During my experiment (vibration analysis), I deliver a knocking force from a shaker (and the shaker itself has an impulse or an half-sine wave signal). The system automatically calculates with the FFT algorithm and gives me the following graph for the input force signal. Note that I was investigating different parameters, and this is not the proper experiment yet. Though the input signal would be probably kept the same for the main experiment.

enter image description here

My question is that: why do we spikes here, in the frequency spectrum?

Thanks for your help.

  • $\begingroup$ What's you x-axis? time or frequency ? what are the units ? $\endgroup$ – Hilmar Jan 18 at 15:59
  • $\begingroup$ Is 50Hz your local power line frequency? $\endgroup$ – TimWescott Jan 18 at 18:43
  • $\begingroup$ @Hilmar x-axis is frequency, and it's in Hz $\endgroup$ – ComradeH Jan 19 at 9:31
  • $\begingroup$ @TimWescott Yes, the local frequency is 50 Hz. But I don't think it is the reason. I do not measure the AC here $\endgroup$ – ComradeH Jan 19 at 9:33
  • 1
    $\begingroup$ If you're measuring something that's high-impedance, has a lot of area, is low-level, or is connected to big power -- yes, you're measuring the local AC there. You pretty much cannot avoid measuring your local AC, unless you hike out to the wilderness with a bunch of battery-powered equipment. Grab an oscilloscope, touch the probe, and look -- you'll see that you're measuring AC. $\endgroup$ – TimWescott Jan 19 at 16:20

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