What is a 2-Pole and a 4-Pole filter?

To understand what is meant by the terms 2-pole filter and 4-pole filter, you first need to know that the word "pole" in the context of filters refers to a 6 dB per octave roll-off.

Well, a 2-pole filter = 12 dB/octave slope, and that's because 6 dB x 2 = 12 dB. Furthermore, a 4-pole filter = 24 dB/octave slope and that's because 6 dB x 4 = 24 dB. So what's important to remember is that a 2-pole filter is synonymous with a 12 dB filter whilst a 4-pole filter is interchangeable with the term 24 dB filter.

What is the difference between a 2-pole filter and a 4-pole filter?​

The most significant difference between a 2-pole filter and a 4-pole filter lies in the distinctive sounds they generate when used on a synthesizer or as an equalizer. A 2-pole filter is more gentle as compared to a 4-pole filter which is much steeper. The end result is that a 2-pole will produce smoother sounds than a 4-pole filter.

You can do a simple experiment where you both set a 2-pole filter and 4-pole filter at a center frequency of 1 kHz and you should hear that a 2-pole will sound brighter whilst a 4-pole will have a darker sound to it.

When should you use a 2-pole filter and a 4-pole filter?​

The choice of deciding between a 2-pole filter or a 4-pole filter in the context of sound design is all about the final sound you are aiming for. There are certain sounds that may require a gentle filter such as keys, leads, strings, pads, brass, and so forth.

Hence, it will be ideal to try out a 2-pole filter first. On the other hand, if you want tighter sounds that have a dark tone then a 4-pole filter will do you more justice than your 2-pole filter.