A subwoofer is a woofer driver used only for the lowest part of the audio spectrum: typically below 100-120 Hz. Because the intended range of frequencies in these is limited, subwoofer system design is usually simpler in many respects than for conventional loudspeakers, often consisting of a single subwoofer driver enclosed in a suitable cabinet or enclosure.
To accurately reproduce very low bass notes without unwanted resonances (i.e., from cabinet panels), subwoofer systems must be solidly constructed and properly braced; good ones are typically heavy. Many subwoofer systems include power amplifiers and electronic filters, with additional controls relevant to low frequency reproduction. These variants are known as "active subwoofers". Passive subwoofers require external amplification.
