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Full Version: what does FFT mag (a.u) means ?
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[attachment=382][attachment=382][attachment=382][attachment=382][attachment=382][attachment=382][attachment=382][attachment=382]hello there
iam a undergraduate student of mechanical engineering from india

for validation of my experimental data iam using phyphox accelerometer spectrum in which
a graph is plotted between FFT mag (a.u) and frequency

iam not able to understand the y axis data
can any one please tell me how can i convert it in db(decibels) or any other common unit magnitude of frequency

i do understand that a.u means arbitrary unit
but what does it signifies?
can it converted to decibels? if yes how, because i have never read about arbitrary unit
i have to quote in a research paper
please suggest any other way for understanding this
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…
(07-05-2021, 08:32 PM)Jens Noritzsch Wrote: [ -> ]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…
ok i think i now able to understand this little bit
it would help me a lot if you please mention some place where i can read more on this
thankyou for you generosity and help sir.
I have seen that you also commented on the final video of our spectra series. Have you watched the second part that goes a bit into detail on Fourier series (Fourier transformations could be understood as a generalisation for non-periodic functions)?

Textbooks on oscillations should typically cover Fourier series and transformations. I personally prefer physics over math introductions in this topic as they are more “vivid”. For a start, Wikipedia should do, too.

… or have I missed the point where you need clarification?