Official Luthiers Forum!

[QUOTE=Rick Turner]Wouldn't it be nice if we could all agree upon a single standard for measuring deflection?   For instance, start with a given thickness and then have a standard fixture and weight and measure for both longitudinal and lateral deflection...   How hard would that be?   It would give us a common language here.[/QUOTE]

Not only would this be a very cool thing Rick and everyone it would greatly benefit us all.

Builders like Ervin Somogyi and others are known to source their tops from places that they can visit and go through hundreds of tops to select the few that are ultra stiff.  I doubt that this "testing" consists of more then feeling and flexing the tops but these guys have the touch and can do this.

For the rest of us we are purchasing a picture on the Internet and sometimes less then this.  We have no idea what the stiffness properties of the wood might be.  So...... we build with what we have and I can't honestly say or know if I have ever had any of the finer material available to me.

In addition - a standardized measurement methodology and the availability of the resulting numbers would also, once and for all, greatly contribute to the advent of a standardized "grading" system where cosmetics alone no longer dictate what is master grade or lesser grades.

A standardized measurement system AND some numbers available to me from a zoot vendor would be a huge improvement.  Knowing in advance that this would increase costs to the vendors a slight amount I would be happy to pay slightly more for zoot "certified" against a standard that is published and understood.

Great idea!

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Ann Arbor Guitars

Shane
Yes 330 is the ballpark endpoint for my L-OO. Remember that is based on
my set up with my weights and bracing the way I brace. Yes the density
fluctuated a bit. I programmed my spreadsheet to automatically calculate
density. It gives me an idea how different species compare.
As far as why longitudinally only...well ES taught me that. You could
certainly do both and keep the data.

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Andy Z.
http://www.lazydogguitars.com

This is somewhat related...here's an interesting web page that I found a bit ago.

Keep in mind that the ratio of stiffness (aka E, Young's Modulus, Modulus of Elasticity - all are the same) to density is primary material factor in determining the frequency response of any piece.

Based on this chart...I'd say that for a certain specific stiffness, there is probably an ideal density and "guitar type" that would result in an optimized instrument. Until I convince work to upgrade my FEA package I am at a standstill..

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http://www.PeakeGuitars.com

Oh yeah...here's the chart:

http://www.amjbot.org/cgi/content/full/93/10/1439/F1

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http://www.PeakeGuitars.com

Parser.

Thanks for the link - an interesting academic paper that I'll read at my leisure. I did like the conclusion though:

"Despite much scientific effort to illuminate the properties of universally accepted perfect instruments and their reproduction, we still rely mainly on the art, knowledge, and experience passed on from one generation of skilled instrument makers to the next. They have the experience to judge the quality of the material for an instrument using eye, ear, and touch - and this often when it is still hidden in the trunk of a tree or in wooden planks."

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

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http://www.chassonguitars.com

I use a vibration test to determine MOE. It's not any harder or easier or more or less accurate than a deflection test at the level we're working at. I just use it because I already have the equipment (a good signal generator, a caliper and a gram scale) and it yeilds information about the damping factor as well as the stiffness.

My feeling is that deflection numbers, while useful, may be less so that we might think. Each of us has a different way of building, and a different idea of what we're after. For example, I tend to make 'fingerstyle' steel strings with 'tapered' bracing. If you looked at the deflection in the center of the top on a completed guitar, mine would tend to be much less than somebody making Bluegrass Dreads with scalloped bracing. I usually also l;eave my tops thicker that many people, because it works for me. If I gave you deflection numbers for the 'bare' top before I braced it, you'd think it was too stiff, and for you, it might be.

The way I have been using the numbers is by comparing with an instrument that I have data on, and that I know worked. One such was a 'standard' classical, with a WRC top that had a lengthwise E value of 7670 megaPascals (that's in kilogram-meter-second units). I assume that it's the lengthwise bending stiffness that sets the limit for thickness under bridge torque. Certainly the crosswise stiffness ought to contribute, too, but it's hard to factor in, and this keeps me on the safe side. Anyway, that top worked out well at a thickness of 2.5mm just in front of the bridge

Since I know that the stiffness is proportaional to the Young's modulus and the cube of the thickness, I multiply 2.5^3 times 7700 to get a result that is close to 120,000. Since the units are mixed this is just an 'index number': it doesn't mean much by itself, but can be used to compare another top.

Suppose I get out a Euro top that has a lengthwise E value of 10,000, and I want to use it on the same model of guitar. All I have to do is divide that 120,000 index number by the 10,000 E value, to get 12, and find the cube root of that, a little less than 2.3, and that's the thickness I need for that top, in mm.

I'll note that the WRC top probably had a density of around 340 km/m^3, whereas the Euro top is more likely closer to 380. 340*2.5 is 850, while 380*2.3 is 874, so the Euro top will most likely be a bit heavier (about 3%), even though it's thinner. In practice that's probably within the bounds of normal workmanship. OTOH, this is a pretty general result: lower density will usually end up giving you a lighter top, so long as the lengthwise E value is not much lower than normal for that density.

Note that your final numbers are only as accurate as your _least_ accurate measurement. For me, that will be thickness on most top blanks, and I suspect that will be the case for most of us. If you can only get three significant figures on the thickness, then that's all you can use on the final calculation, and two tops that have calculated MOEs of 12365mPa and 12443mPa are, for all intents and purposes 'the same'.

I'm in agreement with Al (usually a good thing I think) that the test data is of marginal usefulness. I use it for two things, originally I used it as a better grading system than the standard A, AA, AAA, AAAA, as it has some objectivity behind it. And secondly,as of very recent, I started using the data in a FEA engineering package hoping to be able to capitalize on it. I have great hopes here and am making progress in my limited time I have available for it, but wish I was further along quite honestly.

I also think that there's apples and oranges being talked about in this thread. IMO Andy is doing a completely different thing than what I consider deflection testing. His approach seems to be a tuning method which utilizes deflection and has a lot of merit. I think it should be seriously considered by everyone interested in keeping records of their instruments. Eventually a correlation could be made between good and bad, I think.

Rick and Hesh- yes the formulas are embedded in the spread sheet. they are the same formula that Parser shows in his post above.

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Jim Watts
http://jameswattsguitars.com

Thanks again Jim. Your observation about what Andy is doing is bang on! So you can use this process, in a standardized fashion, to speak to a grading system, such as the density issue many of my customers enquire about and secondly you can use this system to thickness a top to a specific "deflection" as means for what thickness you want prior to bracing, based on your construction process. This starts to build some consitancy into your instruments. With the appropriate data set of course.

Thanks again!

Shane

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Canada

Al, I'm guessing that most of the tops you measure are pretty much the same width and length? It should probably be noted that in order to compare one top to another using your method, the length and width should be consistent.

Also - do you try to build for a consistent stiffness, or for a consistent natural frequency? Vibration modes are usually predicted through some form of f = (k/m)^.5 (where m is mass and K is stiffness)...I guess at some point though, you get to the point where you are splitting hairs and it's just better to sand it a little more if it doesn't sound open enough? I'd also guess that you are using some form of this equation in evaluating the stiffness of the top?

Also, who the heck convinced you to use MPa instead of psi??

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http://www.PeakeGuitars.com

My feeling is that deflection numbers, while useful, may be less so that we might think. Each of us has a different way of building, and a different idea of what we're after.

I think if anyone believe this type of test data is going to give them a magic formula they're mistaken. The usefullness is in gaining consistency. If I want to build two guitars that are similar this would be very useful. If I want to build two guitars that are similar except for one design feature (say changing the bracing) I would want to start with tops thicknessed to a similar "behaviour" so I can learn more about what that one change has done.

I also think that deflection testing at more than one point is valuable. Test first before the top is thicknessed, which should give you an idea how thin to take the top. Then test again after braced and joined to the sides, which may give you some information about brace shaving. Testing again as a final product may be useful but it's a little late at that point to make changes.

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David White, Toronto

"All my favourite singers can't sing."

[QUOTE=Rick Turner] Wouldn't it be nice if we could all agree upon a single standard for measuring deflection?   For instance, start with a given thickness and then have a standard fixture and weight and measure for both longitudinal and lateral deflection...   How hard would that be?   It would give us a common language here.[/QUOTE]

Well, in fact, this is all relatively meaningless as transferable knowledge until we do just that. So, if we were an engineering organization like ASTM, we would appoint a committee, they would design a methodology, and we would all just do it that way. It wouldn't matter any more if it were the perfect way to do it, it would be THE way, and I could tell Shane that I loved top A because it tested to X, or I hated it cuz it whimped out and measured at Y, and those numbers would mean the same thing to everyone on this forum or anywhere in the world.

I nominate Al Carruth, Brian Burns, Rick Turner.   Other volunteers?

JK

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Jim Kirby
kirby@udel.edu

[QUOTE=jtkirby] [QUOTE=Rick Turner] Wouldn't it be nice if we could all agree upon a single standard for measuring deflection?   For instance, start with a given thickness and then have a standard fixture and weight and measure for both longitudinal and lateral deflection...   How hard would that be?   It would give us a common language here.[/QUOTE]

Well, in fact, this is all relatively meaningless as transferable knowledge until we do just that. So, if we were an engineering organization like ASTM, we would appoint a committee, they would design a methodology, and we would all just do it that way. It wouldn't matter any more if it were the perfect way to do it, it would be THE way, and I could tell Shane that I loved top A because it tested to X, or I hated it cuz it whimped out and measured at Y, and those numbers would mean the same thing to everyone on this forum or anywhere in the world.

I nominate Al Carruth, Brian Burns, Rick Turner.   Other volunteers?

JK


[/QUOTE]

I think it would be more individual than that...Top X may work best with your bracing & construction methods while Top Y may be better for someone else's. I don't think there will ever be one solution for everyone..! Might be worthwhile for personal records tho..

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http://www.PeakeGuitars.com

[QUOTE=Parser]

I think it would be more individual than that...Top X may work best with your bracing & construction methods while Top Y may be better for someone else's. I don't think there will ever be one solution for everyone..! Might be worthwhile for personal records tho..[/QUOTE]

But, if you know that your bracing pattern wants a longitudinal modulus of X, an across-grain modulus of Y, and whatever else, you can choose tops that fall in that measureable bracket.

Actually, the logical extension of what you said would lead to the loss of ability to test materail properties in any engineering situation. Obviously, a variety of engineering designs lead to a desire and need for a range of materials that fall within different windows of property values. We want to be able to test materials for those property values in a way that can be transferred to any designer, anywhere, in a completely unambiguous fashion. The need for individuality is tied up in the design, NOT in knowledge of mechanical properties of materials being used in the design.

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Jim Kirby
kirby@udel.edu

Parsar,

I agree with you and that is indeed what I have found. "X" is indeed what one person/company wants and "Y" is the ideal for another. So I think there is still value in standardization but the reality is that it would very difficult to do. What I have been suggesting is that we pick a methodology and a test apparatus and I for one will use it. If there is not an agreed unpon methodology I will talk this over offline with some people and build a setup and just post what I am using so that others can use it as well if they want to correlate with what I have when requesting or at least asking about top wood.

Shane

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Canada

[QUOTE=Shane Neifer] Parsar,

I agree with you and that is indeed what I have found. "X" is indeed what one person/company wants and "Y" is the ideal for another. So I think there is still value in standardization but the reality is that it would very difficult to do. What I have been suggesting is that we pick a methodology and a test apparatus and I for one will use it. If there is not an agreed unpon methodology I will talk this over offline with some people and build a setup and just post what I am using so that others can use it as well if they want to correlate with what I have when requesting or at least asking about top wood.

Shane[/QUOTE]

No, the standardization is not difficult to do. You just need a sanctioned body that is recognized as having the authority to develop the standard, you send them off to develop it, and when they come back with a methodology, you use it. There's nothing complicated about it at all, once you hand some group the authority to do it.

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Jim Kirby
kirby@udel.edu

I think that is the issue Jim. I with standards all of the time in the day job. But they become regulated requirements, even in graded lumber they exist. The lumber of a particular species and a particular grade can be used for these specific structural elements. They are governed. There is likely too many different views to build consensus so indeed you would need a committee and default to their suggestion/advice. Kind of like herding cats me thinks. But it is certainly the way to go. I also think that we can do just that in this thread. For our community we can agree on methodology and those that want to use it can and maybe the tonewood dealers here could use it and at least we would all have the same set up. I can tell you that I can assure all of the Lutz that is sold through this site would have the same methodology if taht would help at all. Anyway, them are my thoughts on that!

Shane

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Canada

I'm not trying to be strident here. I'm only suggesting that we keep sidestepping an issue that comes up all the time in the engineering world, and is recognized there as a number one primo priority. Tests have to be designed so that usable information about material behavior can be transferred from one person to another. There is no perfect test, only an attempt at a perfect test. It's more critical that the body developing the test be recognized as authoritative, and that once developed, the test is used as specified.

Developing tests is a big deal. In fact, in academic engineering, you know how you get tenure without writing science articles for journals? Be the guy who wrote the specs. I think we know who some of those guys are - let's let them write the specs and agree to use them.

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Jim Kirby
kirby@udel.edu

Shane - I agree completely. I think there are people in our community who, if they stepped forward and said "here is the methodology. Use it", we would be glad to use it.

I'm an engineer, but I'm not a structural engineer and I'm one of those guys who got tenure by writing science papers, and I love doing that. There are people here who are the right people. If Al told me how to test a piece of wood, I'd test it that way.

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Jim Kirby
kirby@udel.edu

Some of the names that keep coming up when these topics are discussed are Burns, Carruth and Hurd. Siminoff has discussed deflection testing as well in his books. Those are some of the names out their. The largest obstacle would be regulating the standard, much like your profession Jim is regulated. I am suggesting that we don't need to regulate anything right now and one step towards getting a standard devised is if a group of use adopt something and demonstrate that there is a desire to have some ability to discuss the same thing when we are talking about the stiffness of a top. Maybe the methodology we use won't be what those in know would prefer but reconfiguring one of these set ups once made would not be difficult at all. Anyway I am game to accept some advice on a methodology and apply it and then use it to offer deflection numbers to those taht request it.

Thanks Jim for keeping the torch on this one.

Shane

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Canada

boy did you hit a nil on the head for me Hesh. There are no and I repeat no tonewood or even quality hardwood dealers anywhere within a days drive of me. I am luck to have some of the best tonewood dealers at my order disposal but Like you i never get the chance to pick through stacks.

There are standards of a sort: Young's modulus along and across the grain, damping factors and density probably define 99% of what's structurally/acoustically interesting about a piece of wood. These can be measured by a number of different means, with more or less accuracy. The problem from our standpoint is that as you go for more accuracy the tests become harder to do. The question then becomes, how accurate do we need to be? My feeling is that we don't need tremendous accuracy, and can use a variety of fairly simple test protocols depending on the proclivities of the tester.

One of the nice things about vibration testing is that I don't need standard sizes of wood. As long as the pieces are rectangular and uniform in thickness I can test them. This makes it easy to get data most of the time; I have some problems with Red spruce tops that are trimmed to odd shapes to work around flaws, but can usually test small trim pieces if I want to.    

I use MKS units simply because I have a gram scale, a metric ruler, and a caliper graduated in mm. It's also, of course, how the rest of the world does it, so it's easier to compare my results with those of most publications, such as Haines', that deal with the subject. It's simple enough to look up conversion factors if you insist on having your units in drams/league/fortnight.

In terms of relating wood test results with perfomance, we're only just beginning as far as I can see. There are no 'standards' as yet to tell you what sort of E values, density, and damping, go into making a great OM for fingerstyle, say. Those of us who have used this sort of data for a while find it helpful in our own work, but I, for one, am not at the point of wanting to generalize beyond that as yet. The quicker we can get a lot of folks testing and comparing results the faster we'll home in on useful guidelines, if there are any.

Al, I think you've published your methods of vibration testing.   Could you provide a link?

I have not published that myself. I learned the method from Mort Hutchins, who got it from Dan Haines. Both of them published articles in the Catgut 'Journal' about it, but that was a while ago ('81), and some of you may not have copies. Haines' article is also in the 'Journal of Guitar Acoustics' reprints, which you can get from Tim White at tpwhiteco@aol.com.

Basically, you start with a rectangular piece of wood, of uniform thickness. The more like a 'bar' it is, the better: it should be much longer than it is wide, and much wider than it is thick, so that the vibration modes in the different directions are far apart in frequency. Guitar top and back halves usually work out reasonably well. Find the length, width and thickness of the strip/plate, and weigh it, all as exactly as possible.

I use my signal generator to vibrate the pieces. The stationary lines, the nodes, for a flat bar will be about 1/5 of the way in from the ends, and that's where you put some foam pads. I set the pieces up high enough so that I can put the speaker under the middle of the plate on the bench. I sprinkle a little glitter on and find the lowest lengthwise and crosswise bending mode frequencies. There should be two parallel lines of glitter for each mode, running along the length of the plate for the crosswise bending mode, and across the width for the lengthwise mode.
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|   |      &nbs p; |   |
|   |      &nbs p; |   | Lengthwise mode
|___|_________|___|

The other thing you'll need is some way to 'read' the amplitude. One convienient way to do this is to use a magnetic guitar pickup to sense a piece of iron of some sort stuck to the plate in the center of one edge, where it will be moving for both modes. The iron 'metglass' strips that they use in the little security stick-ons at stores work well, and I just stick it on with tape. The ouput from the pickup goes to a millivolt meter.

Read off the frequencies for each mode where the output of the pickup is at its maximum, and the frequency on either side of that where the output is 3dB lower (70.7% of the meter reading). I usually try to get the meter to peak out at 100mV AC on the resonant pitch, by adjusting the output power of the signal generator (wear earmuffs!), and then change the frequency until it drops to 71mV AC. I enter all of this data in a notebook in a standard format.

When you have the data, it's time for a little math. The Young's modulus in either direction is equal to:
(.946*density*peak frequency^2*length^4)/thickness^3
This assumes you've used MKS units, and gives the output in Pascals. Since most wood will have E values in the 6000-20000 megaPascal range, you will probably want to divide by a million somewhwere in there. I have no idea what the constant (that .946) would be if you use inches and pounds.

The Q value, a measure of the loss in the wood, will be equal to the peak frequency divided by the -3dB bandwidth: subtract the pitch of the lower 3dB down point from that of the higher one.

The speed of sound (=c:for a compression wave) in the wood will be sqrt(Young's modulus(E)/density(rho)). Some folks have proposed a 'figure of merit' for wood of c/rho. The higher the number, the better. Brian Burns likes to put in the damping factor as well.

BTW, if you have a program such as FFT4WAV3 or 'Wavesurfer' (free from http://www.speech.kth.se/software/) you can simply support the plate on the foam pads, tap it, record the sound on your computer, and look at the peaks in the FFT to get the mode frequencies and Q values. The trick there is knowing which of all the peaks you'll see are the ones you'll want.    

This is the real basic basics. There are lots of pitfalls and things that can screw up the accuracy, but this works quite well most of the time. It's a bit of a deal to get set up, so I usually spend part of a day when I get some new wood in.

Anyone like to comment on this dismissal of Young's Modulus?

http://www.lucchicremona.com/EN/tester_tester2.htm