ABSTRACT
The lowpass filters used to define the upper limit of the audio bandwidth are usually second-order with roll-off rates of 12 dB/octave; third-order 18 dB/octave filters are rather rarer, probably because there seems to be a general feeling that phase changes might be more audible at the top end of the audio spectrum than the bottom. Either the Butterworth (maximally flat frequency response) or Bessel type (maximally flat group delay) can be used, and this gives us an excellent opportunity to make a real-life comparison of the two types. It is unlikely that there is any real audible difference between the two types of filter in this application, as just about everything happens above 20 kHz, but using the Bessel alignment does enforce a compromise in filtering effectiveness because of its slow roll-off. I will demonstrate.
