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#31
01-26-2022, 08:58 AM
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#32
01-26-2022, 01:40 PM
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I've tried a few times to make guitars using matched wood, cut from the same flitches, that measured out as 'the same' within measurement error. I use tight quality control in the building phase, looking at thickness, bracing, weights of all parts, resonant modes and so on. Each pair I've made was better controlled than the last. In the latest pair the spectrum charts up to 1000 Hz were so closely matched that you'd have said they were identical. They still sounded different in 'blind' listening tests.
Of course, those matched guitars did sound very similar, and nobody had any marked preference for either one. But they were different, and almost nobody got that wrong. Measurements of the sound output showed that they tended to diverge as you went up in frequency, particularly above 2000 Hz. That range from 2-4 kHz, is where 'normal' hearing is most acute, so that we pick out small differences. At the same time, as you go above about 1000 Hz the maker has less and less up-front control over the sound. There are simple so many resonances, 'coupling' with each other in such complex ways, that it becomes impossible, even in theory, to predict the outcome of a small change in one of them: it's a 'resonance continuum'. Changes in the local stiffness of small areas of the top, in particular, can shift the spectrum in some ways. In that range the output of the guitar is quite directional, so that moving the mic even a few inches can alter the spectrum you measure. It's impossible to say, given the limits on the apparatus I have, and the time it takes to make these measurements, whether the two guitars put out different amounts of sound at a particular frequency in that range, or just put it out in different directions. Probably some of both. Since out ears are much better at that sort of thing than microphones are we can hear those differences, but they're hard to measure. So: as long as we're making them out of wood we can probably get 'arbitrarily close' to the same sound with care, but never match it exactly. The interesting question is, if we found 'matched' tops of, say, Red and Sitka, would the guitar be more or less similar than two made from flitch-matched tops of either species? That's going to take some effort to answer. 
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#33
01-26-2022, 02:10 PM
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That's a great question, and we do have data on that, summarized below. There is a third dimension to the graph, and that is the damping coefficient. It refers to the amount of internal friction that is created when a material vibrates. That friction serves to dampen that vibration, and thus NOT radiate that energy as sound. I would expect that every one of those overlapping dots has a different damping coefficient, because damping is NOT influenced by the other two characteristics. This is a hugely important variable to know and measure, as it varies nearly as much as density and stiffness. When you tap a board, part of what you are hearing as the board rings out (or doesn't) is damping...but it is very difficult to judge by ear, and compare many boards to one another. It can be measured electronically, though, in a couple of ways. The method we use is called BING (Beam Identification for Non-destructive Grading) developed by the CIRAD materials laboratory in Montpelier, France, which happens to be the site of some of the best tonewood research in the world It is also a great place for a site VISIT, on the lovely shores of the Mediterranean, but I won't digress  So. The way we test the impact of each variable in isolation is to start with a population of several thousand boards ( this graph x about 5). If we wish to study the effect of damping, we would choose three boards, all with the same density and stiffness along and across the grain, but with high, medium, and low damping. When we do so, and carefully build out the guitars, we find that the low damping instruments are preferred in by blind listening tests (performed at the Department of Akustics und Haptiks at the Technical University of Dresden, Germany). When we repeat the test, using groups of both high and low density, we get the same result - low damping tops tend to sound better. For the guitar build, we have relied on a GREAT partnership with our friends at Taylor Guitars. For the Dresden studies, all guitars were the 814ce, built on the same day from the same population of back and side woods, to eliminate those variable. We've tested both X and V class. We repeated the damping test with bespoke guitars built by Trevor Gore, a superb scientific luthier from Sydney, and are getting much the same result The above a nutshell of our latest studies, not yet published in reviewed literature. Our earlier study in the Journal of the Acoustical Society of America used selected boards that had all equal damping, but varied across the graph from low density/ low stiffness at one extreme, to high density/ high stiffness at the other. We found a marked preference, with this 814 design, for top wood on the low end of the density and stiffness range. Again, we repeated the test, and came up with exactly the same result with different guitars. Fretboard Journal last issue, # 48 has a story on this for more detail -- this is getting a bit long, and I'll stop now... So, yes, knowing the physical characteristics of the top CAN lead, with careful design and build, to a predictable, optimized, and reproducible result in terms of tonal quality. The guitars will NOT sound exactly the same, of course, there are too many variables, some we may not be able to measure, some just random. But these characteristics are the big ones to look for and measure, I'm convinced Cheers, Dave Olson
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#34
01-26-2022, 02:34 PM
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Low damping, low density and low stiffness are preferred/sound best. Damping is not correlated with the density and stiffness shown in your graph, so it's not possible to say whether Lutz or Adirondack (generally at different ends of the density/stiffness ranges) would be most likely to have those properties. And both Lutz and Adirondack are preferred by luthiers and players, though perhaps for different reasons. Thanks for the numbers. Illuminating. A minor question. Is it possible to predict by the appearance of wood what properties it will have? Something like the number of annual rings/length or bearclaw?
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#35
01-26-2022, 02:43 PM
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Is there any data on how using an aging process "might" affect these variables? In other words, do we know or hypothesize if it has a measurable effect, no tangible effect, or random effect? Best, Mac
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#36
01-26-2022, 04:10 PM
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The researchers tell us that aged wood gradually loses density by virtue of losing hemicellulose, but because this doesn't contribute very much to stiffness in the first place, stiffness doesn't change very much. Damping in aged wood does seem to decrease, in most studies (References: some of the work of Eichii Obatayo in Japan, and Iris Bremaud in Montepelier). So. Take another look at the graph, and imagine Adirondack, harvested 50-90 years ago. Imagine its trajectory, to the left, as it lowers its density while maintaining its stiffness. It starts to encroach upon the Lutz space in that scenario, which is likely why Lutz is sometimes compared with AGED Adirondack. With a decrease in damping, which tends to favor every test guitar we build, well I can see why Old Red is preferred tonewood, and I can see why these old guitars are treasured so. Incidentally, our studies suggest that a particular design has an optimal set of characteristics. In the 814, it seems to be low density and low stiffness. It may well be that another design (top thickness, bracing pattern, shape) will have an entirely different set of characteristics that lead to an optimum. But low damping seems to be an intrinsic good, regardless of design. By the way, you can test that yourself with your own guitar, but I'll save that for another post... Back to work, Olson!
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#37
01-26-2022, 05:03 PM
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What this suggests is that the old belief that "light/stiff" wood always makes the best guitar is incorrect - a guitar built with wood that is too stiff will stifle sound, indeed the guitars at this end were noted to sound "tight" and muted. When it comes to stiffness and density, then, one must build to the wood, OR figure out the optimal wood for your particular design and keep using wood twitch those characteristics- if you want a consistency between one guitar and the next. Not the same, of course, but more similar than otherwise. Damping, however, seems to be a consistent effect across designs so far. In addition to the Dresden study guitars of Sitka spruce, we've built two sets of pilot study guitars, three each - a Lutz topped 714 trio, and the Trevor Gore small body trio, both groups with high, middle and low damping, and the same density and stiffness in all guitars (average values for the species hybrid). Same result - the low damping guitars sounded better in listening studies. Bright, resonant, rich in overtone bloom, as opposed to dull and compressed, with high damping wood. No species has a monopoly on low damping, as far as we can see. And - it is NOT visual. Damping doesn’t exist as a property until the wood is compelled to vibrate and is observed, in contrast to the intrinsic properties of density and stiffness. It has to be measured, and the measurement is fussy! Cheers, DO Last edited by varve; 01-26-2022 at 09:11 PM.
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#38
01-27-2022, 04:42 AM
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Thank you for all this great information Dave. It is very interesting.
Have you looked at how torrefaction affects the parameters you’ve discussed above? My day job gets me involved in biomass-based charcoal and activated carbon manufacture, and I think of biomass torrefaction as progressively driving out the volatiles, with the quantum and size (molecular weight) of the volatiles driven off being related to the temperature and duration of the torrefaction process. But I had not thought of torrefaction as having any significant impact on cellulose, hemicellulose or lignin, the three main building blocks of the wood and, I had thought, the principal determinants of the parameters you discuss. Can you shed any light on this? Are we seeing, for example, some sort of annealing process?
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#39
01-27-2022, 01:13 PM
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Yes, we did address this directly, with a comparison of the 4 extant techniques for thermal modification, as it pertains to spruce tonewood. https://www.researchgate.net/profile/David-Olson-10 A light thermal modification (far less intense than your biomass processes!) will indeed decrease hemicellulose, but leave Cellulose and Lignin alone, accounting for the favorable effects. But as you will read, it can be overdone, and the effects vary dependent upon the technique, all of which are very different. Enjoy! If you are unable to open, please PM DO
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#40
01-27-2022, 01:34 PM
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I've been measuring damping for years using a resonant bandwidth method. In most materials that's pretty much 'it', but wood may be different, as it could well have different damping coefficients at different frequencies. Daniel Haines took the method I use a bit further: rather than simply testing tops, he tested narrow strips at several different resonant pitches, and cut them down in length to get a 'new' family of pitches from each sample. He noted that most spruces had low damping below 2000 Hz, but that the loss factor took a 'dogleg' at that pitch, and started to rise quickly. Sitka, had higher damping at low frequencies, and it fell until the 2kHz point was reached, and rose above that in the 'normal' manner. He could think of no reason for any of this, and felt in may have been an artifact of his measurement method. If the variation in loss is frequency dependent we may have to look at another variable...
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#41
01-27-2022, 01:47 PM
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Last edited by FingahPickah; 01-27-2022 at 02:21 PM.
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#42
01-27-2022, 06:11 PM
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From my research, Picea abies does not sound anything like Sitka...or any other spruce. Of course there can be overlap...and yes the luthier has a lot of latitude with voicing.
In the classical world, Picea abies is the preferred spruce. No mention of Sitka in the classical world that I'm aware of. Last edited by xzy; 01-27-2022 at 10:34 PM.
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#43
01-27-2022, 07:54 PM
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I would love to have a non destructive method for easily determining the instrinsic damping of wood at different frequencies...I'm still looking! Whether using the half bandwidth method, as Alan and Professor Haines did, or the log decrement as we do, we are restricted practically to the first resonant frequency of a free bar or plate. The only way we can test different frequencies then is to test different sizes of samples, as Haines did. By the way, if anyone has a copy of those papers, from over 40 years ago, I'd love to see them. I've never found the actual work! When we started doing this work, more recently, we tested 4 mm thick soundboard blanks, much as a luthier might purchase from a supplier. Such a top has a tap tone of about 80 - 100 Hz, but so many other things affected the sound damping, including the mass to surface area ratio of the material, the air friction due to the roughness of the top, the precision of the node placement, etc. etc. It was difficult, indeed impossible for us at least to get consistent results that we could attribute to damping due to internal friction of the countless tracheids sliding against one another, which is what we really mean by "damping". Instead, we shifted to larger specimens, the 22 mm thick billets, each weighing about a kilo - billets that are subsequently resawn into 2 bookmatched tops after we test them. The natural resonant frequency, and thus the damping measurement frequency of such a billet is around 400 Hz. Suddenly, with a heavier board where air friction is less of a damping consideration, the other stochastic variables died away in importance ... and we were able to get consistent results. Now, we didn't know at the time whether those results were meaningful or useful until we started building guitars with Trevor, and the good folks at Taylor Guitars. Those results are reported above, at least in summary form. 400 Hz is probably as good a frequency as any to measure the damping of guitar wood. Close to concert pitch, right in the midrange of the fretboard, and our ears are fairly sensitive to this frequency, as opposed to the 80 - 100 Hz of a tap tone on a bare soundboard (look up the "Phon equal loundess contour" for the nerdy details, and remember that the dB scale is logarithmic!). So, even though damping is a notoriously difficult measurement to get right, I think we've got a pretty workable solution here for grading tonewood. Cheers, DO
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#44
01-28-2022, 02:09 PM
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Haines published in the 'Journal' of the Catgut Acoustical Society, and those are available on line, I believe from Stanford U. See the issues from May of '79 and '80.
I always used to think that balsa had high damping, until I tested out a 2" square piece of it about 3' long; it's right in the ballpark with spruce. The energy gets sucked right out moving air when you test a thin piece. The 'perfect' test is a myth, of course. Wood varies in stiffness and density from point to point, and to get a really good 'read' on a top you'd need to cut it into 2" squares and test each one. Then you can't make a guitar from it. The equations we use to derive the Young's modulus from vibration tests are approximations that only get close to the 'right' answer. McIntyre and Woodhouse published some articles on all of this, also in the JCAS, in '85 and '86. They gave some hints about using results of several modes in rectangular plates tuned so that the lengthwise and crosswise 'pure' bending frequencies would be the same, to account for shear moduli and Poisson's ratios and get better 'E' values. None of this is simple...
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#45
01-28-2022, 02:42 PM
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Since most classical guitars were built in Europe, I'm guessing they used European spruce because it is generally on the lighter end of the spectrum and because it is locally available. Cedar is generally even lighter than Euro spruce, but it is generally from North America so it probably didn't become more popular until global trade made movement of woods easier and cheaper. I have heard of some luthiers using Sitka Spruce, but it seems like they need to make the tops thinner to save weight. Of course, if they find a light piece of Sitka or maybe even Adirondack Spruce, it will probably perfectly suitable for classical guitars.
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