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Question

I have my data obtained from A Resonant Frequency Testing using impact hammer. The time domain waveforms from both impact and acceleration responses were processed using signal processing software. To get the Frequency Response Function (FRF), the fourier transform was preformed in the signals, but this data is the real part not the imaginary part. So, what I need to do is to plot the mode shapes (Normalized amplitude vs. Length of the specimen) by using the imaginary part of the transfer function. Can I get your help please to get the imaginary part from the real part of my data using Excel?

Fernando Z. · Accepted Answer

Hi Bruno.  If I understood correctly:  Raw Data -> Processed Data -> FRF -> Excel with only real parts  It means, if you have acess only to the ''Excel with only real parts'' it will be impossible to extract the imaginary parts as Prof. Marcos explained.  You must have some of the predecessors to apply FRF to obtain the real and imaginary parts. Indeed, I would ask why do you want real and imaginary data to plot instead of absolute value and phase (e.g. Bode Diagram).  You should also be very careful with discrete Fourier Transforms, if you use them on a computer, you must pay attention to the sample time if you use a FFT algorithm or something like this.

Marcos F. · Answer

Hi Bruno.  You can use these functions provided in Excel:  - COMPLEX( , )  -Note: you may need to change the width of the column in order to see the numbers-  - Built-in functions:  - IMDIV(a,b),  - IMSUM(a,b), - IMPRODUCT(a,b),  - IMPOWER(a,n) to perform complex division, sum, product and power operations on complex numbers a and b, with n as integer.  More details: http://pages.mtu.edu/~tbco/cm416/freqexcel.html  Regards.

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