Chicago plenum, 3-1/2" deep, 4" square junction boxes with air tight (8) 1/2” & (4) 3/4” side and (3) 1/2” & (2) 3/4” bottom knockouts are installed in plenum walls and ceilings spaces where extra depth is needed for dimmers, cameras, speakers, strobes, signals and other electrical and electronic devices. Garvin Industries’ labor saving design eliminates the need to awkwardly install boxes and extension rings to accommodate deep devices.