4-pole filter... why?

[sault]

With some recent discussion happening about four pole filters I figured that I'd do a little research to try and figure out what makes them special. I couldn't really find anything that really explained it, other than that they were more kinda/kinda-not like four one-pole filters.

Besides having more complex treble roll-off, is there a particular reason why they're so popular and garner so much discussion?

[WGTP]

See if I get this right.  1 pole = 6db/octave roll off, 2 pole = 12 db/oct roll off, 3 pole = 18db/oct, 4 pole = 24db/oct.  Look into speaker crossovers for lots of info and debate.  It's a math thing.  You also have a Q for different types of filters.  .5 = Linkwitz/Riley, .7 = Butterworth, etc. 

Basically the 4 pole is very steep, so things get rolled off fast and there is less interaction between bands, but there is lots of phase shift...   

[Skrogh]

Because 4-pole filters sound f***ing sweet!
They are more step (aggressive sounding) than 2-pole and has a more pronounced resonance - giving a greater emphasis on the harmonics around the corner freq.  4-pole filter usually sound wetter and more synthy.

[daverdave]

A 4 pole filter has twice the frequency attenuation of a 2 pole, a 4 pole is typically made up of 2 cascaded 2nd order filters. You get 80dB of attenuation per decade with a 4th. So frequencies in the stopband are attenuated twice as much as with a 2nd order. The phase shift is also doubled, so at cutoff you should get 360 compared to 180 phase shift, and 720 compared to 360 in the stopband.

Not sure I'd be concerned with the phase shift too much unless you're mixing the filtered signal back with the unaffected one.

[Mark Hammer]

They can be described as having a more "defined" sound.  Move a 4-pole cutoff around and the signal says loud and clear "I'm not that anymore, I'm this!"

It needs to be emphasized that the aural beauty of 4-pole filters is most evident with lowpass filters.  Bandpass and highpass may not benefit quite as much from the increased steepness of boundary.

Why?  Well, you need to have some "meat" in the resulting sound, and most of the guitar signal lives in the basement.  A bandpass filter with 4-pole steepness on each side of the passband won't leave you with very much content as it sweeps up, and may provide kind of an aural "shock" as it sweeps down.  A shallower slope leaves some substance and a bit of audible fundamental in there.  In some ways, what one wants is a bandpass filter that is steeper on the treble side than on the bass side, and behaves more like a cascaded 2-pole highpass into a 4-pole lowpass, with their corner frequencies just staggered enough that the passband has acceptable width.

As you can imagine, a 4-pole highpass filter on its own can have limited use in the guitar context.  I can think of plenty of uses for a fixed 4-pole highpass, but a swept one only has limited application.