The VB-A21/VB-R19/VB-S20 Backbox is designed to facilitate easy mounting of Valcom’s large clock/ speaker assembly. The Surface Mount Backbox is preferred when installing on a harder surface such as concrete or cinder block. The Recessed Mount Backbox is common for softer surfaces such as drywall. The Angle Mount Backbox offers a different sound distribution and can be installed on harder or softer surfaces.