FFT amplitude units

[evinouba]

Hello

I very recentrly start using Diadem 2007 and I am interested to create a spectrum of an accelerogram. That is, I have the acceleration m/s2 (sample values N=11999) acquired every 0.0025s.

Using the FFT(one time signal) I choose the peak amplitude but I can understand the Y-axis results (Fourier amplitude), i mean the units! Is there is a scale being done, how could i convert them to read the same values as in the time domain.

thank you

evina

[Brad_Turpin]

Hi evina,

DIAdem 2015 automatically copies the Y channel's unit from the time-based signal to the FFT amplitude channel.  There was actually no "DIAdem 2007", so I'm curious what version of DIAdem you're really using... DIAdem 9.1?  DIAdem 10.0?  DIAdem 10.1?

Regardless, it sounds like you may be stuck copying over that Y unit yourself in the version of DIAdem you have at hand.

Brad Turpin

DIAdem Product Support Engineer

National Instruments

[evinouba]

Hello and thank you for your response

Yes I am sorry, I am using Diadem 10.2. But I think that it normalized the Y-axis values.

Am I correct?

Thank you again

[Brad_Turpin]

Hi evina,

There are a variety of configuration choices you have in the DIAdem FFT dialog for the Y axis scaling of the FFT amplitudes:

Peak Amplitude

RMS Amplitude

Power Spectrum

Auto Spectrum

Power Density Spectrum

None of those affect the unit of the Y axis of the FFT amplitude, but they certainly lead to different Y axis values.  What do you mean by "normalized"?

Brad Turpin

DIAdem Product Support Engineer

National Instruments