title: How to make a multiple notch filter in MATLAB?
body: <p>I'm designing a filter to remove the noise from a piece of audio.  I found the noise frequencies are located at 745 and 1965 Hz in the spectrum but I don't know how to make a multiple notch filter to remove these two specific frequencies </p>
  
  <p>This is my code. I can only remove one frequency in the audio. Is there any way to make convolution of two filters in MATLAB?</p>
  
  <pre><code>%Reading first sample file
  [x1,fs1] = audioread('sample.wav');
  
  %Creating the time span for the file
  t1=(0:length(x1)-1)/fs1;
  
  %Creating the frequency span for the file
  k1 = 0:length(x1)-1;
  f1=k1*fs1/length(x1);
  wg=[744.5*2/fs1 745.5*2/fs1 ];
  
  %Creating  filter
  [b1,a1] = butter(2,wg,'stop'); 
  
  %Performing filtering on file
  x1f = filter(b1,a1,x1);
  </code></pre>