(From the pioneer site)
TS-SW124D Design:Pioneer engineers were faced with the daunting task of designing a full-size, full-power 12" subwoofer that could fit into a small space – not the typical weakling 8" or 10" subwoofers the competition offers. Well, we did it and in the process five new patent-pending designs were introduced to the world. Pioneer engineers labored two years through various design concepts until just one achieved the unprecedented power, clarity and functionality in a speaker that’s only four inches deep. This revolutionary new subwoofer provides the body-rumbling bass of a fullsize 12-inch sub in a package that fits almost anywhere, especially behind a seat! The space between the cab and the seat was originally intended as a quick place to stash things like coats, spare tire jack and emergency equipment such as flares and flashlights. The UD-SW12 Subwoofer Enclosure is designed to easily fit in this small space. Despite the compact spacing, the TS-SW124D still performs like a full-size subwoofer delivering tight, clear bass you can feel.