JBL MTC-424NC Ceiling Speaker New-Construction Bracket for Control 424C/T (Pack of 6)The JBL MTC 424NC is an optional new construction bracket for Control 400 Series ceiling speaker installations. It is used before sheetrock is installed to help define speaker location and support wiring placement behind the ceiling surface. Its flat sheet metal design includes mounting wings for attachment to the building structure. In production style installations, the ring can also serve as a guide for blind cutout work in sheetrock using a router