Short for Subwoofer, though occasionally used as an abbreviation for subgroup. A subwoofer is simply a speaker (woofer) designed to handle the very low frequencies of a speaker system. The concept has been around for many years, but only in the last 10 or 15 years has their use become widespread. With increased popularity of smaller main speakers and much more low frequency content and dynamic range in our recordings, low frequency drivers have become an important part of any speaker system. Many systems may have only one subwoofer (as opposed to the two you would expect). In fact, most of the home theater surround sound technologies (AC-3, etc.) in use today have only one mono sub output. This is based in part on the theory that very low frequencies tend to be omnidirectional so one speaker can cover an entire room. Plus it’s often difficult to produce stereo separation between subwoofers, and in fact any two drivers producing the same frequency range in the same area can tend to interfere with one another as the time arrival of the sounds at the listening position causes certain frequencies to be more or less out of phase with one another, which causes uneven frequency response and even dead spots.