[SciPy-User] butterworth filter on .WAV file

Peter Howard peterhoward42@gmail....
Thu Jul 8 06:41:34 CDT 2010

This tip from Chris, has nearly solved the problem.
Thanks to everybody that has contributed.
I now get sound out that sounds similar to but different from the input - so
the fundamentals are sorted thank you.
However I can only get it to work by experimentation with the frequencies
and gains - it doesn't work with my understanding of what they theoretically
should be. Either the filter design module objects to the coefficients or I
get silence.
Also, (and almost certainly significantly) the filtered output is only on
the right stereo channel.
I wonder if each integer sound sample in a WAV file is split bitwise into
left and right sectors and the type conversion is corrupting half of each

Here's the state of play:

from scipy.io.wavfile import read, write
from scipy.signal.filter_design import butter, buttord
from scipy.signal import lfilter, lfiltic
import numpy as np
from math import log

rate, sound_samples = read('monty.wav')
sound_samples = np.float64(sound_samples / 32768.0)
pass_freq = 0.2
stop_freq = 0.3
pass_gain = 0.5 # permissible loss (ripple) in passband (dB)
stop_gain = 10.0 # attenuation required in stopband (dB)
ord, wn = buttord(pass_freq, stop_freq, pass_gain, stop_gain)
b, a = butter(ord, wn, btype = 'low')
filtered = lfilter(b, a, sound_samples)
filtered = np.int16(filtered * 32768 * 10)
write('monty-filtered.wav', rate, filtered)

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