# RMS values in FFT-plot

Joacim Rådstam

Posted on 07.04.2011 12:51
How is the total RMS calculation defined for the FFT-plot in version 7.0.1? I understand that the first value is calculated for the entire frequency range and the value enclosed by () is calculated for the frequency range currently showing in the plot.
Dejan Cencelj

Posted on 12.04.2011 08:46
[img]http://sk-zalec.org/images/stories/image001.png[/img]
Joacim Rådstam

Posted on 14.04.2011 08:25
I'm unfamiliar with this formulation so I have some additional comments. It would be nice if you could give an online reference for this formulation. Why is the first and last value doubled, x and x[n-1]? The window correction factor for the Hanning window is correct (1/sqrt(1.5)) for the special case of a frequency resolution of 1 Hz. But that's rarely the case, I believe it should be dependent on the resolution df = (sampling frequency)/(window length). The factor for the Flat top window is incorrect. Should be 1/sqrt(3.77) = 0.515 for df = 1. (Using the window defenition in ISO 18431-1.) I really recommend these articles I used for theoretical background information: http://www.bksv.com/doc/bv0031.pdf http://www.bksv.com/doc/bv0032.pdf