

And so perhaps there is some merit in your ideas, and you must build them to find out.

Have you heard Doug Martins Balsa violins? the structure I believe is similar to what you are describing, some of them sound quite good.

He used to post here, but have not seen him in many years 
 

No I haven’t made guitars. Once in a while I am reading things, mostly about inventions. There were very interesting ideas, which I found inspiring, though most of them can’t be applied ‘as is’ on violin making. However, all pivots around new ideas to make the top as light as possible by altering the bracing pattern, or laminating the top or making the frame ultra strong. The best thing is that there is a great acceptance from guitarists.

When I was still working in the US I met Canadian double bass maker James Ham, who presented his balsa cello. Indirectly I took this as inspiration for my project. However, if it were about making a violin which sounds ‘ok’ I’d stop right now. 

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16 hours ago, Don Noon said:

The "as possible" can mean many things.  I can (and have) made some tops very thin and light, no problem with structural collapse (immediately), and I wouldn't say they worked all that well (unless you want to play bluegrass with a microphone and like the "tubby" sound).  Probably could be salvaged to some degree with a massive bass bar, but I think that "thin" can be taken too far and still be "possible".

I didn’t say ‘as thin as possible measured against structural collapse.’;)

However my experience so far experimenting with structural elements showed that a top which I would have thought is by far too thin (or in your terms good for a bluegrass fiddle) can be ‘powered up’ with rib construction, neck angle and bass bar. Currently the top of the new concept violin weights 47g (not counting the edge doubling I made) and is around 2.2-4mm thick and I wouldn’t say it sounds like a bluegrass fiddle. (I really wonder how thin and light one can go.)
 

New experimental results on the new concept violin coming up soon.

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22 hours ago, Don Noon said:

The obvious player, bridge rocking frequency, I have tested... but in test and theory, is is a slow rolloff at best, or about -6dB/octave for a 1st order lowpass filter... far less than what I actually measure.  At the moment, I haven't looked all that deeply into higher order filters and how a structure might perform higher order filtering... but it is a very intriguing idea.

This is a response plot of one of my violins, showing a steep dropoff corresponding to about a 4th order (-24 dB/octave) filter.

rolloff.jpg.99d45fdb5751865524bea3bda949c8f3.jpg

 

I think the bridge hill lie around 2 kHz and drops from a bit over that. Something else contributes to the higher part up to 3 and above 3 kHz. Maybe a different bridge mode (bouncing), radiation properties or something unknown. 

It is easy to get much roll-off by adding mass or using a bridge with much mass. 

The string harmonics drop too with the increasing frequency. For a sawtooth it drops 6 dB per octave. Some shaping of the curve will also appear, related to bow bridge distance, I believe. One get "dips" where the bow play on, or near, "nodal lines" for the string modes.

In addition there may be an effect of mobility loss due to the mass of the bridge and the central region of the plate itself. I think that mobility term goes as -20log(m*f*eta) where eta is the loss factor, m is the mass per area and f is the frequency. Doubling of the frequency gives 6 dB too. 

6 dB is harmonic drop oss, 6 dB is simply mobility (mass law), maybe the bridge resonance, or similar, give 6 dB per octave too, so we are left with 5 dB we do not know where come from.

Damping goes down as the frequency goes up for simpilicity, like eta = 0,02 (or whatever correct internal damping) + 0,8/root(f) in average. 

In this case it may also be some contribution from roll-off in the mic. Good enough for speech, but probably not for high end measurements. 

Edited by Anders Buen
Corrected the dB/octave for string harmonics after lookup. Mobility and damping.
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45 minutes ago, Andreas Preuss said:

Currently the top of the new concept violin weights 47g (not counting the edge doubling I made) and is around 2.2-4mm thick and I wouldn’t say it sounds like a bluegrass fiddle. (I really wonder how thin and light one can go.)

My lightest top is 48g including varnish, and it too does not sound like a bluegrass fiddle.  But neither does it sound or play like a normal violin.  Not terrible, but not normal either.  Players often talk about "resistance", but I think it's more about "inertia"... where mass matters.  I also think that light tops (beyond a point) don't help the high frequencies as much as some of the middle ones, giving a loud but unrefined tone.

8 minutes ago, Anders Buen said:

The string harmonics drop too with the increasing frequency. I have forgotten how much, might be 6 dB per octave or 12 dB per octave.

Fan Tao said string harmonics drop at 6 dB/octave, and he should know.  But that wouldn't show up on an impact response plot.

My cheap microphone I'm pretty sure is actually over-responsive in the high frequencies.  I have compared it to 2 other pretty good microphones, and there is definitely more high end with the cheap one.  In testing for C in short soundpost stock, there are clear peaks over 20kHz. 

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15 minutes ago, Don Noon said:

My lightest top is 48g including varnish, and it too does not sound like a bluegrass fiddle.  But neither does it sound or play like a normal violin.  Not terrible, but not normal either.  Players often talk about "resistance", but I think it's more about "inertia"... where mass matters.  I also think that light tops (beyond a point) don't help the high frequencies as much as some of the middle ones, giving a loud but unrefined tone.

Fan Tao said string harmonics drop at 6 dB/octave, and he should know.  But that wouldn't show up on an impact response plot.

My cheap microphone I'm pretty sure is actually over-responsive in the high frequencies.  I have compared it to 2 other pretty good microphones, and there is definitely more high end with the cheap one.  In testing for C in short soundpost stock, there are clear peaks over 20kHz. 

Ok, this was an impact test. The impact roloff is related to your stick and bridge properties, I belive. The contact physics there play a role I think. Im not able to tell where the rolloff comes in relation to the contact time but they are in some way inversely related. The roloff may even depend a bit on the humidity level in the bridge and the spruce stick.

I think the theory behind it is described in one of the references I gave under my thread on radiation from orthtropic plates. 

I think I have a similar mic to the one you use, a mic that came with my first computer and fine soundcard then. I do think it is a little directive. I used it until I bought finer ones for the Curtin rig. 

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