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#1
11-15-2020, 12:58 PM
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Plate tuning for top and back
Hi all, continuing in a series of questions I've thought of while building
I've read that the back plate of the guitar should sound a half step higher than the top. For the average 6 string nylon or steel. However, this may be hard to accomplish on a 12 string, since the top will be necessarily stiffer and probably have a higher tap tone. I know this is fairly niche, but has the breaking of this convention been investigated for 12 string, baritone, and harp guitars? 
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#2
11-16-2020, 03:55 PM
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Keep in mind that the top on a 12 may not have all that much higher a tap tone. It needs to be something like twice as stiff as a 6, but that only requires making the top 25% thicker, and the bracing 25% taller, so it's not twice as heavy. The bridge plate is larger, of course, as is the bridge, and that helps drop the 'top' pitch some.
Basically, it seems to be a good idea to have the pitch of the 'main back' tap tone higher than the 'main top'. How much higher is somewhat a matter of taste. The closer the two are in pitch to each other, the more strongly they will couple. This enhances the power of the 'main air' mode, and drops it's pitch. In theory, the effect is strongest when the 'top' and 'back' pitches match. In practice, if they're too close you can get strong 'wolf' notes, some of which can be pretty odd. A semitone of separation seems to be 'enough' to avoid problems. People seem to prefer guitars where the back pitch is higher. In some cases, at least, putting the string on seems to raise the back pitch a bit, and may drop the top pitch, although that's not as firmly established. I have also seen guitars where the top pitch has dropped about a half semitone in the first month or so of playing, although I don't always have them around long enough to be able to say that's the rule. Starting out with the back pitch a bit on the low side thus invites convergence in the tap tone pitches, which could put one into wolf territory, so it's probably best to avoid that if you can. Not everybody likes the sound of a guitar with tight top-back coupling. Many 'vintage' instruments have the back higher in pitch by a musical third or more, and if that's the sound you're going for that may be the best way to get it.
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#4
11-16-2020, 11:09 PM
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That's an important question, maybe one which warrants its own discussion. i assume people talk about the pitch of the free plates, so that's what I'm listening to now. I hold the plate a few inches from my ear and tap. If I hold it by a brace, it's really lively. if i hold it by the plate itself, it's not as lively. actually a bit less than I was hoping for. I'm not sure what the benchmark is, but I've approached my minimum structural requirement and I'm going to live with whatever the top pitch ends up being. It's higher than the back by more than a semi tone, but without the bridge, which is quite large in relation to the top. this may drop the top pitch significantly once glued on. I made the bridge with a pot belly. 6" long, 1.5/16 wide on the wings, and 1.75+ wide at thd belly. it weighs about 41g but I could probably get it down to 36 by finishing the holes and beveling the back edge.
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#5
11-16-2020, 11:12 PM
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Plate tuning for top and back
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I should note that my current minimum dimensions are subject to change based on the results of the simulation i posted in the general discussion. right now it's an estimate based on rules of thumb. I expect to be surprised and have room to build lighter.
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#6
11-17-2020, 09:56 AM
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I would consider a braced back to not be free. I do acoustic testing on the top vis-à-vis the Gore method and that is done on only 1/2 plate and you hold the plate on a node, basically a dead spot, which can easily be found by measuring and by feeling and hearing it too. But that's to find Young's Modulus not a particular pitch.
I have a hard time understanding what the pitch of a free, or even braced, plate has to do with an assembled instrument. But it is at least a method and if iterated over time I guess I could see some results.
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#7
11-17-2020, 05:51 PM
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redir wrote:
"I have a hard time understanding what the pitch of a free, or even braced, plate has to do with an assembled instrument." The short answer is; 'not much', but..... The problem is that there are so many variables involved. I've been doing 'free' plate tuning using the Chladni method for a long time. If you make 'identical' tops and backs, using 'the same' wood, and assemble them to matched rims, the finished boxes will have their lowest 'signature' resonant modes (such as the 'main top' and 'main back' at the same pitches, within a very small range, depending on how careful you are to match things. However, the instruments will probably not sound 'the same', although they will be very similar. The problem seems to be that any piece of wood will have some local variation in density and stiffness, which will perturb the shapes and pitches of the higher order modes on the assembled instrument. Since normal hearing is so sensitive in the 2-4 kHz range, where these differences tend to become more pronounced, listeners can readily distinguish the guitars. I suspect this will be the case so long as you work with wood. OTOH, if you have fairly complete information about the assembled modes of a given guitar it's possible to produce a 'tonal copy' that will be 'arbitrarily close' to the original, even though it's unlikely to be 'identical'. If the one you want to copy is one of your own, and you have the 'free' plate information along with data on the wood properties, you could most likely make a reasonable stab at a match using the 'free' plate modes. Again, I feel that 'identical' sound is probably not possible, but the more information you have, and the further back in the process you can start to match things, the more likely you are to have a satisfactory outcome.
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#8
11-17-2020, 09:04 PM
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Al is the right guy to answer your questions. But there are other very successful approaches that do not address the questions at all. It is close akin to a religious belief system IMO.
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#9
11-17-2020, 09:44 PM
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#10
11-18-2020, 11:23 AM
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Not unlike spiritual pursuits, following one’s own path is a wise approach if the pinnacle is your goal.
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#11
11-18-2020, 03:18 PM
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Guitars are physical things, and it is therefore possible for them to be fully described in physical terms. But we ain't there yet. Musical acoustics was pretty well understood by the middle of the 19th Century. A lot has been learned in recent decades via the use of laser interferometry about guitar top modes, and to a lesser extent back and whole body modes. Some day it may be possible to program a robot to build a great guitar, and while everyone who wants one will be able to have one, the romance of building will suffer. I feel lucky to live in an earlier time, when there is lots of room for art in it.
The use that is made by builders of what we do know about guitars can in some respects resemble the old joke about the man looking for his lost keys at night under the lamp post: he didn't drop them there, but he's looking there because the light is so much better. When builders talk or write about the vibration modes of a guitar and its parts, they mostly look where the light is best, which is what we have pretty good information about, can measure, and can affect by changes in design. The problem is that there is much more going on in the dark that determines whether a guitar will be a great or even a very good one. The main focus of the kind of lutherie that is concerned with vibration modes has been things that do matter, but may not matter all that much to, or be determinative of people's subjective impressions of the instrument's sound. This includes things like checking free plates, which are not free when on the guitar; checking top modes on the guitar but without finish or bridge or neck and fretboard attached, and not under string tension; ignoring neck modes and whole guitar modes (body and neck together); and looking at only the first few top or top and back modes because they are the ones we can easily measure and affect (most focus has been on main air, 0,0, 0,1, and 1,0 modes) when we actually know that higher midrange and treble peaks in the guitar's responses have a huge effect on how much people actually like a guitar's sound. We don't know much about which of those peaks are needed, and even less about how to predict that a design will make them happen, except for our knowing which existing designs have most often done the trick. So it's not surprising that there are great builders who ignore all the science and build those designs, and whose instruments are at least as good as any. There's nothing wrong per se about doing the "left-brain" things. They can be helpful to building well. But we do many of them because they are the part we can see pretty well at this time. We do not have the recipe for how to measure all our parts so that when the recipe is followed the result will necessarily be a great or even very good guitar. And some builders' (some very well-known builders) devotion to maximizing response in the first few modes has resulted in their building one-trick ponies that boom in the bass and baritone ranges, but tend to be uneven, out of balance, and plinky in the treble. I personally have read some of the texts on the acoustical physics of musical instruments, and I like to stay up to date, and follow the comments of those who know that stuff better than I do and are doing their own research on it (thanks, Al Carruth!). I think it sharpens my intuitions and has helped me to nudge my guitars in the direction I want them to go. OTOH, I know one first-rank builder who has covered his ears when I talk about musical physics because he fears that it could corrupt his intuitions. Hasn't seemed to hurt his results. Different strokes. Use the methods that appeal to you. But don't overestimate what is known, or ignore your intuitions.
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"Still a man hears what he wants to hear, and disregards the rest." --Paul Simon Last edited by Howard Klepper; 11-18-2020 at 03:24 PM.
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#12
11-18-2020, 05:19 PM
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#13
11-19-2020, 03:08 PM
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I have been away from building for a couple of years, other pursuits muscled it out of the way, but I have a background in a mechanical test lab and want to measure some parameters while building. If something pops out of it that would be fine but I am more of the opinion (not sure if I got it from Bruce) that if I tape the plates as I go I want to not hear a sudden change in tone, sort of a sonic discontinuity. I think maybe Allen's reading tea leaves would do the same. In 20 or 40 more guitars I may think differently but for now walking is good for me, if I get to the point of running that would be fine also.
__________________
Fred
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#14
11-22-2020, 03:31 PM
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I've been mulling replies for several days, and even started typing a few times. After a page or two I give it up. The problem is that I started working with Carleen Hutchins back in 1980, and met Fred Dickens (who was the first person I know of to plate tune on guitars) at about that same time. That's a lot of experience and discussion to summarize in a post. I'll try to describe my current thoughts as briefly as I can, but I make no promises.
Back at the beginning (in the '70s), not much was known about how these things worked, and it was hard to find out. Carleen had $20,000 worth of B&K lab equipment for making spectrum charts on violins, and it was a laborious process. She would run spectra on the 'free' plates, and try to optimize them as best she could, but without any knowledge of that the actual modes were it was a shot in the dark. Laser holography opened that door, and lead to the re-discovery of Chladni testing. Affordable and useful test equipment for that was still hard to get, but at least it was possible on a luthier's income. Now, of course, it's almost trivial. There were two strands in the research. One was to see what older authorities, such as Savart, had to say. He had actually looked at Chladni patterns on some Strad violins, and gotten pitches. Once in a while somebody got a chance to look at a 'great' instrument and come up with a bit more such data. Needless to say, since it's less common to disassemble guitars, we don't have much of that. Another was to look for correlations in our own work. 'Correlation is not causation', of course, but there is no causation without it. Correlation, backed up with some sort of plausible mechanism can be a place to start. As has been pointed out, if you knew enough about all of the parts of the instrument, and about how they are coupled to each other, you could predict the final vibration modes of he completed instrument. It's been said that the only branch of physics that's more complicated than acoustics is quantum mechanics, so getting the point A, describing the instrument, to point B, predicting the sound, is extremely difficult, even with the best modern computers. Remember the state of the art in the '80s and '90s? Even now it's hard for computer models to cope with a guitar, even without factoring in the local variation in the wood. For me, at least, with my limited math chops, experimentation seemed to be the best way forward. The most productive ones in terms of plate tuning were 'pair' experiments. Instruments were made that were as well matched as I could manage in ways that were thought to be important, and players and listeners were asked to judge the results. One of the first 'pairs' tested the notion that 'free' plate pitches would predict the final sound. Two classical guitars were made from redwood and mahogany that was cut 'in flitch', and the frequencies of the first ten modes of the tops and backs were matched to within 2Hz. The weights and so on were also very close, and the modes of the assembled guitars also matched well in pitch. The problem was that they didn't sound the same: they were very similar, but most players and listeners preferred one of them over the other. Part of this outcome was due to a failure on my part: I didn't use perfectly straight grained wood for the fan bracing. There was a wave caused by a distant knot that made the grain in the braces arc vertically from one end to the other very slightly. Some of the fans arced 'up' and others 'down'. This necessitated profiling the braces a bit differently in order to get the pitches of the modes to match well, and the mode shapes were a bit different. The preference was for the top that had the 'better' mode shape for the 'ring+' mode. Keep in mind that the pitch of the modes 'encodes' information about the overall relationship between mass and stiffness, while mode shapes encode information about their distribution. The absolute pitches of the 'free' plate modes say little about the pitches of the assembled modes of the guitar. What can we learn from the shapes? At this point we're moving into conjecture, but with some justification. Discussion with other makers who use these methods, and particularly with Mark Blanchard (who is smarter than I am, and keeps better records) has lead to the hypothesis that the free plate modes are indicators of the mass and stiffness distribution in the plate that set up the higher order modes of the assembled guitar. This fits in with a paradox that was pointed out years ago. Most guitars of reasonable quality work very similarly in the low frequency regime; you have to mess up royally to make one that departs very far from 'normal' in this range. The differences between 'average good' guitars and 'great' ones seem mostly to be in the high frequency area, particularly between 2-4 kHz., where 'normal' hearing acuity is greatest. The problem is that there are so many resonances of the top, back, and air, in that region that they overlap; you're in what is called a 'resonance continuum'. The response is so complex that the maker has no prior control over what happens in any detail. All you can say with any assurance is that, if there are twenty peaks in the output in the octave between 2kHz and 4 kHz, there are twenty resonances, but it's possible (and even likely) that none of them is at the pitch of any peak, and you can't say whether any given peak is primarily an 'air,' a 'top' or a 'back' peak. So, if the maker has no direct control over what it is that makes them sound good how is it that some makers can consistently make better guitars than others? What we have observed is that guitars that have a larger number of well-formed 'free' plate modes, particularly in the top, tend to be preferred by players and listeners. A recent 'pair' experiment also showed that small differences in the shapes of higher order 'free' top modes were carried over into changes in the shapes of the high frequency modes of the assembled top, and could be seen in the output spectrum of the guitar. In that pair of mahogany/Red spruce OMs the spectra below 1000 Hz were virtually 'identical' but they diverged at higher frequencies. In 'blind' tests listeners were easily able to distinguish them as 'different', but there was no consistent preference for one or the other. The idea, then, is that bracing is a necessary evil. We could make the top stiff enough to hold up under the string load by making it thicker, but that would be too heavy. We use bracing to get stiffness without commensurate weight, but if it's not profiled carefully it will restrict the ways the top can vibrate at higher frequencies. We can use Chladni patterns to see where we might want to remove material from the braces to get the correct 'balance' of stiffness everywhere. When well done this can enable the top to work better at high frequencies, producing a 'clearer' sound. The design of the guitar as we have it has been quite well optimized over centuries simply by trial and error. A standard design, carefully made of good wood, will generally end up being a better than average guitar. Some attention paid to getting the 'signature' modes, the 'main top', 'main air', and so on, to work well, with elevate the quality further. When we made a top plate tuning video a few years ago the producer wanted to bill it as a 'scientific' approach to lutherie, but I wouldn't let him. What we're doing here is just barely 'science', simply because it's a tough row to hoe. 'Free' plate tuning, by itself, is simply a 'tech' version of the old 'tap tone' method, that gives more information. As such, it's useful, although it has the drawback of taking a lot more time than most tap tuners spend. When we made the video, he wanted me to talk about backs. I'm still having problems getting those to work right..... 
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#15
11-22-2020, 06:04 PM
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Alan I hope you write a book some day. I've thought of this before, scraping the internet for Alan posts and assembling it into one but I'm too busy building guitars
 So from what you said here, "Needless to say, since it's less common to disassemble guitars, we don't have much of that. " Are you saying that they actually did Chladni tests on Strad tops after removing them from the violin? And when you say, "We can use Chladni patterns to see where we might want to remove material from the braces to get the correct 'balance' of stiffness everywhere." Are you saying that you run the Chladni test on the fully assembled guitar?
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