A subwoofer (or sub) is a speaker designed to reproduce low-frequency sound. A sub can be passive (externally powered) but generally includes a built-in amplifier (active subwoofer). The low frequencies, known as bass or sub-bass, range typically between 20-200Hz in consumer products. A sub is never used on its own and is intended to enhance speakers by widening or extending the bandwidth beyond that which a speaker is capable of producing on its own. While many humans can hear audio down to 20Hz, most people can not hear in stereo below 80Hz, making the placement of a subwoofer less critical and more flexible than that of a speaker.