Notification Appliances and Evacuation Signaling
Audibility and intelligibility
Public mode requires 15 dBA above the average ambient, or 5 dBA above the maximum ambient lasting more than 60 seconds, whichever is greater, with a general floor of 65 dBA and 75 dBA at the pillow in sleeping areas (private mode). Public-mode sound pressure is capped at 105 dBA (120 dBA only with AHJ approval) to prevent startle and hearing damage. Voice systems must be intelligible - measured by STI or CIS - not merely loud; volume alone in a reverberant lobby can be useless.
Visibility
Visible appliances are rated in candela under UL 1971 and installed per their listed room-spacing ratings: roughly 15 cd for a small room, 30 cd for a 30 ft x 30 ft room, higher ratings for corridors and large spaces, with increased candela for ceiling heights above about 10 ft. Mounting is typically 80-96 in to the bottom of the appliance. Sleeping areas need 110 cd at the pillow for wall-mounted appliances. Appliances in the same field of view must be synchronized.
Circuits and modes
Notification appliance circuits are supervised, usually power-limited. Notification is by area: general alarm, or zoned/partial evacuation per the sequence of operations. Audible-only devices are not acceptable where the code requires visible notification, such as accessible spaces, sleeping rooms, and high-noise areas.
Traps
- Assuming louder is better and exceeding the 105 dBA public-mode cap.
- Substituting audibility compliance for intelligibility in a voice system.
- Forgetting synchronization, or ignoring that a candela rating assumes a ceiling height.
- Mixing public and private mode thresholds.
Rule of thumb
Build the room grid from the manufacturer's published spacing tables first, then confirm audibility with a 10-ft check and the intelligibility method the AHJ accepts.