Passive radiator

A passive radiator is a driver-like diaphragm with a cone and suspension but no motor (no voice coil or magnet). It’s mounted in an otherwise sealed enclosure and driven only by the air-pressure swings the powered woofer creates inside the box — so it replaces the port of a vented/bass-reflex design with a tuned mass instead of a tuned column of air.

Why use one instead of a port

Same goal as a vented box — reuse the driver’s rear energy to extend low-end output near a tuning frequency — but the radiator’s tuning is set by its moving mass and compliance rather than port length/area. That buys practical advantages in a small cabinet: no port to grow long or to chuff/whistle at high excursion, and the box can be smaller for a given tuning. The trade is an added moving part to tune and a sharper rolloff below tuning (like a vent). It’s a distinct fourth alignment alongside infinite-baffle, sealed, and vented.

Seen in the wiki

chesky-audio-lc2 uses twin 8″ passive radiators with a reworked 7″ woofer to reach a rated 40 Hz in a ~13″ bookshelf cabinet — the compact-box case passive radiators are built for.

Related

loudspeaker-enclosure · loudspeaker-driver · thiele-small-parameters · chesky-audio-lc2