Passive radiators with floppy suspensions are best.
Speaker Sub Grills and Accs: Focal Utopia W Series 10 / 11 Passive Radiator For Car Or Home Audio Use New -> BUY IT NOW ONLY: $149.99 on eBay!
Grab your music and join the party with Riddim BT by The House of Marley. This ultra-portable Bluetooth speaker really packs a punch with its dual 1.5” drivers and dual passive radiators paired with enhanced DSP (Digital Signal Processing).
Image 1/3: At long last we've found the [http://www.sonos.com/shop/products/play3|advertised] [http://en.wikipedia.org/wiki/Passive_radiator_(speaker)|bass radiator], a passive speaker cone secured in the very back of the case that forms an inexpensive alternative to a powered subwoofer.
Edifier Exclaim Bi-Amped 2.0 Speaker System (e10) Each upper section of the speaker contains two 11/2 inch midrange/ tweeters, and a 11/2 inch x 3 inch passive radiator Edifier
Speaker Parts and Components: Ess Tempest And Model 8 Speaker Repair Kit - 8 Woofer And 10 Passive Radiators -> BUY IT NOW ONLY: $49.45 on eBay!