WAS PEERLESS 830481 Identical to a 10” XLS driver but without a magnet and the voice coil has been replaced with a weight. Passive radiators replace the port by adding a mass equivalent to the volume of air in a 10” tube. This eliminates air velocity noises and enables very low tuning F without ridiculously long or narrow port dimensions. Suit small enclosures. You can fine tune this mass by adding weight to the M5 threaded hole enabling excellent control over the final design. Made in Denmark by Scan-Speak.