A filter pole refers to the order or steepness of a filter’s response curve. Filters are used to selectively attenuate or boost specific frequency ranges of an audio signal, and the number of poles in a filter determines its slope, which is measured in decibels per octave (dB/oct).
Each pole of a filter corresponds to a slope of -6 dB/oct, so the more poles a filter has, the steeper its roll-off becomes:
- 1-pole filter: -6 dB/oct slope
- 2-pole filter: -12 dB/oct slope
- 3-pole filter: -18 dB/oct slope
- 4-pole filter: -24 dB/oct slope
Filters with different numbers of poles have different sonic characteristics, and the choice of filter pole count depends on the desired sound and application. For example, a 1-pole or 2-pole filter would have a more gentle roll-off and a less pronounced filtering effect, which may be desirable for subtle sound shaping. On the other hand, a 4-pole filter would have a steeper roll-off and a more aggressive filtering effect, which can be useful for creating resonant and pronounced filter sweeps.
In modular synthesizers, filters with different pole counts are available as separate modules or as features within a single module, giving you the flexibility to choose the appropriate filter type for your patch.