A subwoofer is a specialized loudspeaker dedicated to reproducing extremely low frequencies. Because many “full-range” speakers can’t handle frequencies below 45-50Hz, a subwoofer can be added that will output the low frequency information normally missing. In a typical 5.1 setup, the “.1” (or “point one”) refers to the subwoofer.
The first commercial subwoofer was developed in the 1970s, while the first documented use of a subwoofer in a pro recording environment was during the mixdown of Steely Dan’s Pretzel Logic album. Early subs were huge (up to 34 inches), but today, thanks to advances in technology, subs with speakers of 8″ or 10″ are available, though the larger the driver, the lower the sub typically can go. Today almost all subwoofers are active or self-powered via an onboard amplifier.