A slope, also known as the filter roll-off or filter order, refers to the rate at which a filter attenuates frequencies beyond its cutoff frequency in a modular synthesizer. The slope is typically measured in decibels per octave (dB/oct) and indicates how rapidly the filter attenuates frequencies outside of its passband.
The most common filter slopes found in modular synthesizers are:
- 6 dB/oct: This is a gentle slope, often associated with first-order filters, such as one-pole RC filters. These filters provide a subtle frequency shaping effect, allowing some frequencies beyond the cutoff point to pass through with less attenuation.
- 12 dB/oct: This is a steeper slope, typical of second-order filters like the classic 2-pole state variable filter or the classic Moog ladder filter in its 2-pole mode. The 12 dB/oct slope provides a more pronounced frequency shaping effect, and it is commonly used for a variety of sound design tasks.
- 18 dB/oct: This slope is less common but can be found in some modular filters. An 18 dB/oct slope is associated with third-order filters and offers an even steeper roll-off than 12 dB/oct, giving more aggressive filtering effects.
- 24 dB/oct: This is the steepest slope commonly found in modular synthesizers, associated with fourth-order filters, such as the classic 4-pole Moog ladder filter or the diode ladder filter. The 24 dB/oct slope provides a very sharp roll-off and is often used for creating resonant filter sweeps, strong bass sounds, and more dramatic sound shaping.