This forum uses cookies
This forum makes use of cookies to store your login information if you are registered, and your last visit if you are not. Cookies are small text documents stored on your computer; the cookies set by this forum can only be used on this website and pose no security risk. Cookies on this forum also track the specific topics you have read and when you last read them. Please confirm whether you accept or reject these cookies being set.

A cookie will be stored in your browser regardless of choice to prevent you being asked this question again. You will be able to change your cookie settings at any time using the link in the footer.

what does FFT mag (a.u) means ?
#1
Exclamation 
                                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 
Reply
#2
Do perhaps these threads, https://phyphox.org/forums/showthread.php?tid=1193 or https://phyphox.org/forums/showthread.php?tid=1217, answer your questions?
Reply
#3
(07-05-2021, 03:34 PM)Jens Noritzsch Wrote: Do perhaps these threads, https://phyphox.org/forums/showthread.php?tid=1193 or https://phyphox.org/forums/showthread.php?tid=1217, answer your questions?

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 
so i have to be sure about this 
please suggest any other way for understanding this
Reply
#4
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…
Reply
#5
(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…
Heart 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.
Reply
#6
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?
Reply


Forum Jump: