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Very hesitant to dive deeply in the Gilet/Gore books (don't want to afford them right now either) or theory but can understand how the frequencies of the plates could effect the ultimate tone of a guitar. Yes, looking for any easy way out of the calculations and heavy science involved but I do think it would be fairly easy to measure the frequencies of both back and top before assembly as I have both microphones and software to analyze such things.

Is there any merit to doing so? Or much the guitar be measured more as a whole before readings can be ascertained? Another motivation is to understand the theory a little better then ultimately use my own senses to do the tuning independent of exacting measuring devices.

Wow I've been doing some reading regarding the above subject and the info is all over the map with, to me, the most poignant advice being, build a lot if guitars and get a feel for what works for you, either a strict science of plate resonance or the feeling you get in your nut **** that tells you the guitar is what you and it are looking for.

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Thank You and Best To All

No the free plate frequencies do not tell you anything about the final frequencies because the support conditions change so dramatically to fully supported around the edge.

I have the books but haven't spent a lot of time with them - so a little knowledge...

From what I think I understand, Trevor doesn't tune plates before assembly besides thicknessing them to predetermined value based upon measured mechanical properties of the plates and dimensions of the sound box. When the guitar is assembled, you have a coupled system - all of the components are hooked together & affect one another, including the air mass inside the box.

On the other hand, I believe Alan Carruth does tune his free plates & he has a DVD about it.

Hope this helps.

Kevin Looker

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I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

This is not exactly true, the free plate frequencies can allow one to determine the flexibility and density and predict results. If you record these values you can create similar plates on later builds, or modify to stiffer or more flexible plates. I record the taps at each step of assembly and have a spread sheet created that I store the frequencies in this is becoming more valuable as my data base gets larger. It works for me as different methods do for other builders, is it right or wrong I don't know, but so far the results seem promising. After only 19 guitars I am still learning a lot on each build and the methods are evolving.

Fred

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Fred Tellier
http://www.fetellierguitars.com
Facebook page http://www.facebook.com/pages/FE-Tellier-Guitars/163451547003866

Good advice Fred, I got all the gear to record - I'll see if I can get that to work, in some way, for me.

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Larry,
Because you are dealing with some mighty different variables, measuring the top hz before bracing might not be the tonal magic 8 ball that you want it to be.

I have heard of at least one noted luthier who will run a chladi test on all unbraced tops.

Those that close the ring at a size of a dreadnought will become a D.
But if it doesn't close the ring, then the top is cut to the next smaller sized top and retested.
If the resonant ring closes, it becomes an OM. if, not the top is cut to the next size smaller and tested again.
In this case, the resonance of the unbraced top, used with chladni patterns, guides the luthier to know if the stiffness of the top is equal both with and across the grain. Without the cladni test, you will know resonant hz, but not how the top acts at a given hz.

All of this can be measured and tested with free software, a speaker and an amp.

But you said you didn't want to get into the science that heavily.


Before I dabbled in the chladni science, I did attempt to brace the top and back to match hz. Then when the box was closed, the bridge would bring the top down in hz far enough to couple the two plates wonderfully.

If they are too close or too far apart, the tone can be weird. Just right and you have something that sounds right nice.


If you are at the point where you are curious about and ready to use technology to help your builds, I highly recommend doing so. Just a little more info and reading will pay off with your understanding of how wood works and how you should work the wood.

Read that book. Then after youve built a whole bunch of guitars, you will have a much better idea than had you not taken the extra steps.

Good luck,

Dave

As usual, this gets complicated.

In theory, if you could, say, build a really complete mathematical model of the guitar, then it would be possible to use it to predict the final resonant frequencies of the assembled instrument from the free plate frequencies. You'd need to input an insane amount of information about every piece. In practice, it's quicker just to build guitars.

If you make two guitars that are exactly the same in every respect, including the free plate mode frequencies, then the lower order modes of the assembled guitars will have the same frequencies. In this case, small differences in free plate mode shapes will translate into subtle differences in the sound. So far I have not had the time to build enough pairs to get two that had the same free mode shapes as well as frequencies, so I can't be absolutely sure, but I suspect you'd end up with 'identical' sounding guitars.

Two guitars of the same size and shape, with matched tops, but different B&S materials, will end up with somewhat different assembled modes, due to differences in the wood properties. Bigger changes make bigger differences.

'Free plate tuning' is thus something of a misnomer, but we're sort of stuck with it. It's more like tuning up a car engine than it is like tuning your guitar strings. What we're tuning is not so much the pitches of the plates, with a view toward ending up with some predictable finished mode frequencies. Rather, in my mind, we're establishing a sort of 'harmony' of stiffness and mass distribution that will allow the plates to work to their maximum potential within the design parameters. A top with scalloped braces will tune differently in some respects from one with tapered or straight bracing; the mode pitches will be different, and will bear different relationships to each other depending on a bunch of factors. What seems to carry over is that tops with 'good' mode shapes that are well defined and have reasonably high Q values tend to end up making 'better' sounding guitars, even though they will sound different depending on all the variables.

Now, as has been said, the mode pitches do tell you something about the relationship of mass and stiffness in a plate. Lightly built tops have modes that come in at lower pitches, and with some experience you can tell when things are starting to get dicey. It works the same the other way, of course: a top that had modes that are very much higher than your normal one is likely to be too stiff, unless it happens to be really light. Over time you get to know what the mode pitches are telling you.

Free plate tuning is a technical development from tap tuning. We're a long way from being able to do the science to be able to call it 'scientific' even in a loose sense. If you wanted rigor, you'd have to be able to specify in advance what 'good' sound is. Good is a value judgement, and not amenable to pre-definition. We have to work from good sound backwards, and the free plates are several long steps back.