Polk Audio LSi9 Bookshelf Speakers (Pair, Ebony) Pair of bookshelf speakers that delivers superior linearity, definition and sound imaging Equipped with two 5.25-inch dynamic balance aerated polypropylene composite drivers Contains a rear-firing power port for precise deep bass Equipped with one 1-inch high performance ring radiator tweeter Cast aluminum driver baskets resist flexing and "ringing" for ultra-tight bass and coloration-free midrange.