JBL MTC-424MR Ceiling Speaker Mud Ring Bracket for Control 424C/T (Pack of 6)This optional bracket is used in Control 400 Series ceiling speaker installations where pre install planning in sheetrock is required. It helps reserve the speaker location before the ceiling surface is installed and provides a point to secure wiring behind the sheetrock. The MR plaster ring design includes a circular offset that serves as an edge guide during plastering. In production style ceiling work, it can also assist with consistent cutout