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Hi there, 

I am not quite sure about the units in the Fourier Transform. What does a.u. stand for? The only a.u. that I know of is A.U., astronomical units... But I doubt that they are the same.

Thanks for your time and the app!

Hi José.
a.u. - arbitrary units. It is when you do not know well units of the original signal. For example for sound when the microphone is not calibrated. When we know the original signal we may use corresponding units. For example for the accelerometer the unit is m/s², so FFT spectrum should have unit m/s² * s = m/s ...
a.u. nevertheless is always valid :o)
Thank you for answering this, Mikhail. Something has apparently stopped me while preparing in answer that has been pretty much along your lines…

Fourier transforms are not uniquely defined and may vary by a constant factor, so attributing a definite unit to the result does not make much sense.
Oh! I understand. Thank you for your quick response!