modelfrf, experiment, naturel frequecies, magnitude

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When I plot the experiment data with modalfrf, the units on the vertical axis are not correct. I gave the result for the experiment with figure 1 and the numerical results with figure 2. I think figure 2 is correct and figure 1 is wrong. Where is my mistake?
B1=xlsread('excel.xlsx',1);
x = B1(1:10000,3:3);
f = B1(1:10000,4:4);
dt = 0.0001;
Fs = 1/dt;
N = 10000;
t = dt*(0:N-1);
wind = hann(N/2);
[frf,f] = modalfrf(f,x,Fs,wind);
plot(f,20*log10(abs(frf)))
  2 Comments
Gloria
Gloria on 18 Jan 2021
Thanks for reply. two figure must be match. The values of x axis true bu obviously I have a mistake when I change displacements which are time depentt to dB.

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Accepted Answer

Mathieu NOE
Mathieu NOE on 18 Jan 2021
hello
I plotted the time data for channels combos 3/4 (and 1/2)
both shows single (low) frequency sinus type data - so this is not what I expected for a modal frf - I was expecting to see random, random burst , sine dwell (chirp) , but not a lonely fixed freq sinus
not a surprise that the computed frf is completly misleading
also the amplitude of x and f are of very different orders of magnitudes : x is in the 10^-5 and f in the 10^5 , so a ratio like this cannot give a frf modulus like the "good" figure 2 would show between 10^-7 and 10^-4.
  7 Comments
Mathieu NOE
Mathieu NOE on 19 Jan 2021
you have to introduce some wide frequency range excitation signals , like random (white noise ) or chirps , but do not rely on "operationnal" background noise or narrow band signals like sines to get accurate models
have a good day

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