AP21-BH2 is a self-contained, plug and play extra loud 2-buzzer signal system for larger factories, warehouses, auto and truck shops with loud environments. It includes (2) high quality, low-voltage, 4-inch, high output buzzers (100dB), which are pre-wired with 90ft of cable for easy installation. Mounting hardware is included. No wiring is required. Simply hang the enclosure and the buzzers, plug it in, and enter program. These buzzers must be mounted at least 10ft above employee work areas.