code: def erb(freq, Hz=None):
    
    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
docs: ``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.``