4-Pole Lowpass Butterworth Filter

This is a 4-pole lowpass Butterworth filter with a cutoff frequency of 250 Hz and a gain of about 2.5 that we use to filter the microphone input. A Butterworth filter has the flatest frequency response in both the pass and stop bands but not as quick a drop off between pass and stop bands, as say, a Chebychev filter. We found the latency of this filter to be about 1.5 ms. However, phase delay varies based upon the input frequency. The phase delay is important to know because we must produce anti-noise quickly to cancel out the low-frequency sounds of the road and engine.