Bass guitar cabinets may include a single speaker (typically 12" or 15"), multiple speakers of the same type (common formats include 2x10", 4x10" and 8x10"). Some bass cabinets use multiple different-sized speakers, such as a mixture of 12" and 15" speakers. In rare cases, some large bass cabinets (known as "bass bins") incorporate folded horns to boost the bass response. Bass amp cabinets may have a horn or tweeter built into a speaker cabinet which contains one or more woofer drivers. When a cabinet includes such a horn, the enclosure may also have an attenuator knob, for turning the horn volume up or down. Some more expensive bass cabinets with a woofer/horn approach may have additional features, such as circuitry to protect the speakers or horn from overloads or a biamplification option.