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Hi folks,

a non wood related thread from me for once

i was wondering what teqniques if any you guys were using to introduce a frequncy tone to you tops for chaladni testing etc.

i have seen a few methods from just putting a speaker under the top to my own method attaching peizo,s to the bridge plate area and inttrodincing a tone from a freaquncy genorator at that point.

also do you ristrict the movement of the sides or jut do it once the top is braced and let the top vibrate freely.

i like to do it once the top is attached to the sides before putting the back on that way i can take more wood off the braces if i have to.

also what are you looking for in the patterns.

its still early days for me using this method so i dont have many points of referance yet.

any input would be welcome this is a topic that is greek to many luthiers and there is little info on the about how to do this on the net.

Thanks,

joel.

Hi Joel,

I have been testing for the past couple builds after seeing a presentation of a talk Mark Blanchard gave at the last Healdsburg festival. What I have taken from that talk, my own observations, and some discussions with other people is this:
I am doing most of my testing on the free plate before it is glued to the rims. I test the glued up top with no braces or rosette, then I test it with the rosette and hole in and cut and cut to shape and thicknessed, and then I begin adding braces and still testing. I am doing this while also keeping deflection results. Mark spoke about being able to see in the shapes and frequencies some important stiffness distribution aspects of the top while testing the rough top. I do the deflection testing in conjunction with the chladni testing to hopefully also begin to see how they correspond to each other and be able to assess a top before any work has gone into it. Once I start adding braces I am still testing the shapes. There is no definite one group of correct patterns although there is some concensus on shapes that are more important. There is one frequency which seems to occur in the mid 200's which will show a ring and a half type shape, which is a circle in the lower bout and a line following the upper transverse brace closely. (This assumes there is an upper transverse brace, I have not tested a guitar without one) Generally, it is thought that it is important to have the circle in the lower bout be closed. Usually when I begin testing it is not closed, it opens up in one area. It can be hard to know where to shave on the braces to remedy this but a small trick is to add a piece of sticky tack (like to hold up posters) on points on the braces and this is a crude way of mimicing the removal of mass from that same brace. I know that seems a little counterintuitive, but it works well for me. Aside from that one frequency shape I am mostly keeping records for my own use. A theme of Mark's talk was that the data you keep is important mostly to you and as it applies to your building style and guitar shape. Once you have data on 5 or more of the same guitars you can begin to see where the shapes and frequencies of the better guitars land in relation to the less successful ones. It becomes a very nice quality control tool. Very similar to observing tap tones, but in a more concrete form(unless you have excellent pitch memory!)
In terms of the actual physical aspects of the testing, this is what I have been doing. I have a Maddell tone generator which I run through an amplifier and a speaker. I suspend the top on foam blocks at 3 or 4 points (the foam cut to pyramid shapes) and I have been using aluminum glitter as the medium. I used poppy seeds first, which are more sensitive but have a tendency to bounce right off of the plate, especially if there is a radius to it. I start the frequencies in the 30's or so and slowly increase until I start to see some vibrations in the medium. It is important to move the speaker around as you do this. Each frequency has a specific point on the top that the speaker needs to be pointed at for it to fully emerge. Some are much more picky than others. Some of the major ones it seems will activate anywhere and others seem to need to be pinpointed within an inch or so of space. Once you get some activity and a shape begins to emerge, move the foam blocks so they are under the lines of the shape you are getting. This will sometimes slightly change the shape and frequency, but not by much. I do this up to about 450 or so. The ring+ mode is usually 230-260 but can vary, as they all can. Always wear ear protection as the sine wave can hurt your ears with exposure(and can disorient mice as I found out this winter). There is a lot of information in the archives here and at the MIMF, most of it written by Al Carruth and Mark Blanchard who know by far the most about it and who inspired me to attempt it. Mark put a photo of some basic shapes for one of his guitars on one of the discussions in the archives here and I keep that as sort of a basic guideline when looking for mine. Sometimes they are similar and other times not so much so.
This is the longest post I have ever written and I am sure there is much more information people will fill in with. I hope this helps.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

I do pretty much what Burton does, along with tuning the resonant frequency of the back so that it ends up about a half step lower than that of the top, with the bridge on. I check again after a few weeks, since the top's frequency drops a bit and I don't want the top and back too close together with respect to their resonant frequencies. Some builders prefer a stiff back, so this doesn't apply if you're of that persuasion.

I use a PA speaker, an old stereo receiver, and AudioTest on my clunker shop ibook to excite the plates.

One little clarification on Burton's post, though. If I understand this little piece correctly, adding mass such as poster stickum mimics lowering stiffness (not mass), which you get for real later on when you actually remove brace material in that area. Maybe a minor point, but it's an important conceptual one, IMHO.

Read all you can find around the web from Alan Carruth. He's the guru on the subject.

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formerly known around here as burbank
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http://www.patfosterguitars.com

IMO it's much more useful to test the plate modes 'free'; before they're glued to the rim, rather than afterward. Once the top is glued down it will show pretty much the same set of mode shapes as any other top, unless you're using some really weird bracing pattern. The frequencies, also, will tend to be quite similar. I've seen 'the same' assembled modes on guitars that were supposedly 'matched' but sounded noticably different. However, those guitars had different 'free' mode shapes.

From what I can tell right now, 'closing' the 'ring+' (AKA 'ring-and-a-half') mode may not be as important as just getting a nice, smooth shape. It seems as though having the lower bout node lines running more or less parallel to the edge, without a lot of inward 'wiggles', gives a good sound, even if the mode is 'open' in some place.

I, too, am correlating Mark's unbraced plate mode data with measurements of stiffness along and across the grain. I've only just started doing this, so I can't say much as yet. Thanks to Mark for digging that info out of his records, and sending it along to me.

Although generating mode shapes using a hand held speaker calls for a lot of power, and is noisy, it has some advantages over driving with an attached piezo or magnetic driver. One is that you are not adding any mass, which can mess up the mode shapes and frequencies. Another is that not all of the modes drive well from any one location, and it's time consuming to have to move an attached driver.

Your computer has excellent pitch memory and pitch detection if you record the taps and convert to the frequency domain. This is helpful even if you like the glitter pattern technique because it makes it easy to find the mode frequencies. It also shows relative amplitude of the modes and the Q. It may also be useful at providing some insight about what the higher frequency modes are doing.

Interesting,
Thanks for the reply,s

The reason i posted this is becouse there are basicaly two schools of on the subject.

the free plate school and the fixed plate school.

to me the second seems to make more sense but i can see the benifit of both methods.

i cant wait for somogyi,s book hopefully this will matter a bit clearer to me.

Joel.

I can't wait for Al Carruth's book! I meet Al at the ASIA symposium in 2007. He was able to explain tap tuning so I could understand it, so I'm sure he would be able to explain chaladni patterns to a dummy like me.

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Say what you do, Do what you say.

I too thought that fixed plate made more sense but an observation and a hypothesis have convinced me that free plate tuning is the most promising approach. First, the observation that it is very hard to make much difference in the pattern once the top is mounted to the rims. Shaving a brace by quite a bit will make a noticeable change in the tone of the instrument but the Chladni pattern my not change much at all. That makes it hard to draw any conclusions about the shape of the pattern and the final sound. If you are measuring something that changes significantly less than the result you are attempting to control, you are going to have difficulties using that measurement.

Second, is a hypothesis that I have about why free plate tuning is effective. The idea is that you want to get the plate to vibrate in a similar way when it is free as it will when it it fixed to the rims. The rims will force the top to vibrate with a closed ring and a half regardless of the shape of the braces. Now, suppose that the braces on a plate are such that the free plate Chladni pattern is not closed and the shape is irregular. Once you mount this to the rims the pattern will be closed. The rims will make the top vibrate in one way while the braces want to force the top to vibrate another way. The braces are interfering with the vibration of the top. Suppose, again, however, that the free plate vibrations are also nice and closed with a gentle regular shape. Once this top is mounted on the rims, the rims and the braces will be trying to get the plate to vibrate in the same way. They are not fighting each other.

I suspect that this cooperation between the braces and the rim makes for a good sounding top. Actually, my argument is negative logic. I suspect that when the braces are fighting the rims, the top won't sound as good. That seems more obvious to me.

I have no hard data to back up this hypothesis (hence I call it a hypothesis instead of a theory) and I'm having trouble coming up with an experiment that will test it. This is where science and "gut feeling" come together. However, gut feelings can be very wrong and any hard data or even just a compelling argument would cause me to shrug, say "oh well" and move on to some other idea.

I think tuning a free plate to have Chladni patterns that resemble the patterns the guitar will have when the top is glued to the rims does a lot to increase the efficiency of the instrument. it makes a guitar that is more resonant and therefore tends to have more volume, sensitivity and sustain. There is less damping so you just get "more".

I also think that efficiency should not be confused with "good tone" or "sounding better". I have heard very sensitive, loud guitars that produce a tone that is not really all that pleasing to the ears. I have also heard somewhat subdued guitars with a tone as sweet as honey.

I is my experience that nice mode shapes primarily improve efficiency and the relationships between the modes, as indicated by their frequencies, say more about the tonal quality of the instrument.

Mark

It seems that we're on the same page in our thinking. Well, perhaps a nearby page. I suspect that the braces and rims fighting each other affects the tone as well. My reasoning is that interference between the two is going to be quite irregular and could cause certain pitches to have peaks in overtones that can make for less desirable tone. There may even be ring modulations (sum and difference of frequencies) and those can be quite discordant. But, I agree that efficiency is the main gain you get by matching the free and mounted patters.


Good point. I think we've all heard those guitars. Perhaps I should choose my words more carefully.


I don't have enough instruments under my belt to make that observation yet. I suspect, however, that the modes represent frequencies where the top is more efficient. If those frequencies boost or dampen certain harmonics and those harmonics are dissonant with each other, you could see the effect you describe. This makes sense to me too.

PS. Al presented your GAL talk to our local lutherie group. It was very good and it was the closest thing I've ever seen to a recipe for making a good sounding guitar. I'm tempted to follow your process as closely as I can just to see if the results sounds good and, perhaps, maybe even sound something like a Blanchard. Not that I'm interested in making Blanchards, mind you, you seem to be doing a good enough job of that already.

But even if that's true for a closed ring and a half mode, what about the other free plate modes? They're not closed. What happens to the notes that have frequencies closer to those modes than the ring and a half? Are they resisting the rims?

The fact is i still dont understand this properly but i think as whole there needs to a lot more resaerch done.

hopefully we are getting closer to making this a more exating science.
i the mean time we all seem to be doing very well with our own techniques

i think i will stick with my peizo method but will look into the diferences between free plate and fixed more closely and try to make some sense of this in my own mnd.
i will also check to see if i get different reading using the standard speaker method and my peizo method.

Thanks for the reply,s folks it given me alot to think about,

Joel.

Good question. The other modes change a lot more or go away when the top is mounted to the rim. Any mode that has a lot of movement at the rim will be severely altered once mounted. That, in my opinion, makes it difficult to draw correlations between the shape of these mode on a free plate and the shape once mounted and, more importantly, the sound of the resulting instrument. Mark Blanchard believes, as he said earlier, the frequencies of these other modes rather than their shapes seems to be more predictive.

Another important aspect of free plate tuning is that you want to find something that you can measure while the plate is accessible that correlates to the final sound. If you had a test that could only be done on a completely finished guitar and that test showed something about how the guitar sounds, you might be able to use this to gain insight into how the instrument is working. But, that test would be too late in terms of trying to build a guitar with the sound that you want. Free plate tuning has the advantage of possibly showing you something about the final sound at a point when you might be able to do something about it.

This is an important point. For me its not so much about understanding how each individual mode relates to certain aspects of the tone of the finished guitar. I think it would be nearly impossible to do that. The thing is, there is a relationship between the structure of the top and the modes and frequencies it produces. If you change the structure, the mode set will change as well. If you carve a certain brace in a certain area the mode set will change. If you carve a different brace in a different area the mode set will change in a different way. I would argue that changing the structure also changes the tone, and that carving one brace in one spot will have a different effect than carving a different brace in a different area. Because of all this it is possible to correlate the mode set with the tone of the finished guitar without really understanding why or how it works. It is possible to find a mode set for braced free plate that tends to produce a guitar with "good" tone, and likewise it is possible to recognize mode sets that tend to produce less likable instruments.

In my experience, figuring out which mode set is a good one and which is a bad one is best done by trial and error. Build guitars, keep records, and sort the good ones from the bad ones.

Mark

Mark Blanchard wrote:
"In my experience, figuring out which mode set is a good one and which is a bad one is best done by trial and error. Build guitars, keep records, and sort the good ones from the bad ones. "

The modes of the free plate are the modes of the free plate, the modes of the assembled guitar are not. The two structures are related, but they are not the same, and going from one to the other mathematically is possible, but not easy even now. The weak link in all free plate tuning schemes, whether by tap tones or Chladni patterns, is that leap from the free to the bound plate, and we're really not much closer now to bridging that gap rigorously than we've ever been.

We do have a lot of tantalizing clues, though. I may have showed John the results of my 'matched pair' experiment that ended up making two classical guitars that sounded different. There was no difference I could measure at the time between the assembled boxes, but they did sound different. The one thing I could come up with was that the one people liked better had 'ring-type' modes that were more 'closed'.

Recently I loked at the records of all 15 of the 12-fret 000 style guitars I'd made. All of them had, of course, a 'ring' mode on the assembled top, but the free plates were quite a bit different from each other. You could imagine that, in sme way, the sides had to do some work to get the modes that were not 'naturally' closed or nicely shaped to close when the top went on the box. There's always a difference in pitch between the 'free' ring+ mode and the ring mode on the assembled top, and it makes some sort of intuitive sense to think that the more work the sides had to do to get the modes to close, the more pitch shift there might be. So I looked at the pitch shift and how it corresponded to the closure of the 'free' plate modes. The rersult was, as you might expect, more 'interesting' than I had thought.

It turned out that large pitch shifts correlate most with what you could call 'irregular' mode patterns. That is, a 'closed' 'ring+' mode that was not smooth, but had sharp 'spikes' on it, tended to shift pitch more than a smoother mode that might have been 'open' in one spot. In other words, the more parallel to the edge the node line of the lower 'ring' on the free plate were, the less pitch shift. This also seemed to correlate pretty well with the 'quality' of the guitars, although that's more subjective, of course.

I have thought of a simpler system I can use to test 'ring' mode closure and it's correlation with assembled modes, but have not had much time to work on the experiment. Preliminary results seem to indicate that the biggest difference in the output of the 'guitar' in this test is in the _high_ frequency range; the low-end spectrum is not much effected. This correlates with my own impressions of 'well-tuned' guitars: the low end doesn't get better, but the high end does (that was what happened on the 'matched' classicals, for example). This has me a little worried: it's the result I expected to see, so I wonder if I'm not fooling myself. I've got to try to prove myself wrong, and that's going to take a lot of time with the setups I have now.

Somebody on another list suggsted that we should be able to figure all of this out with 'only' a few million dollars of funding. In his world, that might be peanuts, but for us it's the moon. We just have to keep plugging along as best we can, with the realization that the answers may always be out of reach, simply because the system we're dealing with is more complicated than the funding we can get will suffice to figure out. I think of it as 'job security'.

wow thanks guys this is the type of info i was trying illicit from the post.

as for the funding problem Al i think that half the fun of guitar building is the fact that we are trying answer these questions.
i think that in 10 years time they may well look back at this time the golden age of guitar building where the most research and progress was done and we learned the most about acoustic theory in guitars.
Even though we all have our methods they are basicaly pointing in same direction.

Thanks for all the info i will be taking this account when making observations on future guitars.

Joel.

Take heart Al, Rutherford figured out the structure of the atom using experiments made of wood, brass and tin foil. The researchers working on atomic physics were a couple dozen guys in half a dozen laboratories. They went from not being sure that there were such a thing as atoms to self sustained fission reactions. It's only with the Manhattan Project did physics get big and expensive. Good science does not have to be expensive science.

Plus my brother works at CERN so if you get too frustrated with slow science, we could probably crash a couple guitars together at the speed of light to let off some steam.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

Mike Maher wrote:
" Good science does not have to be expensive science."

True, and it's getting less expensive all the time to work at the needed level. However, those early nuclear physics researchers were actually getting paid to do research, even if they weren't getting paid much. We do get some help from universities from time to time, when a mechanical engineering student needs a project that will get them up to speed on vibration measurement and modeling techniques, but that's pretty hit or miss. For the most part this stuff gets done in 'spare' time by individual makers.

The other problem, and it's a biggie, is that there is no journal publishing this research, so it's really hard to keep track of what's going on. The Catgut journal used to take articles on guitar acoustics, but since they merged with the Violin Society they only accept stuff on bowed strings. The Acoustical Society of America publishes articles on instrument acoustics when they get them, along with all the other stuff that is of less use to us. GAL and ASIA generally want things 'simplified' a bit, and at any rate they are not juried journals. We need somebody to ressurect the old 'Journal of Guitar Acoustics'.

Speaking of the "Journal of Guitar Acoustics" I picked up issue #6 from September of 1982, subtitled "The Chicago Papers" from the 08' GAL. They were passing out copies of it as freebies and I have to say that it is fascinating reading, even if a lot of it is over my head. It makes me want to brush up on my math skills, but it has lot of interesting things to say about plate vibrations. I don't have the technical chops to take it all in, but the summaries do offer me food for thought and the bibliographies show me where I can deepen my knowledge to the point where I can maybe do some of my own experiments and come up with some practical applications. Anybody else get one of these Journals at the GAL and care to comment?

Oh! I suppose you'll want to eat, too.

Some people are just never satisfied!

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Quick question for you Chladni pattern folks - are you tuning with the bridge on or off? Free plate, I mean.

Mattia,

I am doing it with the bridge off. I think most other free plate tuners are also.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

The reason I ask is I can't find the references/notes I made based on a discussion (more of an interrogation on my part) with Al Carruth yonks ago, where he indicated he used to measure with the bridge on (as described in the Big Red Book/GAL article from 2001 or thereabouts), but now shoots for a top 'ring and a half' mode that's about a semitone lower than the back, which tends to rise by a semitone once the bridge goes on.

...or maybe it was precisely the other way around. I've got a set of plates I'd like to glue up this weekend, but I'm waiting until tomorrow to make lots of noise.