Polypropylene Cone. Spider Plateau Venting for Targeted Mechanical Cooling of the Voice Coil. Asymmetrical Linear Drive System for Proper Linear Movement. 2" Voice Coil for Higher Powered Handling. 8 Layer Aluminum Voice Coil Road Thunder subwoofers are the continued evolution of high performance subwoofers from MTX. Designed with one goal in mind, to be loud, these subwoofers offer extreme performance in an economical package. Based on the TR55 series subwoofer, the Road Thunder subwoofers use a larger voice coil and motor structure to generate more output.