Notification Appliances and Evacuation Signaling
Audibility: measure, then add
Public mode requires the alarm to be 15 dBA above the average ambient (or 5 dBA above the maximum ambient lasting more than 60 seconds), measured at 5 ft (1.5 m) above the floor. Private mode requires 10 dBA above ambient. In sleeping areas, NFPA 72 requires 75 dBA at the pillow and, for commercial occupancies, a low-frequency 520 Hz signal, because high-frequency horns do not reliably wake people.
Worked example: a machine room measures 78 dBA running. Public-mode alarm must reach at least 93 dBA there - so the appliance schedule must be checked against the noisiest condition, not the quiet room next door.
Visibility and intelligibility
Visible appliances are rated in candela per their listing: roughly 15 cd covers a 20 ft × 20 ft room, and required candela rises with room size, ceiling height, and for sleeping areas. Mount them 80 in to 96 in above the finished floor. Audible and visible appliances that can be seen or heard together must be synchronized - never mix unsynchronized manufacturers on one circuit.
Voice systems add the requirement that the message be intelligible, not just loud; adding more speakers into the same area can actually reduce intelligibility through reverberation.
Traps: taking dBA readings at the appliance instead of at 5 ft; using a generic "15 dBA over 60" answer where the code says average ambient; thinking a louder horn fixes an unintelligible voice message; forgetting 520 Hz in residential/sleeping applications; and forgetting that the last appliance on a NAC sees the lowest voltage.
Rule of thumb: design NACs for no more than about 10% voltage drop to the end of the circuit.