Units for Magnitude after Fast Fourier Transform (FFT) is done

Hi all,

I am currently doing vibration analysis for Air Compressors.

I have collected the vibration data signals using accelerometers which gives acceleration units in m/s^2.

My query is that I do not know what is the unit of the Magnitude in the frequency domain plot through the followings codes;

X1 is my Magnitude axis (Y-axis) and f1 is my frequency axis (X-axis).

Any form of help to answer my question on the magnitude is appreciated. Thank you.

Answer 1

[Star Strider]

The magnitude units are the same as the time-domain amplitude units, unless you transform them in some way (such as squaring to diisplay power). However the fft function splits the spectrum into negaitive-frequency and positive-frequency regions (most easily seen using fftshift as you have done), and this divides the energy of the signal so that half the amplitude appears in the negative -frequency region and half in the positive-frequency region. So the true amplitude is 2 times the displayed amplitude.