There are five types of cabinet designs for building a subwoofer box. By far, two are the most popular: sealed and ported.
The sealed box is often seen in stores as a front- or down-firing speaker in an enclosed cabinet. When these are powered by an internal amplifier built within the cabinet, they are called powered subwoofers. When an amplifier is not built in, the subwoofer is a passive. Both are designs for large speakers, from 12 to 18 inches.
Ported subwoofers produce more bass with less amplifier power than sealed designs. They are also more difficult to build, owing to the addition of the port, or hole, that is in the box to augment the reproduction of very low frequencies. Ported designs are larger than a sealed box. They use the same loudspeaker, but require more precise adjustments, referred to as tuning, and more bracing within the cabinet for rigidity against vibrations from the production of low-bass frequencies. Ported subwoofers are also either powered or passive, depending on whether they include a built-in amplifier. Ported, vented and bass reflex are used interchangeably to describe this type of speaker.