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I glued my back to the rim a week or two ago and it seems the main frequency dropped by about 3 1/2 steps (roughly 35 HZ). If I would have glued the top on first, would it have dropped a similar amount?

I realize, from reading Al's posts and others, that the final top frequency is dependent on other factors like coupling with the main air frequency but I wanted to know if this same affect would have happened if I had chosen to glue the top first.

For the record, my back dropped from 242 HZ to about 207HZ which is a G#. If my top dropped similarly, it would be about an F#.

Slightly off topic, but the humidity inside my home has been in the low 30% range. When my top dries this much it doesn't have the nice ping it does at higher humidity. Last night I placed my top and rim outside on the back porch for 3 or 4 hours where the humidity is nearly 70%. When I brought them inside the back/rim fit my 16ft radius perfectly again and the top rang loud and clear again. I didn't realize humidity affected the tuning this much.

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Formerly known as Adaboy.......

Hummm???? If refering to the freq the back was at when loose compared to attached keep in mind that when loose the back was a free plate. Once glued to the rim it is no longer a free plate.

The order of gluing should not make any difference.

Again, there is no easy way to predict the exact assembled mode frequencies from the free plate mode info: too many variables.

Thanks for trying to answer my post......which was worded poorly. I'll try asking again, but it may be hopeless. Would the top have dropped 3 1/2 steps if I had glued it to the rim first?......or is there any consistancy to how much the frequency drops when glued to the rim (only one plate glued to rim)?

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Formerly known as Adaboy.......

Darryl, you need to remember that this is a system. You can tune a top, back or whatever else you want and when you glue them all together they are something completely different.

If you are following someone's instructions and wonder why it is not working out the way they say it will, direct the question to them.

If you are asking why a plate changes tap tone when glued to another structure, imagine a tuning fork and what note it rings. Now glue the teeth to a garbage can. Should the resonance of the tuning fork stay the same, go up, or go down? Does it matter?

The point is that the resonance will change when you add or remove mass. As for how much the resonance will or should change when adding the mass depends on many factors: stiffness of plate, stiffness of sides, mass of both, is Darth Vader really Luke's father...
You can see that there are many factors at play here.

Are you an engineer.. by chance?

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Chris Oliver
Infinity Luthiers
...in the shop.

live every minute...

Darryl Young asked:
"Would the top have dropped 3 1/2 steps if I had glued it to the rim first?......or is there any consistancy to how much the frequency drops when glued to the rim (only one plate glued to rim)?"

It's impossible to say: you're trying to compare apples (free plates) to oranges (plates glued to something). Yes, the siffness and mass of the free plate, and how those are distributed, do determine the resonant pitches of the assembled structure to some extent, but the mass and stiffness of the other parts comes into play as well. It's just too complicated to sort out without some fairly heavy-duty computer programs.

It's my opinion, FWIW, that the exact pitches of things, at ANY stage, are less important than other things. In the 'free' plates I pay much more attention to the shapes of the modes than the frequencies, so long as the frequencies are not too weird. Once tyhe thingf is together (and ALL together) it's the relationships between the mode frequencies that seems to count for more, and those don't have to be exact. For example, getting the 'main top' and 'main back' modes close in pitch to each other tends to maximise the strength of the coupling between them. This lowers the pitch of the 'main air' mode, and makes the 'bass reflex couple' stronger. This is good up to a point, but if the modes are too close together in pitch it can give you real problems. I talk about getting the 'main back' mode a semitone higher than the 'main top', but that's just a guidline, and it caqn be a full third higher and still be helpful.

In the end, the most important thing for you is to figure out what you like. Unless you're trying for some particular thing to see how it works, don't get too worked up about it. Just take notes, keep good records, and try to figure out over the next few builds what works for you. It might be something totally different from what I like. There are lots of good ways to make guitars.

I agree completely!

Now as for the resonant frequency of the back actually dropping when gluing it up...that's surprising to me. Anytime you restrain the edges of a plate you are making it stiffer, which should raise the frequencies of vibration. I would also expect - and will pay attention next time I glue a top up - that the resonant frequency of the back would be raised again by gluing the top on. It seems like this partially traps the volume of air inside the box which should act to effectively stiffen the top and back plates.

As an aside, here's a good article on Helmholtz resonance:
http://www.phys.unsw.edu.au/jw/Helmholtz.html

Best,
Trev

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