code: def mel(sr, n_dft, n_mels=128, fmin=0.0, fmax=None, htk=False, norm=1):
      
      return librosa.filters.mel(sr=sr, n_fft=n_dft, n_mels=n_mels,
                                 fmin=fmin, fmax=fmax,
                                 htk=htk, norm=norm).astype(K.floatx())
docs: [np] create a filterbank matrix to combine stft bins into mel-frequency bins
      use Slaney (said Librosa)
  
      n_mels: numbre of mel bands
      fmin : lowest frequency [Hz]
      fmax : highest frequency [Hz]
          If `None`, use `sr / 2.0`