import pyfar as pf
import matplotlib.pyplot as plt
impulse = pf.signals.impulse(40e3, sampling_rate=40000)
impulse, N, ideal = pf.dsp.filter.low_shelf_cascade(
    impulse, 4000, "upper", -60, None, 4)
pf.plot.freq(ideal, c='k', ls='--', label="ideal")
pf.plot.freq(impulse, label="actual")
plt.legend()