title: effect of normalizing on mel-spectrogram
body: <p>what is the effect of normalizing (using mean and std of each channel) on the mel-spectrogram?
  this is a image of <a href="https://i.stack.imgur.com/EBZS9.png" rel="nofollow noreferrer">mel-spectrogram with 20 mel filters and 206 frames</a> and this is a <a href="https://i.stack.imgur.com/i2C1D.png" rel="nofollow noreferrer">normalized mel-spectrogram with same shape</a>. we see that some properties appeared after normalizing the mel-spectrogram.
  does it remove some properties from the original mel-spectrogram?</p>