chnfft2 Calculate average from command

[Nacho89]

Hi,

I have some doubts about the Chnfft2 "calculate average from" command. When I read the Help of this command, it says: "DIAdem first calculates the amplitudes in the specified amplitude type and then calculates the average, third/octave with these amplitudes"

What I don't understand is if DIAdem calculates the average and then automatically makes the third or octave of the averaged signal. If it is like that, When it makes the third or when it makes the octave analysis and why it chooses one or another.

Finally, does anybody know where can I read some interesting documentation about the third/octave analysis and how DIAdem makes it?

Thank you

[Otmar]

Hello Nacho,

There is some background information about the FFT in this document. I am not sure if it covers your question, but it might be a worthwhile read:

http://forums.ni.com/t5/DIAdem/FFT-calculations-in-DIAdem/m-p/1071519#M11524

For your specific question, I'll defer to one of our R&D experts to answer.

    Otmar

[Nacho89]

Hello Otmar,

Thanks for the documentation about the FFT, it helped to me to understand some stuff about the FFT in DIAdem.

Concerning to the ChnFFT2, I have finally understood how it averages signals. First of all, you must decide with Frequency response function you want to use. In my case, I have used the H1 function. The use of this function allows you to avoid the possible numerical error of the H0 FRF if the denominator reach zero at some point. If you have noise in your signal, H2 is the recommended FRF. 

Averaging when using the H1 FRF means averaging the numerator and denominator separately and then make the division. Thus you have the peak-amplitude FRF.

Finally you must decide which type of amplitude do you want to have and make the appropiate conversion ( to power spectrum or to RMS amplitude for example ).

Regards.

Ignacio Medina