Boston Acoustics 110-002709 Single 4.5" Subwoofer Replacement for M340 Speaker (Certified Refurbished) Certified Refurbished product is tested & certified by Boston Acoustics to work like-new. The product includes all original accessories, and is backed by a 90 day warranty. Black/Silver color, 4.5" Subwoofer Replacement for M340 Speaker Aluminum shorting rings for dramatically lower distortion and a clearer sound Smooth bass response, Polypropylene woofer material Employs mass loading to dampen harmful resonances and achieve a smoother frequency response at the top of the driver's range