title: MATLAB fft y axis meaning
body: <p>I have a time varying signal (a), which I take the fft of. I need to multiply a frequency dependent weighting factor by the fft's y axis value; however if I do:</p>
  
  <pre><code>xdft = fft(a);
  xdft = xdft(1:length(x)/2+1); % only retaining the positive frequencies
  freq = Fs*(0:(L/2))/L; 
  </code></pre>
  
  <p>And plot(freq,xdft) I get a peak fft value (y axis)of ~2000 at the correct frequency of the signal. But original signal peak value (amplitude) was ~46. I need to know how the numbers relate so I can weight the fft values.</p>