An apparatus for mounting a flat panel radiator into a ceiling grid. The ceiling grid system includes openings defined by main beams and crossbeams. The main beams are secured through hanger wires to a hard ceiling. The main beams and the crossbeams have flanges with the crossbeams resting on the flanges of the main beams. The mounting apparatus has a frame including a top plate, a side plate, and a bottom plate, containment elements and isolation elements attached to the frame, and an acoustically transparent scrim attached to the bottom of the frame. The flat panel radiator is mounted between containment elements and isolation elements. The radiator panel can be fabricated from a honeycomb core. The containment elements may be made from a stiff to resilient material, whereas the isolation element will always be resilient to vibration isolate the radiator panel from the ceiling grid system both mechanically and acoustically. The flat panel sound radiator assembly can also be installed in wall partitions.