If you are using a square wave to drive the beam, you are probably
getting resonance at the harmonics in the square wave (at odd multiples
of the fundamental).  As you increase the frequency you would get a
smaller resonance at the third, fifth, seventh, etc. harmonic.  So, for
example, if the beam were resonant at 1000Hz, you would get a response
at a fundamental of 143Hz, 200Hz, 333Hz, etc.


 On Thu, 11 Feb 1999, Chua Bee Lee 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