Solid State Logic X-EQ - Manual (Page 23)

Gaussian
No ringing or overshoot in the time domain, but slow roll-off in the
frequency domain.
Butterworth
Characterised by having a maximally flat magnitude
response, i.e, no amplitude ripple in the passband.
Chebychev
Characterised by having an equiripple magnitude response, meaning
the magnitude increases and decreases regularly from DC to the
cutoff frequency.
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