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I'm not a Chladni guy so maybe someone can help me out, I don't really understand how the free plate frequencies relate to a completed guitar. I would expect the mode shapes and frequencies to be very different on a fixed plate vs. free plate, much higher on a fixed plate.
Do you figure that the fixed plate frequencies will jump "x" Hz or something when you glue it to the rim. Has anyone botherd to figure out how much it jumps, I know it'll depend on body size, particular mode, etc..?
I suspect it'll jump up a lot.
Thanks,
JIm

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Jim Watts
http://jameswattsguitars.com

If you do not change many variables you will start seeing some trends pretty quickly, as in after a few guitars. The shape and stiffness of the rim, and the weight of the entire box, influence the jump. The jump is higher on a smaller, or stiffer box.

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Build log

Jim raises the Standard Objection, and I've been thinking about that some of late.

First, of course, that same objection would hold for any method of 'free' plate testing; tap tones, deflection, whatever. In every case, the thing you are measuring would be much different on the completed guitar, so what's the use? And, of course, if we can justify one, we might be on our way to justifying any of the free plate testing methods.

I don't think anybody would say that things like the top thickness and graduation, bracing pattern, or brace profiles have _no_ effect on the sound. Quite the contrary: the way we all obsess over this stuff suggests that we think it's very important. The fact that there is a whole cottage industry of shaving braces on completed tops, with a lot of testemonials as to efficacy, shows that the braces do have some effect on the tone. Given that, then, the issue becomes one of finding the 'best' way to determine the proper bracing scheme or profiles as we work.

Obviously, one can just go in with a a chisel/plane/sandpaper after the fact and chop away. That works, but it's not easy, particularly for guys like me with fat arms who like to build classicals. Some method that allows for shaping things _before_ you put them together make a lot of sense. So what's the best way?

One big advantage of deflection testing is that it does give you the stiffness directly. Since stiffness is the thing that limits how thin you can make the top it's good to know when you're in safe territory on that. Of course, the 'free' stiffness doesn't tell you what the 'assembled' stiffness will be, but with a few builds under your belt you should be able to predict one from the other closely enough.

However, in my mind, deflection tests, whether done 'by hand' or with weights and micrometers, only give you part of the information you'd like to have. Vibration involves both mass and stiffness, and deflection testing only tells you about the stiffness.

Tap tones have the distinct advantage of being simple to do and low tech. They have the distinct disadvantage that there's a lot going on, and it can be hard to sort out. One big advantage of Chladni testing, IMO, is that it sorts all of that stuff out for you.

The other big advantage is that it's easier to record, and to share and teach. You can take pictures, or draw them, and file the information away, send it over the internet, and show it to other people.

The biggest problem, I think, is simply that we're very early in the development of this method. Different shapes, and different bracing systems, even different woods, will act differently, and we have a long way to go to sort out the general rules. At the moment, the best we can say is that you need to figure out for yourself what works on your designs. After that we can encourage people to share the information they have so that maybe we can start the sorting out process.

I try very hard to avoid saying that this is a 'scientific' method of guitar making. It's a 'technical' improvement on tap tone tuning, IMO. At some point in the future, when we have enough data, we might be able to generalize better, and also to think of ways in which the shapes we see relate more directly to the sounds we hear. There's a lot of research to do before we can do that, though. I look at it as 'job security'.

Alan,

I asked a question, but I think you missed it because it was at the bottom of p. 1. Anyway, I wanted to ask if you are going to do a Chladni testing DVD for classical guitar, as you said you might?

Thanks,
Max

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Max Bishop
Brighton, Michigan

Darryl,
I thin the brace side of the top. For stiff spruce, a typical graduation for one of my classicals would be on the order of 2.2 mm thick in the center of the lower bout tapering to 1.9 mm at the periphery. This slight and gradual transition is fine for springing in braces on a dished workboard. If I tweak the outside face after bracing, I make sure that the transition is smooth so you don't detect low spots.

Sorry Max. I'd need to get together with my videographer first, and then there would be some Mighty Preparation to do. I would not have time to even start for several months, so don't hold your breath.

Thanks Gary!

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Formerly known as Adaboy.......

Thanks for all the contributions, everybody!

I appreciate the help.

Cheers
John

What I have found more helpful is to string the top up and voice it with the strings on prior to gluing it to the guitar. How do you do that? Make a "dummy" guitar. I glue my bridge on after I've made the braced soundboard. Insert the soundboard in your dummy guitar and string it up. No need to close the back. The volume will not suffer, in fact it will be very loud from the open back side. Now you can easily reach in the box and modify your braces. This is the fastest way to learn how to voice a guitar. Once you got it voiced how you want it you can remove it and continue with the build. Keep in mind that you can't polish a terd. If it is a bad guitar you are not going to make it better. The sound character will remain the same, the voiceing just adjusts it. I can also clamp my back in the dummy guitar if need be. If you screw up the top in a dummy guitar just toss it and learn from it. If you had done that with the guitar already built you would have to rout the top off! Wouldn't you like to know ahead of time how it is going to sound prior to gluing it to the guitar?

Hey David,
Got any pics? Sounds like a great idea.

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Jim Watts
http://jameswattsguitars.com