Jack Devereux

Basic Acoustics Resource

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4 hours ago, curious1 said:

I would say in this example the higher frequency is not the result of excess stiffness but less mass.

Performance wise, IMO, this is a fault of ultra light materials. 

If we have two bars of identical dimension the one with the higher frequency is not necessarily stiffer. We can say it is stiffer only if the two bars are also the same weight.

If they are the same frequency the heavier one is stiffer.

density and stiffness should not be lumped together interchangeably.

In your last sentence, do you mean to say "density frequency and stiffness should not be lumped together interchangeably"?  If you wrote what you meant, then I'm confused.

Thanks,

Jim

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6 hours ago, Jim Bress said:

Given the five variables, density, arch height, thickness, plate weight, and stiffness, how would you (anyone) rank their effect size on the outcome on achieving your acoustical target? >>>>

A better question might be in what order during a build do factors come into consideration?  I would rank density as first to decide arch height. >>>

.  Am I in the ball park here?  

The acoustic function of increasing arching height is to suppress low frequencies and enhance high frequencies (see attachment). 

 

IASS2016_mendez_etal_vibro_final_1475755095.pdf

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curious1   
15 hours ago, TimDasler said:

I was taught to consider density when choosing arching height. A low density piece needs the support of a higher arch, and ultimately it will be easier to get to your target weight and flex with normal graduations. Conversely, a more dense piece should get a lower arching, which will also make it easier to get to your target weight and flex without going thinner with the graduations.

 

14 hours ago, Davide Sora said:
 

Although it may seem logical,  I'm not entirely convinced of this theory.

If we take the case of two archings of equal height and equal thickness, the one with denser wood will be heavier.

So to reduce weight and increase efficiency it may be necessary to raise the arching with thicker wood to be able to decrease the thickness and so the weight.

Instead, raising the arching with the low density wood would probably result in an excessively high tap tones frequency i.e. an excessive stiffness.

In my empirical working experience this works, but maybe somebody with clearer ideas about physics might disprove.....

 

 

7 hours ago, curious1 said:

I would say in this example the higher frequency is not the result of excess stiffness but less mass.

Performance wise, IMO, this is a fault of ultra light materials. 

If we have two bars of identical dimension the one with the higher frequency is not necessarily stiffer. We can say it is stiffer only if the two bars are also the same weight.

If they are the same frequency the heavier one is stiffer.

density and stiffness should not be lumped together interchangeably.

 

3 hours ago, Jim Bress said:

In your last sentence, do you mean to say "density frequency and stiffness should not be lumped together interchangeably"?  If you wrote what you meant, then I'm confused.

Thanks,

Jim

"density and stiffness should not be lumped together interchangeably"

is just what I wanted to say.

 

if you look at the bold quotes above, I think they are equating low density with low speed of sound or modulus of elasticity. 

There are good structural reasons for making the arch higher for wood of low stiffness but the reasons are less obvious to me for wood of low density alone.

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lpr5184   

density and stiffness should not be lumped together interchangeably"

 

So in other words low density doesn't always mean low stiffness...makes sense.

I have some light and medium weight Sitka coming and even though it may be low density it is probably still quite stiff. At least I hope so.

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Don Noon   
2 hours ago, lpr5184 said:

So in other words low density doesn't always mean low stiffness...

There is, however, a very strong correlation between density and absolute stiffness (modulus of elasticity).  

When you become concerned about vibrations (which as instrument makers, we should be), then various flavors of stiffness/weight come into play, and things get more complicated.

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carl1961   
1 hour ago, Don Noon said:

There is, however, a very strong correlation between density and absolute stiffness (modulus of elasticity).  

When you become concerned about vibrations (which as instrument makers, we should be), then various flavors of stiffness/weight come into play, and things get more complicated.

Just curious if anyone has any one thought of Paulownia wood? it is lighter than spruce and more flexible.

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You all are making me feel stodgy and primitive.   For violins I stick with spruce and maple, and don't have any thoughts of wandering off to other materials.

On multiple threads across the board right now, people seem to be just itching to replace materials and methods that have work wonderfully for very long stretches of time with whatever newness pops into their thoughts momentarily.    That's the part that makes me feel stodgy.    My initial and persisting reaction to all these proposed new fangled materials -- from Gorilla Glue, to Acetone cleaner, to balsa wood tops, to adjustable metal posts etc ---  is just 'ugh!!!',  and 'why???'.   

But the part that makes me feel primitive is all the tech talk.   I just don't have the impulse to measure the wood density, or speed of sound, or try to measure the elasticity.  I just don't feel the urge.  Even if I did, I have no way of seeing that information as meaningfully useful.   I don't know how such numbers help me build.

So then,  in comparison to these fancy approaches, me own methods seem naive and primitive.    I first of all pick traditional types of wood, from traditional sources, with a traditional 'look' to it.  Not a hint of innovation.   Then, within that, I like to find wood that I think 'speaks'.   That's the test I actually use.   And, I don't like wood that feels either heavy or rigid to me as I handle it.   Again, naive.  But I trust my instincts.     By 'speaks', all I mean is that if a rap a knuckle or scratch a fingernail on the wood, the wood readily converts the stimulus to sound.   And I much prefer a sense of 'liveliness' and 'clarity' in this.   That's it.  No fancy tools or measurements.  Eye and ear.   Primitive.    (and stodgy)

 

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curious1   
9 hours ago, lpr5184 said:

density and stiffness should not be lumped together interchangeably"

 

So in other words low density doesn't always mean low stiffness...makes sense.

I have some light and medium weight Sitka coming and even though it may be low density it is probably still quite stiff. At least I hope so.

 

6 hours ago, Don Noon said:

There is, however, a very strong correlation between density and absolute stiffness (modulus of elasticity).  

When you become concerned about vibrations (which as instrument makers, we should be), then various flavors of stiffness/weight come into play, and things get more complicated.

Yes and yes.

 

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curious1   

We shouldn't use 'density' as a catch all for  density and stiffness.

Stiffness and density are two different characteristics. 

Sorry for being a preachy pain in the ass.

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curious1   
18 hours ago, Jim Bress said:

Given the five variables, density, arch height, thickness, plate weight, and stiffness, how would you (anyone) rank their effect size on the outcome on achieving your acoustical target?  I realize there's some inter-dependencies among these variables.  I would think arching and stiffness would be in the one and two slots, and density may dictate the bounds in which these can be achieved within "normal" parameters.  The last statement forces density into some form of acceptable range.  Plate weight, density and thickness are not independent, so that muddies the water.  A better question might be in what order during a build do factors come into consideration?  I would rank density as first to decide arch height.  Then thickness to achieve a target (range?) of stiffness, with plate weight and thickness as being parameters to stay within while trying to achieve some stiffness.  Am I in the ball park here?  

That's well thought out.

Stiffness/arching, density/graduations/weight

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6 hours ago, carl1961 said:

Just curious if anyone has any one thought of Paulownia wood? it is lighter than spruce and more flexible.

I use it for my tops, backs, fingerboards, necks and blocks of my violins and violas but most people say they're really not violins and violas.

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carl1961   
16 minutes ago, Marty Kasprzyk said:

I use it for my tops, backs, fingerboards, necks and blocks of my violins and violas but most people say they're really not violins and violas.

Well Thanks. LOL if you using it for all the wood then that must be some Awesome wood. I remember seeing someone bending the board a lot and it did not break, looked like rubber wood

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52 minutes ago, curious1 said:

That's well thought out.

Stiffness/arching, density/graduations/weight

Ok got it.  Thanks!  In my order of operations I've been using density as a surrogate for stiffness to decide whether to make my arching a little higher or lower.  After that I don't think about density.  How do you measure stiffness before making chips fly to make a decision on arching? 

Thanks,

Jim

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49 minutes ago, carl1961 said:

Well Thanks. LOL if you using it for all the wood then that must be some Awesome wood. I remember seeing someone bending the board a lot and it did not break, looked like rubber wood

Its not clear to me that the "best" wood for violins is also the best wood for violas.  Violas make some lower pitch notes (C string) than a violin but they are only marginally larger.  

So you could argue that you should make the plates thinner, use lower arches, and/or lower speed of sound wood than what violins use.  On the other hand nobody agrees what  violas should sound like so it probably doesn't make much difference how they're made.

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5 hours ago, David Beard said:

But the part that makes me feel primitive is all the tech talk.   I just don't have the impulse to measure the wood density, or speed of sound, or try to measure the elasticity.  I just don't feel the urge.  Even if I did, I have no way of seeing that information as meaningfully useful.   I don't know how such numbers help me build.

So then,  in comparison to these fancy approaches, my own methods seem naive and primitive.   

 

 

David,

I've been through this "cursus" and am heading back to basics. I often ask myself, "What did Strad do?" Maybe that's why I have less to say these days.

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curious1   
1 hour ago, Jim Bress said:

Ok got it.  Thanks!  In my order of operations I've been using density as a surrogate for stiffness to decide whether to make my arching a little higher or lower.  After that I don't think about density.  How do you measure stiffness before making chips fly to make a decision on arching? 

Thanks,

Jim

Speed of sound (which has been discussed in other threads) and or modulus of elasticity (speed of sound^2 x density).

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curious1   

I'm not advocating that anyone should be a slave to numbers but they can provide a somewhat objective perspective. 

For instance, if you had a back with a high modulus and you left the arching high you may encounter difficulties when graduating it. There is a limit to how thin you can take a plate even if the wood is dense. At a certain point you risk the structural integrity. Also, at a certain point making it thinner does not reduce the stiffness because of the inherent strength of the shell (arch). Consequently the stiffness of the finished instrument will be outside the normal range of a violin.

What effect this has acoustically we can discuss to the ends of the earth.

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4 hours ago, Marty Kasprzyk said:

I use it for my tops, backs, fingerboards, necks and blocks of my violins and violas but most people say they're really not violins and violas.

I forgot to mention that I use 0.8mm thick 3 ply model aircraft birch plywood for the ribs.  Its more crash resistant than curly maple.

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1 hour ago, curious1 said:

I'm not advocating that anyone should be a slave to numbers but they can provide a somewhat objective perspective. 

For instance, if you had a back with a high modulus and you left the arching high you may encounter difficulties when graduating it. There is a limit to how thin you can take a plate even if the wood is dense. At a certain point you risk the structural integrity. Also, at a certain point making it thinner does not reduce the stiffness because of the inherent strength of the shell (arch). Consequently the stiffness of the finished instrument will be outside the normal range of a violin.

What effect this has acoustically we can discuss to the ends of the earth.

This is true, but how do you determine which is the point of no return where you risk compromising structural integrity?

I think we can discuss this to the ends of the earth too, without being able to refer to specifical cases.

 

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curious1   
36 minutes ago, Davide Sora said:

This is true, but how do you determine which is the point of no return where you risk compromising structural integrity?

I think we can discuss this to the ends of the earth too, without being able to refer to specifical cases.

 

If I had a piece of spruce with a modulus in the 9,000 -10,000 range and I was graduating it less than 2.4mm I'd start to worry if it could hold its shape.

I think an experienced maker would know when they are headed into the red. The trick is giving it a name.

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16 hours ago, curious1 said:

If I had a piece of spruce with a modulus in the 9,000 -10,000 range and I was graduating it less than 2.4mm I'd start to worry if it could hold its shape.

I think an experienced maker would know when they are headed into the red. The trick is giving it a name.

May I ask why?

I mean, regardless of type arching with wich you are dealing with?

I would be pleased to find out the trick to give a name to the maker.....:);)

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curious1   

The ability of the wood to hold the arch's shape should be a function of both the wood's density and its stiffness. If I had a piece of wood of both low density and low stiffness I would need to leave the arch higher to account for the low stiffness and thick to account for the low density.

This conversation may take awhile so it is probably worth describing how I measure stiffness (speed of sound^2 x density). It is a back of the hand method but accurate enough for violin making.

 

 

 

 

 

 

IMG_1949.JPG

IMG_1950.JPG

IMG_1947.JPG

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2 hours ago, curious1 said:

The ability of the wood to hold the arch's shape should be a function of both the wood's density and its stiffness. If I had a piece of wood of both low density and low stiffness I would need to leave the arch higher to account for the low stiffness and thick to account for the low density.

This conversation may take awhile so it is probably worth describing how I measure stiffness (speed of sound^2 x density). It is a back of the hand method but accurate enough for violin making.

Thanks for the papers, seems an interesting way to predict stiffness, it may well be worth trying to insert it into "numbers" to be verified.

How do you measure the speed of sound?

I stick with Don Noon system, seems similar to that used in the papers you submitted, do you think it is sufficiently accurate? (sorry Don for asking, just to hear someone else's opinion.....:))

 

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curious1   
2 hours ago, Davide Sora said:

Thanks for the papers, seems an interesting way to predict stiffness, it may well be worth trying to insert it into "numbers" to be verified.

How do you measure the speed of sound?

I stick with Don Noon system, seems similar to that used in the papers you submitted, do you think it is sufficiently accurate? (sorry Don for asking, just to hear someone else's opinion.....:))

 

Yes, I use Don's method of measuring the speed of sound in the wood which is the same as the method in the study above.

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