QSC's SB-5218 and SB-7218 subwoofers offer unique solution that addresses requirements of digital motion picture soundtracks. Featuring dual 18-inch low frequency transducers mounted in ported enclosures, these systems extend response to lowest audible frequency. The two custom 18-inch transducers were developed especially for cinema use. The woofers features 4-inch voice coils and vented pole pieces to ensure cool operation, even at high power levels. Cooler temperatures increase driver lifespan and decrease power compression at high drive levels. Undercut pole pieces provide a symmetrical magnetic gap, reducing second harmonic distortion. Enclosures are constructed of high-quality MDF panels and feature single woofer chambers. The separate chamber for each transducer makes the enclosure stronger, provides rigidity, and prevents cone over excursion in the rare event of a driver failure. Enclosure loading is not lost for the remaining transducer. Large, fully radiused ports ensure smooth airflow, especially at higher drive levels. This prevents potentially audible port turbulence noise. Both internal and external port openings are flared. With symmetrical port loading, the bass ports are evenly spaced on each side of the transducers, making internal pressure more uniform across the back surface of the woofer. This prevents the cone from being displaced to one side or another by unbalanced air pressure, reducing the chance of driving the voice coil out of the center of the gap at high drive levels.