The speaker cabinets, which hold these drivers can be closed-back or open-back, along with variations such as a semi-open back 4x12 in cabinet, which may have a baffle deflecting two of the four speakers. Closed back cabinets may be acoustic suspension or bass reflex. Bass cabinets are usually closed-back or use a bass reflex port or vent to boost low frequency response. The purpose of the cabinet is match the acoustic impedance seen by the driver to the driver's own electrical/mechanical impedance. Cabinets are best used when they are matched to the driver(s) being used; casual substitution of another driver which can be mounted in a cabinet will only accidentally make changes in a beneficial direction.