Speakers are typically housed in a speaker enclosure or speaker cabinet which is often a rectangular box made of wood or sometimes plastic. The enclosure's materials and design play an important role in the quality of the sound. The enclosure generally must be as stiff and non-resonant as practically possible. Where high fidelity reproduction of sound is required, multiple loudspeaker transducers are often mounted in the same enclosure, each reproducing a part of the audible frequency range (picture at right). In this case, the individual speakers may be referred to as "drivers" and the entire unit is called a loudspeaker. Drivers made for reproducing high audio frequencies are called tweeters, those for middle frequencies are called mid-range drivers and those for low frequencies are called woofers. Extremely low frequencies (16Hz-~100Hz) may be reproduced by separate subwoofers.