1st try to visualise vibration modes

[ScottC.]

Well I gave it my first try with my A. Picado classical guitar.
1. I had a hard time pinning down the exact frequency.
2. T1 came in at 97-98Hz
3. T2 came in at 197-199Hz
4. T3 came in at 230Hz when measured back side up, I could really hear it resonate when it hit 230Hz, top up it was not clear. Is the back or top supposed to be up to measure the back? The guitar has only three ladder braces on the back and that's it.
Any ideas or suggestions welcome.
Thanks, Scott C.

[Trevor Gore]

What you see on the back at 230Hz is the B(1,1), because you are exciting the back directly. The T(1,1)3 is a response seen in the top due to a particular way it couples with the back, hence many of us rather lazily calling it the back response. The T(1,1)3 can be quite strong with a high compliance back, but a guitar is unlikely to be like that unless specifically designed to be. So more often than not the T(1,1)3 is quite a weak response, tending to non-existent with a non-live (stiff, heavy or both) back. Normally the T(1,1)3 will be found at a frequency close to, but usually slightly higher (due to coupling repulsion) than the B(1,1). Of course, you can have a B(1,1) without a T(1,1)3 if the coupling is not very strong.

[ScottC.]

Thanks, would a back with only 3 ladder braces live?
How do I narrow down the frequency for the T1 & T2?
Scott C.

[Trevor Gore]

Thanks, would a back with only 3 ladder braces live?

Impossible to say without knowing Young's modulus for the back, its thickness; height and width of the bracing and their Young's moduli, how they're shaped etc.. The Design book is all about getting that sort of thing in your chosen ball park.

You can tell if you have a live back on a built guitar by checking to see if you have a T(1,1)3 in the tap response. And, of course, any peak you see in a tap response you should be able to find a Chladni pattern for.

How do I narrow down the frequency for the T1 & T2?

Assuming you have a sig gen with the frequency resolution, it's down to how well you can pick "max bounce" of your poppy seeds, leaves etc. Alternatively, tapping usually gives you very repeatable peaks.

[ScottC.]

Thanks, I'll try the tapping tomorrow.
Scott C.

Thanks, would a back with only 3 ladder braces live?

Impossible to say without knowing Young's modulus for the back, its thickness; height and width of the bracing and their Young's moduli, how they're shaped etc.. The Design book is all about getting that sort of thing in your chosen ball park.

You can tell if you have a live back on a built guitar by checking to see if you have a T(1,1)3 in the tap response. And, of course, any peak you see in a tap response you should be able to find a Chladni pattern for.

How do I narrow down the frequency for the T1 & T2?

Assuming you have a sig gen with the frequency resolution, it's down to how well you can pick "max bounce" of your poppy seeds, leaves etc. Alternatively, tapping usually gives you very repeatable peaks.