Lowell's conically shaped 8XD4 Series backboxes are engineered to accommodate an 8-in speaker in either new or existing construction. The slightly tapered, conical shape allows these backboxes to be stacked for space efficient storage and transit. Popular series mounts a wide selection of Lowell grilles and mounting accessories. Series 8XD4 backboxes are certified US steel construction (22-gauge) with a white powder coat epoxy finish and include a polyurethane foam disc. The disc meets UL flammability CAL-117 and is affixed to the bottom inside surface to reduce acoustic and mechanical resonance. Models are furnished with factory installed 8-32 screw retaining clips for grille mounting. 8XD4 features one planished (0.5") knockout on top to preserve leak-free integrity of enclosure in return air plenums and four (0.5-0.75") side knockouts placed at 90-degree intervals.