ISUB BMW4 Subwoofer 4 ohms ISUB BMW 4 is a subwoofer that delivers powerful bass, thanks in part to its neodymium motor, a high-performance Focal technology. With the ISUB BMW 4, sound is natural and installation quick, thanks to the spacer integrated into the chassis, which allows perfect integration into original locations. Thanks to its 4 Ohms impedance, it's ideal for use with your original amplification.