High efficiency backward cavity design. Increase in the same volume in the low-frequency extension and electro-acoustic conversion efficiency, the same time significantly reduce the bass standing wave interference, BGS-218B subwoofer can provide the full energy of ultra-low frequency output. Completely acoustic design of the box structure, access to a high electro-acoustic conversion efficiency, and control design the perfect system, the frequency range is accurate, simple and honest and strong low-frequency full clean.