def erb(freq, Hz=None):
  """
    ``B. C. J. Moore and B. R. Glasberg, "Suggested formulae for calculating
    auditory filter bandwidths and excitation patterns". J. Acoust. Soc.
    Am., 74, 1983, pp. 750-753.``
  """
  if Hz is None:
    if freq < 7: # Perhaps user tried something up to 2 * pi
      raise ValueError("Frequency out of range.")
    Hz = 1
  fHz = freq / Hz
  result = 6.23e-6 * fHz ** 2 + 93.39e-3 * fHz + 28.52
  return result * Hz
``B. C. J. Moore and B. R. Glasberg, "Suggested formulae for calculating
    auditory filter bandwidths and excitation patterns". J. Acoust. Soc.
    Am., 74, 1983, pp. 750-753.``