import pyfar as pf
signal = pf.signals.impulse(4096)
signal = pf.dsp.filter.bell(signal, 2e2, -20, 3)
signal = pf.dsp.filter.bell(signal, 6e3, 20, 3)
upper = pf.dsp.soft_limit_spectrum(
    signal, limit=10, knee=6, direction="upper")
lower = pf.dsp.soft_limit_spectrum(
    signal, limit=-10, knee=6, direction="lower")
pf.plot.freq(upper, label='upper limit')
pf.plot.freq(lower, label='lower limit')
ax = pf.plot.freq(signal, ls='--', label='original')
ax.set_ylim(-25, 25)
ax.legend()