In speaker design a passive radiator is an element that is designed to move sympathetically with the energy in the cabinet. They generally resemble a low frequency driver or woofer, but have no voice coil or any element to actively generate sound. Often they are employed in speakers instead of a port to create a bass reflex type of design. The extra mass of the passive radiator actually lowers the resonant frequency thereby allowing smaller cabinets to reproduce lower frequencies than they would otherwise be capable of. However this extra mass also causes the radiator’s movement to be hard to dampen, which in turn can lead to bass hangover and a more “boomy” sound.