Vibration is a continuous process, but we only acquire a data ‘snap shot’ over a short period of time. We assume that if the ‘snap shot’ sample were duplicated and linked together they will make a ‘movie’ and resemble the real world continuous signal from which we can calculate an FFT.

However whatever period we chose as the ‘snap shot’ duration it is unlikely that we will capture full complete cycles of all frequencies. This means that there will be ‘glitches’ in the ‘movie’ at the joins and an FFT will generate spurious spectral lines as a result. To remove this effect the beginning and ends of the ‘snap shot’ are mathematically brought to a zero slope they will match more smoothly and produce a more meaningful FFT. This mathematical function which produces this effect is termed ‘windowing’. There are different forms of the weighting curve, and go by the names given by their designers - Hanning/Hamming, Kaiser-Bessel or Flattop.