Have had, picked up to implement the following filters

Chebyshev filter
Bessel filter
Butterworth filter

https://en.wikipedia.org/wiki/Audio_...d er_or_slope

Included slopes, though listed to the 10th, generally perform at
limits to a decreased complexity, if at all, of their 5th order.

Foobar VST Fx - don't recall the author. Is effectively configured
for splitting the XONAR soundard to an octal signal path, at four
equivalent stereo pairs, each of four subsequent divisibilities, both
from the above three and generally characteristic 4 band-pass types
(sub, low, med, hi).

In any event, a good introduction to filtering designs.
http://www.eeeguide.com/higher-order-filter-design/

Like the sound from the Chebyshev model at a reduction, from 4th order
default), to 3rd order, stacking of design bandpass ordering. Also
configs for a reference pass-thru, which I used on the soundboard's
sub;- Not a sub-fan, since 12" passive bass radiators (think drum
skins) are an effectuation of 2ea 4" active drivers in enclosed or
non-ported build designs (reminds me of the Bose 901 thing).