SB21RDC-C000-4 Large surround dome for increased acoustic output. Dual balanced compression chamber for improved dynamics. Copper cap for reduced voice coil inductance and minimum phase shift. Saturation controlled motor system for low distortion. Non-reflective rear chamber with optimized damping for improved dynamics. Flow optimized vented pole piece for optimum coupling to rear chamber. CCAW voice coil for low moving mass. Long life silver lead wires. Low resonance frequency.