Z-18 V.3 D1 - Sundown Audio 18" 1500 Watt RMS Dual 1-Ohm Z3 Series Subwoofer Z-18 V.3 D4 - Sundown Audio 18" Dual 4-Ohm Z3 Series Subwoofer Specifications: DC Resistance (Re): 1.7  | Resonance Frequency (Fs): 31.8 Hz | Equivalent Volume (Vas): 104.2 L | Electrical Q Factor (Qes): 0.5 | Mechanical Q Factor (Qms): 5.5 | Total Q Factor (Qts): 0.5 | Voice Coil Inductance (Le): 2.0 mH | Force Factor (BL): 17.8 NA | Moving Mass (incl. Air Load) (Mms): 503.8 g | Suspension Compliance (Cms): 49.5 m/N | Sensitivity: 89.6 dB 1w / 1m Box Recommendations: Sealed Enclosure Suggestion: 3.0 cu. ft. | Ported Enclosure Suggestion: 4.0 cu. ft. | Displacement: 0.25 cu. ft. | Depth: 11.25 in | Outside Diameter: 18.425 in | Cutout: 16.692 in | Recommended Tuning: 32 Hz | Recommended Port Area: 64 sq. in.