what does FFT mag (a.u) means ?

[Jens Noriʇzsɔɥ]

I hoped that these questions would be already answered in the threads: “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.” and “The Fourier transform of the recorded acceleration gives you an impression of the relative intensities of frequencies, that's why it is given in arbitrary units (a.u.). If you put your smartphone on a washing machine rotating at 1200 rpm, you would see a sharp peak at 20 Hz, for instance.” (supposing you know what an FFT is)

Anything could be converted to decibels: at the end it is just a relative data unit for a logarithmic scale. So, if you define a reference value in a (best) fixed frequency experiment you could invent your own decibel scale…