Notification Appliances and Evacuation Signaling
Audibility and intelligibility
Public-mode audible appliances must produce at least 65 dBA or 10 dBA above the ambient (whichever is greater) throughout the occupied area; private mode is lower. In sleeping areas, the sound level at the pillow must reach 75 dBA - and the tone matters: sleeping-area alarms must use a low-frequency (520 Hz) square wave where required by the code edition in force, because high-frequency horns are far less effective at waking people. Levels must not exceed about 110 dBA at minimum hearing distance. Where voice evacuation is used, intelligibility (not just loudness) is the design target.
Visible and other appliances
Visible notification appliances are required for accessibility (ADA) in common areas, restrooms, corridors, and sleeping rooms. Candela ratings increase with room size and with the need to penetrate the sleep state - sleeping areas require substantially higher candela than corridors or meeting rooms, and the exact value comes from the code's candela/room-size tables. Visible appliances in the same viewing area must be synchronized so flashes do not create a seizure hazard. Typical mounting is 80 in. to 96 in. above finished floor. Textual notification, tactile devices, and combination appliances round out the design.
Circuit realities
NACs are power-limited, supervised circuits; end-of-line devices and voltage-drop calculations determine whether the last appliance on the run actually reaches its rated output. Adding appliances to an existing NAC without recalculating voltage drop is one of the most common field failures.
Worked example
A hotel corridor has 60 dBA ambient. A horn rated 85 dBA at 10 ft is proposed at 30 ft spacing. Required: 65 or 60+10 = 70 dBA minimum. At 30 ft, you lose roughly 6 dB for each doubling of distance from the 10-ft rating point - 10→20→40 ft is two doublings, so 85 − 12 ≈ 73 dBA. It passes on paper, but barely; verify at the far end of the corridor with a meter.
Traps
Forgetting the 520 Hz rule; using corridor candela in sleeping rooms; skipping synchronization.