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## Main Question or Discussion Point

Hi, I need some help with an experiment:

I have a cantilevered steel beam, fixed at one end and free at the other.

When I excite the beam with forced vibration at the fixed end, resonant frequencies are evident. The first is at 10 Hz and the second is at 60Hz.

Performing analytic calculations, the first two natural frequencies are indeed 10Hz and 60Hz.

When I do a bump test (give the beam an initial displacement and use an accelerometer to measure its subsequent natural vibrations) the peak vibration frequencies occur at 7Hz, 20Hz and 33Hz (I view the accelerometer data in the frequency domain)

Why is there this difference? Is it becuase of what the accelerometer is measuring? Is it because of where the accelerometer is positioned on the beam? Is it because of the way the Fourier transform is performed?

Any help ASAP would be much appreciated.

I have a cantilevered steel beam, fixed at one end and free at the other.

When I excite the beam with forced vibration at the fixed end, resonant frequencies are evident. The first is at 10 Hz and the second is at 60Hz.

Performing analytic calculations, the first two natural frequencies are indeed 10Hz and 60Hz.

When I do a bump test (give the beam an initial displacement and use an accelerometer to measure its subsequent natural vibrations) the peak vibration frequencies occur at 7Hz, 20Hz and 33Hz (I view the accelerometer data in the frequency domain)

Why is there this difference? Is it becuase of what the accelerometer is measuring? Is it because of where the accelerometer is positioned on the beam? Is it because of the way the Fourier transform is performed?

Any help ASAP would be much appreciated.