Dicrepancies in FFT values at lower frequencies

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Murali Krishnan 2023-11-22

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https://ww2.mathworks.cn/matlabcentral/answers/2050762-dicrepancies-in-fft-values-at-lower-frequencies

评论： Mathieu NOE 2023-11-27

I have been trying to compare the time domain and frequency domain results from Matlab and ansys for a vibrating system. Although the values agree generally, at lower frequencies there appears to be a significant difference.Especially at 0, the amplitude varies significantly (in the time domain data,at time 0, both systems had the same amplitude). Why would that be? I have exported data from excel sheet for this analysis.

%Import time domain data from  excel sheet
file = '....nt.xlsx';
sheet= '...';
data = xlsread(file,sheet);
time = data(:,1);
mat_amp = data(:,2);
ans_amp= data(:,3);
%% Frequency domain plot
L = length(time);
n=2^nextpow2(L);
Ts = time(2)-time(1);
Fs = 1/Ts;
f= Fs*(0:(n/2))/n;
ft_mat = fft(mat_amp,n);
M2 = abs(ft_mat/n);
M1 = M2(1:n/2+1);
M1(2:end-1)=2*M1(2:end-1);
ft_ans = fft(ans_amp,n);
A2 = abs(ft_ans/n);
A1 = A2(1:n/2+1);
A1(2:end-1)=2*A1(2:end-1);
figure(2)
plot(f,abs(M1),'-*',f,abs(A1)) ;
title("Frequency Response-Beam with Mass")
xlabel("f (Hz)")
ylabel("Amplitude")
grid on;
legend('Matlab', 'Ansys');
figure(3)
plot(f,20*log10(abs(M1)),'-*',f,20*log10(abs(A1)));
title("Frequency Response-Beam with Mass")
xlabel("f (Hz)")
ylabel("Amplitude in dB")
grid on;
legend('Matlab', 'Ansys');