An imprecise term concerning how low in frequency a loudspeaker or subwoofer can still operate to produce usable output. A typical bookshelf-sized speaker may manage a bass extension of 80 Hertz (say, at -10dB), a good floorstanding speaker may manage 30 or 40 Hertz, an inexpensive subwoofer 40 Hertz, a middling one 25 to 30 Hertz, an expensive one 16 Hertz.