These vibrating type bells are designed for use as fire or general signaling devices. They have low power consumption and high decibel ratings. The unit mounts on a standard 4" (101mm) square electrical box for indoor use or on a model BBK-1 or HC-BB weatherproof backbox for outdoor applications. Weatherproof backbox model BBK- 1 or HC-BB, Stock No. 1500001.Main FeaturesAC and DC modelsDC models are motor driven, polarized, and have built in transient protection for supervised alarm circuitsAvailable in 6", 8" and 10" sizes