For Jbl Xtreme Series 4.5 Inch Special Bass Passive Radiator Assistant Woofer Radiation Metal Diaphragm For 3-5inch Speakers Use - Speaker Accessories - AliExpress
2pcs For JBL 2"inch 59mm Bass radiator Passive Speaker Home Audio parts | eBay
Passive radiator (speaker) - Wikipedia
What are Passive Radiators in Speakers? | Audio Video Unlimited