How a subwoofer works

A subwoofer plays the lowest notes in the music, the deep bass below about 80 Hz that a pair of small speakers can’t reach. It is one large driver in a stiff, heavy box, with its own amplifier built into the back panel.

It usually takes a line-level signal from the pre-out on your amplifier, or a tap off the speaker wires when there is no pre-out, and its plate amplifier drives the cone. The crossover knob sets the note where the sub takes over from the main speakers, and the level and phase knobs blend it in, so the bass seems to come from the speakers and not from a box in the corner.