A subwoofer is a woofer, or a complete louspeaker dedicated to the reproduction of bass audio frequencies, typically from 150 Hz down to 20 Hz. Subwoofers, also known as "subs", are used to augment the low frequency playback of main loudspeakers. Subwoofers are constructed by mounting one or more woofers) typically between 8" and 21" in diameter in a well-braced wood or plastic cabinet. Passive subwoofers have a subwoofer driver and enclosure; active subwoofers have a built-in amplifier.