FFT

Hi,

I am trying to perform FFT for ambient vibration data and My code calculates such an absurd value like 6.2172e-17 for the first value of Fourier Amplitude Matrix. However, other amplitudes are reasonable and if I plot these amplitudes in loglog figure, I get meaningless plot.

for j=1:3

      a{j}=textread(names{j},'%f');

      a{j}=detrend(a{j},'constant');
      a{j}=detrend(a{j});

      if ((60*60)/length(a{j}))==0.02

      A{j}=fft(a{j},2^nextpow2(length(a{j})));

      A1{j}=abs(A{j}(1:2^nextpow2(length(a{j}))/2))*0.02;

      %A1{j}(1)=A1{j}(2);

      Fs=50;

      f=Fs*(1:2^nextpow2(length(a{j}))/2)/(2^nextpow2(length(a{j})));

      figure (j)
      loglog(f,A1{j},'b');
      title(names1{j});
      xlabel('Frequency (Hz)');
      ylabel('Fourier Amplitude');

      elseif ((60*60)/length(a{j}))==0.01

      A{j}=fft(a{j},2^nextpow2(length(a{j})));

      A1{j}=abs(A{j}(1:2^nextpow2(length(a{j}))/2))*0.01;

      %A1{j}(1)=A1{j}(2);

      Fs=100;

      f=Fs*(1:2^nextpow2(length(a{j}))/2)/(2^nextpow2(length(a{j})));

      figure(j)
      loglog(f,A1{j},'b');
      title(names1{j});
      xlabel('Frequency (Hz)');
      ylabel('Fourier Amplitude');

      end

end

The cell array are assigned in order to perform 40 FFT calculations for 40 different data texts. What makes Matlab calculate such an absurd value?

Answer 1

[Dr. Seis]

Your first amplitude represents 0 frequency. When your data has 0 mean, your first amplitude should also be 0.