Notification Appliances and Emergency Communications
Audible and visible requirements
Public-mode audible appliances must produce at least 15 dBA above the average ambient sound level (or 5 dBA above the maximum ambient of 60 seconds or longer, whichever is greater), and no more than 110 dBA at the minimum hearing distance. Private-mode signaling uses a lower threshold. Sleeping areas require a low-frequency 520 Hz square-wave sounder because it is far more effective at waking occupants. Visible appliances are rated in candela per UL 1971 and have prescribed mounting heights (roughly 80 to 96 in. to the top, at least 6 in. below the ceiling) and area coverage from the listing tables. Visible appliances in the same room or area must be synchronized.
Voice and intelligibility
Voice systems must be intelligible, not just loud: an intelligibility score of at least 0.70 on the common intelligibility scale (roughly 0.45 STI) in the occupied areas. Emergency communications systems add priority rules, message control, and mass notification integration, which must be defined in the sequence of operations.
Traps
The two frequent failures are exceeding 110 dBA while chasing the 15 dBA-above-ambient rule, and forgetting the 520 Hz requirement in sleeping areas. Another is assuming louder equals intelligible.
Worked example
A 60 ft by 60 ft room using wall-mounted 15 cd visible appliances at 20 ft by 20 ft coverage needs a 3 by 3 layout: nine appliances, all synchronized.