The per-section Q values that realise a Butterworth pass filter of the given slope: a
12 dB/oct filter is one maximally-flat 2nd-order section (Q = 1/√2); a 24 dB/oct filter
is two cascaded sections (Q = 0.5412, 1.3066) whose 4th-order response is maximally flat
(−3 dB at the corner, twice the roll-off). Both the curve maths and the engine's biquad
cascade read this one table, so a slope means the same thing everywhere.
The per-section Q values that realise a Butterworth pass filter of the given slope: a 12 dB/oct filter is one maximally-flat 2nd-order section (Q = 1/√2); a 24 dB/oct filter is two cascaded sections (Q = 0.5412, 1.3066) whose 4th-order response is maximally flat (−3 dB at the corner, twice the roll-off). Both the curve maths and the engine's biquad cascade read this one table, so a slope means the same thing everywhere.