Dayton Audio PA465S-8 18" Pro Subwoofer 4" VC 8 Ohm 11 mm of Klippel-verified Xmax Inside/outside 4&quot; diameter voice coils for maximum motor efficiency Ultra-rigid Kevlar-paper cones that resist deformation Extensive motor venting for elevated power handling and cool operation Copper shorting ring and pole cap minimize distortion up to the limits of excursion