In the audio realm the term filter order is synonymous with filter poles. Both terms are interchangeable with one another.
- 1st Order = 1-pole Filter (6 dB filter)
- 2nd Order = 2-pole Filter (12 dB Filter)
- 3rd Order = 3-pole Filter (18 dB Filter)
- 4th Order = 4-pole Filter (24 dB Filter)
As such a 1st order filter (1-pole) has a roll-off rate of 6 dB per octave meaning it attenuates frequencies by 6 dB per octave starting from it’s cut-off point.
E.g if the cut-off point for a low-pass filter is set at 100 Hz this means the frequencies which are are below 100 Hz will be attenuated with a roll-off rate of 6 dB per octave. The same is true for the rest of filter orders i.e 2nd Order, 3rd Order , 4th Order and so forth.
Quality Factor of Filter Orders
An ideal 1st order filter (1-pole filter) is less likely to exhibit resonance phenomena. This is the reason why some EQ plugins which try to emulate a pure 1st order filter (1-pole filter) will not have the option of tweaking the amount of resonance (quality factor).
On the contrary 2nd order filters (2-pole filters) allow the user to define the amount of resonance which is often denoted by a knob labeled Q in short for quality factor. The same applies to 3rd order, 4th order filters etc.