import pyfar as pf
import numpy as np
import matplotlib.pyplot as plt
center_frequency = 1000
num_fractions = 1
octave_filter = pf.dsp.filter.fractional_octave_bands(
    None, num_fractions, 44100,
    (center_frequency, center_frequency))
lower, upper, frequencies =                 pf.constants.fractional_octave_filter_tolerance(
        center_frequency, num_fractions, tolerance_class=1)
ax = pf.plot.freq(octave_filter.impulse_response(2**14),
                  color='k', label='Octave filter')
ax.fill_between(
    frequencies, lower, upper, facecolor=pf.plot.color('g'),
    alpha=.25, label='Class 1 Tolerance region')
ax.set_ylim(-40, 5)
ax.set_xlim(.5*center_frequency, 2*center_frequency)
ax.legend()