The 869 series are designed for flush or panel mounting. For flush mounting, they mount to a standard 4 in. square back box with extension ring (total minimum depth of 3 in.). Back box and extension ring not supplied. For panel mounting, use only the supplied mounting plate without back box. The 869 series electronic horns are designed for use in corridors and manufacturing sites. It is used wherever a distinctive, audible signal is required for timing, scheduling, general alarm, and warning applications.