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Nick Allen

What to do with spongy American spruce?

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7 hours ago, Don Noon said:

With low-density, high RR plate wood, the signature modes can get higher than you might want and the plate weight can get very low.  More mass in the bar (without extra stiffness) keeps the frequency down.  Mass in the bar also (I think) keeps the bass foot from moving at the higher frequencies, where the more efficient movement comes from the treble foot.  More punch.  And keeping the stiffness low allows more bass response.

It's all a balance of mass and stiffness and what you want in the response.  I started out trying to maximize stiffness/weight, mostly by reducing the weight to a minimum, including the bass bar.  Those fiddles didn't play quite right... too lightweight, instant response, and tone overly strong in the midrange.  More mass in the right places helps.

Why use low-density high RR wood in the first place?

If you're stuck with it I suggest using lower arches.

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16 minutes ago, Marty Kasprzyk said:

Why use low-density high RR wood in the first place?

If you're stuck with it I suggest using lower arches.

I'd go the opposite. The lower density the wood, the more it needs a strong, acute cross arch and height. The denser it is, the better it can weather being low and flat...

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14 minutes ago, Christopher Jacoby said:

I'd go the opposite. The lower density the wood, the more it needs a strong, acute cross arch and height. The denser it is, the better it can weather being low and flat...

High arches increase stiffness which increases all the various mode's frequencies.

On the other hand I would rather have high arches than flat feet if there wasn't a draft going on.

 

 

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1 hour ago, Marty Kasprzyk said:

Why use low-density high RR wood in the first place?

If you're stuck with it I suggest using lower arches.

I think you can still get a lighter plate+bar with the high RR plate and heavy bass bar, which theoretically could have benefits to radiativity... although small and perhaps below the level of perception.  

I definitely have migrated away from the ultra-low density spruce, without any loss of power... although I haven't gotten to the Goldsmith density range yet.

If I'm "stuck with" excessively low-density wood, I cut it up for corner blocks.

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On 11/16/2019 at 2:52 PM, Nick Allen said:

The mode 5 was pretty low at a more standard graduation. About 10% off, funnily enough. 

If you think all of your Chambers wood was split from the same round of log then you should have several wedges that are similar in numbers.  Chances are he kept the two to four premium wedges from that round leaving possibly up to eight other lessor but usable pieces for selling.  That' o.k. though - you may even have the two to four premium wedges there with you already.  If that's the case your job of figuring what can be good for sound turns into a little bit more work assuming of course you bought a lot of wood in the first place.

With a low mode 5 you may want to try a shorter, narrower D.G. model - those wider bouts of your Strad plan may be causing the low mode 5.   

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1 hour ago, Marty Kasprzyk said:

High arches increase stiffness which increases all the various mode's frequencies.

On the other hand I would rather have high arches than flat feet if there wasn't a draft going on.

 

 

Got the flat feet!

i was speaking from my own adverse experience with a great deal of .33 and .34 spruce I used in 2008-2012 or so. Modes aside, those instruments’ lives are drastically shortened by having such light tops. A dozen of the 30-40 fiddles I made those 4 years have come back through my life, and the models with flat arches are warped and distorted beyond their years. Sunken bass effhole wings, massively open treble effholes, saddlebacked tops, etc. 

I do now work with spruce in the .38-.48 range happily, but the fiddles made with pronounced cross arch from that wood (and those left Sacconi numbers or thicker to boot) are still in reasonable sculptural shape in comparison! 
lastly, though.... some of those warped fiddles are sounding fantastic. A neck reset, an arch correction here and there, their owners are using them professionally...

low density beware, and handle with an exaggeratedly strong cross arch...

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17 hours ago, Christopher Jacoby said:

Got the flat feet!

i was speaking from my own adverse experience with a great deal of .33 and .34 spruce I used in 2008-2012 or so. Modes aside, those instruments’ lives are drastically shortened by having such light tops. A dozen of the 30-40 fiddles I made those 4 years have come back through my life, and the models with flat arches are warped and distorted beyond their years. Sunken bass effhole wings, massively open treble effholes, saddlebacked tops, etc. 

I do now work with spruce in the .38-.48 range happily, but the fiddles made with pronounced cross arch from that wood (and those left Sacconi numbers or thicker to boot) are still in reasonable sculptural shape in comparison! 
lastly, though.... some of those warped fiddles are sounding fantastic. A neck reset, an arch correction here and there, their owners are using them professionally...

low density beware, and handle with an exaggeratedly strong cross arch...

Interesting,...I never felt comfortable using low density spruce for reasons mentioned above, my best sounding fiddles were made with .40 plus spruce.

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23 hours ago, Christopher Jacoby said:

I'd go the opposite. The lower density the wood, the more it needs a strong, acute cross arch and height. The denser it is, the better it can weather being low and flat...

An advantage of using a high radiation ratio RR (speed of sound /density, c/p) wood over using a wood with a lower RR is that it allows the plate to be made thinner thus lighter for the same mode frequencies.  The lower moving plate weight makes the instrument louder.

A high RR can be achieved by having various combinations speed of sound  and density and often a wood with an especially low density will tend to have a high RR.  So from an acoustic output point of view a thin plate made from low density wood is good.

Unfortunately the strength (fracture strength or creep resistance) of wood is generally proportional to density so a plate made with low density wood is weaker than one made from high density wood.  The bending strength of plate is also proportional to its thickness cubed so a thinner low density wood plate (great for sound output) is relatively very weak.  Cracks and/or deformation will quickly occur.  So there is a trade off between acoustic output and instrument life dependent upon your wood and thickness choices.  

This is not a new observation.  John Schelleng, in his landmark 1962 paper "The Violin as Circuit" (1) showed the derivation of the radiation ratio criteria for picking wood and he stressed the benefits of picking a high RR wood.  However he also said:

     "Strength of wood is another consideration...it appears that a different wood parameter, strength/cp measures the upper limit of sound pressure produced" where strength is                       the bending, shear, tension strength across the grain, etc."

Perhaps we should be picking our top plate wood on the basis of cross grain bending strength/cp to avoid cracks which seem to be so common in old instruments.  Maybe we should also think about how long we wish our instruments to last.

 

1. The Journal of the Acoustic Society  of America, Vol. 35, No. 3, 326-338, March 1963,  Reprinted in CASJ Vol. 4, No. 3 [series ll], May  2001 

 

 

Can you imagine the fine print on the bottom of a soloist concert program saying:  "So and So sold their Strand, paid off his/her student loans, house mortgage, car loans, and all the credit cards  and is now using instruments made by __ __ .   They're especially loud you can easily hear them and only cost just a few thousand dollars so he throws them away (often into the audiences) and buys a new one every year and doesn't carry any insurance on them and doesn't worry all the time about theft or forgetting them in taxis.  The reduced stress and anxiety has greatly improved the quality of his life and it carries through with a care-free spirt for performing which we hope you enjoy tonight."

 

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Going back to the OP, and not to be flip, but my feeling is you should stick to wood you positively like, not try to figure out how to adjust and accomdate wood that you describe in a negative way, like spongy.

That's my actual reposnse to the OP's scenario.

So to be flip, use the spongy stuff to build tool case that hold the knives and gouges you use to carve violins from wood you actually like.

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2 hours ago, Marty Kasprzyk said:

Perhaps we should be picking our top plate wood on the basis of cross grain bending strength/cp to avoid cracks which seem to be so common in old instruments.  Maybe we should also think about how long we wish our instruments to last.

One problem is that wood continues to shrink significantly crossgrain as it ages, and almost nil shrinkage along the grain.  With the purfling, ribs, and lining aligned as they are, they would tend to put the crossgrain top in tension over time... unless the glue fails.  Even the process of gluing tends to build in crossgrain tension.

Naturally I can't pass up an opportunity to note that torrefied wood is unlikely to shrink any more, and also is likely much more resistant to creep.  I have a violin with .30 - .31 density spruce that has survived without observable distortion for 8 years so far, and it's not particularly thick or highly arched.

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3 hours ago, Marty Kasprzyk said:

An advantage of using a high radiation ratio RR (speed of sound /density, c/p) wood over using a wood with a lower RR is that it allows the plate to be made thinner thus lighter for the same mode frequencies.  The lower moving plate weight makes the instrument louder.

Marty, I'm sure you can answer this:  What is the theoretical gain in dB's if you make the plate 10% lighter?  Are you likely to hear it?  (That would be like comparing a 65g top with one that weighs 58.5g, which seems like a lot to me).

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10 hours ago, Don Noon said:

One problem is that wood continues to shrink significantly crossgrain as it ages, and almost nil shrinkage along the grain.  With the purfling, ribs, and lining aligned as they are, they would tend to put the crossgrain top in tension over time... unless the glue fails.  Even the process of gluing tends to build in crossgrain tension.

Naturally I can't pass up an opportunity to note that torrefied wood is unlikely to shrink any more, and also is likely much more resistant to creep.  I have a violin with .30 - .31 density spruce that has survived without observable distortion for 8 years so far, and it's not particularly thick or highly arched.

Maybe that is one of the advantages of arching, that an old instrument, even one with low arching can survive without splitting, it just sinks a tiny bit?

Also, maybe that's another advantage of using hide glue, it can accommodate movement by hygroscopic action?

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On 11/14/2019 at 10:47 PM, Nick Allen said:

Or am I stuck with a spongy wood and a spongy sound?

I remember Davide mentioning to me or someone here that when scraping your wood keep going until the tap tone just starts to diminish in volumn/loudness and stop there.

Keep in mind he more than likely uses master grade tonewood.

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20 hours ago, Don Noon said:

Marty, I'm sure you can answer this:  What is the theoretical gain in dB's if you make the plate 10% lighter?  Are you likely to hear it?  (That would be like comparing a 65g top with one that weighs 58.5g, which seems like a lot to me).

The answer seems to be dependent upon how you make the plates lighter. Here's three examples:

If you substitute the wood with one having a 10% lower density and keep the thickness the same there would be about a 0.5dB gain.  I doubt it would be noticeable.

If you just make the plate 10% thinner to reduce its weight by that much there would be about 1.5dB gain.  I do think this would be noticed but the reduced thickness would greatly reduce the plate's stiffness (because of the thickness cubed) and the various mode frequencies would drop.  The instrument would sound a little louder but also less bright.

If you wanted to keep the same sound character you could increase the sound output by using a wood with a higher radiation ratio (RR= speed of sound/density).  Let's compare  two different RR woods.  The first one is a rather poor wood with  speed of sound of 5000m/sec, density of 0.417g/cc giving a poor RR value of only 12.  The second wood has an impressive speed of sound of 6000m/sec, a lower density of 0.375g/cc giving a very good RR value of 16.

 Notice that the second wood has a 10% lower density than the first like in the first example.  However it has a high speed of sound which allows it to be also made thinner. So the plate made with great wood would be about 25% lighter and would produce about 2dB increase which is very helpful. But this is comparing really poor wood with really good wood.  

One unit of RR increase gives only approximately 0.5 db more output. This doesn't seem that big an inprovement but maybe the difference between good violins and great (loud and bright) violins isn't very big either.

 

 

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Marty, how do you feel like the player/bow play into this?  In my experience, the theoretical 2db scenario will make a violin that's quite loud under the ear and responds very well to light bowing, but bottoms out easily.  Medium or lower RR wood ends up making an instrument that takes a heavier bow arm and ends up sounding louder.  Does that line up with the math at all?

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54 minutes ago, Advocatus Diaboli said:

Marty, how do you feel like the player/bow play into this?  In my experience, the theoretical 2db scenario will make a violin that's quite loud under the ear and responds very well to light bowing, but bottoms out easily.  Medium or lower RR wood ends up making an instrument that takes a heavier bow arm and ends up sounding louder.  Does that line up with the math at all?

This would fit somehow my thought regarding "heavier" violins like the Cannone or the Vieuxtemps dG. I don’t know their RR and I don’t claim that stiffer wood sounds quieter at a higher bow force input, but I think it has rather to do with a higher overall stiffness of the top (which is influenced by arching, wood prop and grads).

 

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5 hours ago, Marty Kasprzyk said:

The answer seems to be dependent upon how you make the plates lighter. Here's three examples:

If you substitute the wood with one having a 10% lower density and keep the thickness the same there would be about a 0.5dB gain.  I doubt it would be noticeable. (etc, etc....)

How are you working this out, Marty?

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17 hours ago, JohnCockburn said:

How are you working this out, Marty?

I was afraid somebody  would ask.  Normally I do this all quickly in my head (just kidding).

In the Schelleng reference I mentioned earlier he states that impedance i is equal to the square root of the product of stiffness S and mass m:

i= (Sm)^0.5

Stiffness S of a plate  is proportional to the thickness t cubed times the elastic modulus E :  

S = Et^3

mass is proportional to the thickness t times the density p:

m = tp

so substituting in the above impedance equation we get the impedance equal to the plate thickness t squared times the square root of the product of elastic modulus E times the density p:

i = (Et^3tp)^0.5  = t^2 (Ep)^0.5

in the first example I gave the wood was kept the same so the elastic modulus E and the density were constant.  The thickness t was reduced by 10% to achieve the 10% weight reduction that Don was curious about so the new impedance i is proportional to the new thickness (0.9t) squared:

i ~ (0.9t)^2  ~ 0.81t^2

admittance a (how easy it is to move) is the reciprocal of impedance i (how hard it is to move) =

a = 1/i

the ratio of the admittance a of the thinned plate to the original plate is therefore:

a new/a old = t^2/0.81t^2  = 1/0.81  = 1.23

So the thinned plate with 10% less weight has 1.23 times higher admittance than with the original thickness and their admittance ratio is 1.23.

I've assumed (could be a mistake here) that a vibration peak amplitude of a vibrating system is proportional to the admittance of the system therefore thinning also increases the amplitude ratio  by the same 1.23 ratio.

But Don asked about decibel change and whether or not we would notice it.  So it is necessary to convert the above 1.23 ratio into a decibel change.  Rather than doing the log math I just picked off the dB change from a graph in Arthur H. Benade's book "Fundamentals of Musical Acoustics", second revised edition, 1990, Dover Publications.  This graph on page 227 is attached and I hope the copyright police won't notice.

 

 

In the second example I gave I considered a 10% reduction in weight achieved by using the same thickness plate but with a 10% lower density wood.  We know the wood's elastic modulus E is generally directly dependent upon density p times some constant b:

E = bp

if we ignore all the constants and substitute this in the above equation for stiffness S we find that the stiffness of the plate is proportional to its density p times its thickness t cubed

S = pt^3

substituting this stiffness S into the earlier impedance i equation gives:

i = t^3 (Ep)^0.5  = t^3(pp)^0.5  = t^3p

again the admittance  a equals the reciprocal of impedance i,

a= 1/i

Since the thickness t didn't change the ratio of admittances a from using lower density p wood is just the ratio of their densities:

p/0.9p = 1/0.9 + 1.11

and again reading off of Benade's graph for a 1.1 ratio of amplitudes gives an increase of about 0.5dB.  So after all this work all I get is a lousy T shirt saying 0.5dB.

 

 

The third example used woods with different radiation ratios RR of 12 and 16 and the plate thicknesses were adjusted to give the same mode frequencies.  The radiation ratio RR equals the wood's speed of sound c divided by its density:

RR = c/p

The wood's speed of sound c in turn is equal to the square root of its elastic modulus E divided by its density p which can be rearranged.

c= (E/p)^0.5         C^2 = E/p             E = pc^2

so the elastic modulus E is politically correct squared.

 

to be continued...

 

Screen_Shot_2020-01-17_at_8_12.50_PM.png

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Using admittance to determine violin loudness is a lot like saying that a 4 ohm speaker will emit twice the power of a 8 ohm speaker.  That is true... if the amplifier is capable of pumping out twice the current without losing voltage.

In terms of the violinist, my interpretation is that the lower impedance instrument does not magically make more dB's for free, but the player has to put that energy in by (likely) increasing the bow speed. With the instrument sucking power out of the string at a higher rate, there is likely some  difficulty getting the Helmholtz motion of the string started as well.

This all might not be too handy if you need to play a long note, or play quietly, or if you want quick string response... or if the player tends to be a high-pressure bowing type.  

I think it is more useful to think in terms of energy efficiency, as the player is very sensitive to how much effort they need to put in vs. what comes out.  There, I think the "benefits" of high RR and light weight tend to diminish, and the practical problems of playing become more important.

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On ‎1‎/‎17‎/‎2020 at 3:12 AM, Marty Kasprzyk said:

The answer seems to be dependent upon how you make the plates lighter. Here's three examples:

If you substitute the wood with one having a 10% lower density and keep the thickness the same there would be about a 0.5dB gain.  I doubt it would be noticeable.

If you just make the plate 10% thinner to reduce its weight by that much there would be about 1.5dB gain.  I do think this would be noticed but the reduced thickness would greatly reduce the plate's stiffness (because of the thickness cubed) and the various mode frequencies would drop.  The instrument would sound a little louder but also less bright.

If you wanted to keep the same sound character you could increase the sound output by using a wood with a higher radiation ratio (RR= speed of sound/density).  Let's compare  two different RR woods.  The first one is a rather poor wood with  speed of sound of 5000m/sec, density of 0.417g/cc giving a poor RR value of only 12.  The second wood has an impressive speed of sound of 6000m/sec, a lower density of 0.375g/cc giving a very good RR value of 16.

 Notice that the second wood has a 10% lower density than the first like in the first example.  However it has a high speed of sound which allows it to be also made thinner. So the plate made with great wood would be about 25% lighter and would produce about 2dB increase which is very helpful. But this is comparing really poor wood with really good wood.  

One unit of RR increase gives only approximately 0.5 db more output. This doesn't seem that big an inprovement but maybe the difference between good violins and great (loud and bright) violins isn't very big either.

 

 

What causes the changes in speed of sound in the wood? Is it figure or rays? Does more figure or rays slow the speed of sound?

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On ‎1‎/‎16‎/‎2020 at 10:17 PM, uncle duke said:

I remember Davide mentioning to me or someone here that when scraping your wood keep going until the tap tone just starts to diminish in volumn/loudness and stop there.

Keep in mind he more than likely uses master grade tonewood.

Umm yeah, I love Davide's videos, but what's will all that tapping? It looks too mystical to me.

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Just now, sospiri said:

Umm yeah, I love Davide's videos, but what's will all that tapping? It looks too mystical to me.

To be the best one has to walk like the best.

What was that comment from another post?  Maybe this is one way Davide reaches his own personal level of incompetence.

I glad to meet his acquaintance here at the forum.  Invaluable. 

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13 hours ago, Marty Kasprzyk said:

>

>

The third example used woods with different radiation ratios RR of 12 and 16 and had speeds of sound of 5km/sec and 6km/sec the plate thicknesses were adjusted to give the same mode frequencies.  The radiation ratio RR equals the wood's speed of sound c divided by its density:

RR = c/p

rearranging this shows the density p w=equals the speed of sound divided by RR:

p = c/RR,   the two examples are p= 5/12= 0.417g/cc for the first wood,        and p = 6/16g/cc = 0.375g/cc for the second wood. 

The wood's speed of sound c in turn is equal to the square root of its elastic modulus E divided by its density p which can be rearranged.

c= (E/p)^0.5         C^2 = E/p             E = pc^2

The  elastic modulus E for the first wood is equal to its density times its speed of sound c squared (I'll drop all the zeros and units):

E = 0.417(5)^2 = 10.4

The elastic modulus E for the second wood is

E = 0.375(6)^2 = 13.5

These examples show that the second wood has both a lower density and a higher elastic modulus which causes it to have a higher speed of sound.  It was desired that both plates have the same mode frequencies f.  The better 16 RR wood with its lower density can also be made thinner which will further reduce the plate's weight compared to the 12 RR wood.  We saw earlier that the frequency f is proportional to the square root of the plate stiffness S/plate mass m ratio

f ~(S/m)^0.5

We saw before that the plate's stiffness S is proportional to its elastic modulus E times its thickness t cubed:  S~Et^3

The plate's mass m is proportional to its density p times its thickness t:  m ~pt

f ~ (S/m)^0.5            f~ (Et^3/pt)^0.5             f~ t(E/p)^0.5    

Recall that the wood's speed of sound c is equal to the square root of its elastic modulus E divided by its density p:    c = (E/p)^0.5

Thus the various mode resonance frequencies f of a plate is simply equal to its thickness t times its speed of sound c times some constant b:

f= btc  

A plate made from our 12RR wood with a speed if sound c of 5km, thickness of 3mm and the constant b is 30:

f = (30)(3)(5) = 450Hz resonance frequency for some particular mode.

A second plate made from our better 16RR with a speed of sound c of 6km can be made with a thinner thickness t to achieve the same mode frequency of 450Hz:

t = f/bc  =  450/[(30)(6)] = 2.5mm

So our better 16RR wood can be made only 2.5mm thick instead of 3.0mm thick for our poorer 12RR wood and still achieve the same mode resonance frequency.  (As a side note it should be mentioned that this is one downfall of the "plate tuning" concept where any wood can achieve the same mode frequencies therefore the violins can have the same sound character.  This is true however a better RR wood can achieve a greater sound output as we will see next.)

 

The stiffness S of the plate is proportional to the elastic modulus E of the wood times thickness t cubed so the stiffness S of our 12RR wood is:

S ~(10.4)(3.0)^3 ~ 280.8

The mass of our 12RR wood plate  is proportional to its density p times thickness t:

m ~(0.417)(3.0) ~  1.251

The impedance i of the plate is equal to the square root of its stiffness S times its mass m,  i = (Sm)^0.5

so the impedance i of our 12RR wood plate is:

i ~ [(280.8)(1.251)]^0.5   ~  18.7 

 

Likewise our other plate made with the better 16RR wood has a stiffness S :

S ~ (13.5)(2.5)^3  ~ 210.8

and the mass m of the plate made with better 16RR wood is proportional to its density p times its thickness t:

m ~ (0.375)(2.5) ~ 0.938

The impedance i of our 16RR wood is:

i ~ [(210.8)(2.5)]^0.5  ~ 14.1

 

Admittance a is equal to the reciprocal of impedance i so the ratio of the admittances of the plate made with the 16RR wood to the one made with 12RR wood is 

18.7/14.1 = 1.33

Going back again to Benade's graph we see that this 1.33 admittance ratio should give us about 2dB increase in sound output.  So going from lousy 12RR wood to great 16RR wood gives you just a small improvement in sound output.

 

So finally my advice to makers is to forget about all this shit and just make your plates to weigh about 65g with reasonable wood like Don said.

 

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