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PostPosted: Sat Feb 15, 2014 7:30 pm 
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Posts: 21
I'm assuming that by 'patterns' you mean brace layouts, dimensions, and profiles, rather than things like 'Chladni patterns'? I'm also assuming that, unlike a factory, you use some up-front quality control methods, varying the top thickness, for example, depending on the stiffness? If so, to both questions, then I would not be at all surprised if you're ending up in the ballpark, and can dial things in with a little brace tweaking at the end. There are lots of ways to build a good guitar, and each of us tends to find something that fits our own way of working.

johnparchem asked:
"Would the tap frequency of a brace of a given size be proportional to its stiffness?"

The resonant frequencies of an isolated piece of brace stock of a certain size that's not glued to anything will depend on the relationship of the Young's modulus to the density of the material. You can use this to determine the Young's modulus when you know the size and mass of the piece. Once you glue that brace down it's part of the structure, and the pitches that you hear when you tap on it will depend on the whole structure, with that brace as one part, of course.

There have been builders who have measured the stiffness and density of their brace stock, and worked out over time the 'proper' vales for their builds. By keeping track of what they did, and what worked best, they would determine that, say, they wanted to use dense and stiff stock for the UTB, and lighter wood for the tone bars. They might even figure out what worked best on tops of different densities and stiffness. To each his own.


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PostPosted: Sat Feb 15, 2014 8:43 pm 
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Joined: Fri Mar 03, 2006 7:09 pm
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Location: Hegins, Pa
Yes I am speaking of brace patterns and shapes. Also the patterns evolved over time. I also have the luxury of getting tracings off old production patterns used at Martin years ago. After 10 years of building and keeping a log , I am happy with the end result.

There are times you get a piece of wood that has a very good feel , stiff etc. I can't say that I believe that tapping a top can tell you more than the top is not cracked. Of all the guitars I made and tops I braced I learned the before and after tones are often quite different.

Thanks for your input Alan.


I get to Martin on a regular basis and they do build a decent guitar , little is done outside patterns and I have never seen any top or back tapped.

That doesn't mean one can't build a better sounding guitar. I have studied the pre war Martins and we have the luxury of seeing the past design and the long term effect of stress.

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John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
Board of Directors of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com


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PostPosted: Sun Feb 16, 2014 5:44 pm 
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Joined: Sat Jun 22, 2013 5:21 pm
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Lots of folks have built lots of guitars over the past few centuries. All of them have tried 'improvements' from time to time, and most of them didn't work, but some did. Any time somebody came up with an improvement that worked, everybody else copied it. The result is that, so long as you stick with 'normal' materials, it's pretty hard to improve on the existing designs, since most of the improvements that worked are already in there.

You can, however, 'tweak' things. In particular, you can work with the wood you have, rather than the 'average' piece of wood. Again, because the standard designs are so good, there's a fair amount of leeway in that regard, particularly if you're not trying for a specific sound.

I don't get that much out of tapping either, but Chladni patterns; there's a different story!

For the most part the bracing patterns and profiles on my guitars are pretty similar too, but it's all in the details. Sometimes a shaving or two here or there is all it takes to improve a guitar, particularly if it's 'close' to begin with.

I don't remember where I saw it, but somebody suggested that those old Martins from the 30s may have been better because, when the company cut back during the Depression, they kept the best workers. Decent wood, good design, and careful workmanship go a long way.


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PostPosted: Sun Feb 16, 2014 6:14 pm 
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Location: Hegins, Pa
that was a statement I heard David Musselwhite say. can't say it was his statement but the argument is a sound one. building guitars in my opinion is much like a recipe and the end result is going to be based on a subjective input. Does the builder and buyer like the tone.
I agree that the design has evolved and it is most difficult at this point to reinvent it. I have read a number of books by well named authors . Some I can and some I cannot agree upon.
If there is one thing I can pass on to the newbies is to pay attention to the details. If the joinery isn't right , you can't force it. A forced joint is doomed to fail. I will say that for those that are starting, you have to start at the beginning and keep notes and start a building log. Pay attention on the joints and the fit. To make a good sounding guitar is easy to make it look pretty that isn't
If you chase theory you loose practicality .

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John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
Board of Directors of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com


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PostPosted: Sat Feb 22, 2014 9:23 pm 
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Joined: Mon Aug 19, 2013 11:33 pm
Posts: 36
hi John

What a heavy topic.(tone tapping everything) I really agree with you on this. I don't know much at this point in my guitar making other than keep it simple . But I do not what to tap a brace to a specific tone only to know it will change when it's glued and assembled into an instrument that also needs to age. That's a great point that you made concerning the ageing process. you also mention when the woods are good and the joints are correct and alignments are true to dimension , proper glues and a sound design that's it .Thanks John

The new newbie Stephen Colby


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PostPosted: Wed Feb 26, 2014 10:06 am 
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Joined: Thu Oct 18, 2012 10:36 am
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Thanks so much guys. As background, I am building my 1st acoutic guitar, targeting a 000 size based on reverse-engineering an existing guitar I love. I took a lot of pain to do as complete layout as possible of the guitar especially soundboard thickness and brace dimensions and shape with mold impressions. I have some background in acoustics engineering and materials and I bought a number of books on guitar building. I picked up on the tap-tune method from the Roger Siminoff books but hit a wall as I could not extract the natural frequencies of the glued-in braces as he shows in his books and video. I might have gone down a "gold-plated rathole" trying to pursue this method.

Given my lack of experience, I am in progress making a "throw away" top that I could make mistakes on regarding clamping methods, fixture development, equipment and tool selection and process steps. I think I can detect obvious defects in my top and braces through visual inspection or listening to taps but I hope to get the assembled top pretty darn close before I glue it to the sides. Would using a soundboard deflection method for carving the braces be a good approach or do you think I would be better off just replicating the brace shapes of the target design?

Thanks again for everyone's help. -Blatz24-


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PostPosted: Wed May 14, 2014 11:23 am 
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Copying the basic layout of an existing good design (one that you like!) is a great place to start. However, you have to realize that an exact copy is unlikely to sound like the original, if only because you won't be using the same set of wood. If you knew the Young's modulus of the original top along and across the grain then copying the overall thickness would make some sense, but, again, copying the exact graduations would probably not work out as well as you'd hope.

There are all sorts of reasons why the original maker might have varied the thickness of the top from place to place. Some of of them may have had to do with voicing, but for all you know he/she was sanding out a ding, or just zoned out for a few seconds with the sander running. There are some systematic things you can do to tilt the sound in the way you want. For example, on my builds I've found that thinning the top behind the bridge tends to bring up the bass, while thinning outside of the bridge wings helps the trebles more. YMMV.

At any rate, I've tried to make 'tonal copies', using wood from the same flitch, copying the thickness, mass, bracing, 'free' plate tap tones, etc., and still have not made a pair that sounded exactly the same. I think I'm closing in on it, but these things are incredibly complex, and it may simply be hopeless. Of course, the closer you get to copying everything, the closer your result will be. But, again, since two slices of wood of the same species that look similar can have E values along the grain as much as 40% different, with the cross grain stiffness being even more variable, you can't count on getting it right without some measurement of the actual value.

David Hurd made a stab at this in his book: "Left Brain Lutherie" He uses deflection maps of the completed top to help him profile his bracing. He gives some math for backing out the wood properties. I'm not engineer enough to be able to say whether it works well or not. I note that his readings of long-grain properties do seem to work better than cross-grain on older instruments: I suspect that 'cold creep' is messing up the cross grain results.

A number of folks have used temporary mounting on rigid rims to 'tune' tops. It seems to work for some of them, at least; I have no experience with it so I can't say much. I do think that the resonant modes you would see on such a rig could be substantially different from what the same plates on an instrument would give you. The rig would lack a lot of the coupling that you'd see on a real instrument, and the details of that coupling that really matter. This is why small changes in something like saddle height or bridge pin mass can sometimes have a big effect on the tone: you may have shifted a resonance enough to go into, or out of, the half-band of another resonance, and altered the coupling strength and pitches of both substantially.

Naturally almost anything you do to get control over things can help. With enough experience you can use the results from a rigid rim to predict more or less what the top will do when the guitar is assembled, particularly in the low range. If you can find other people who do the same thing you might be able to exchange data, and progress faster. I suspect that there are a lot of details that you'd have to get just right to be confident about the comparability of your data, but these are not insurmountable problems.

Again, one of the things I like about 'free' plate tuning is just that it's so easy to compare data. Knowing what the data means is the hard part!

As John says, probably the most important thing for a beginner is to pick a good design, and then built it carefully. Don't get too hung up over some of the detail design stuff: I'm not sure whether 'tucking' the bridge plate makes a lot of difference, but whether you choose to tuck it or not, do a good job at it. Whatever you do, keep good records! Try to come up with a model of how you think the guitar works, and record everything you can think of that might bear on that. The better your model is, the more likely you are to find correlations between the inputs and outcomes. After a while, you'll figure out a method that works for you, and then you can write your own book!


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