title: Librosa : MFCC feature calculation
body: <p>Given a audio file of <code>22 mins (1320 secs)</code>, Librosa extracts a MFCC features by 
  <code>data = librosa.feature.mfcc(y=None, sr=22050, S=None, n_mfcc=20, **kwargs)</code></p>
  
  <blockquote>
    <blockquote>
      <p>data.shape
      (20,56829)
      It returns numpy array of 20 MFCC features of 56829 frames .</p>
    </blockquote>
  </blockquote>
  
  <p>My question is how it calculated 56829. Is there any calculation to achieve this frame ? and What is the window size for each frame ? </p>