From MXR;

The Bass Envelope Filter delivers classic, analog, envelope filter
sounds in an easy-to-use and compact pedal designed specifically for
bass. Separate Dry and Effect controls let you dial in the perfect mix
of effected and direct signals, so you can bring the funk without losing
the low end. Its wide range of filter tones can be shaped with the
Decay and Q controls, and a simple twist of the Sensitivity knob can
tailor the pedal for your attack—as well as adjust for passive or active
basses. The Bass Envelope Filter's power supply circuitry is designed
to withstand over-voltage or polarity mishaps, and its true-bypass
switching preserves your tone in bypass mode. It's all packaged in a
durable, lightweight, Phase 90-size aluminum housing for the ultimate in
portability and pedalboard space-saving dimensions.