These could be superharmonics of the natural resonance vibration modes. To
avoid nonlinear vibrations you should use lower dirving forces and/or
change  the design.

At 21:34 11.02.99 +0800, you wrote:
>Dear All,
>       I have been working on folded beams resonators using square waves
>to drive them. As I adjusted the driving frequency towards the resonance
>value, I observed resonance at smaller amplitudes  at lower frequencies
>prior to the one that I was expecting. I would appreciate if someone can
>provide some insight on that.
>
>       Thank you.
>       CHUA Bee Lee
>       Department of Mechanical and Production Engineering
>       National University of Singapore
>
>
>

        Dr. Roumiana Paneva

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