code: def gammatone(freq, bandwidth):
    
    bw = thub(bandwidth, 1)
    bw2 = thub(bw * 2, 4)
    freq = thub(freq, 4)
    resons = [resonator.z_exp, resonator.poles_exp] * 2
    return CascadeFilter(reson(freq, bw2) for reson in resons)
docs: ``A. Klapuri, "Multipich Analysis of Polyphonic Music and Speech Signals
      Using an Auditory Model". IEEE Transactions on Audio, Speech and Language
      Processing, vol. 16, no. 2, 2008, pp. 255-266.``