Audibility and Visible Coverage in a Conference Room
A 30 ft × 40 ft conference room has a 10 ft ceiling and a 60 dBA average ambient sound level, with peaks of 68 dBA lasting more than 60 seconds. The room is public mode. The contractor proposes one wall-mounted combination horn/strobe centered on the 30 ft wall at 10 ft above finished floor. The horn is rated 85 dBA at 10 ft. The strobe is listed at 60 cd. The room has no partitions, and the owner wants the minimum number of appliances that meets NFPA 72 audibility and visible-signaling requirements. The room is used for training and has a projector screen on the opposite 30 ft wall. The fire alarm control unit supports synchronized NACs from the same manufacturer.
1. Determine the required sound pressure level and whether the proposed horn meets it at the farthest listener.
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Public-mode audibility requires 65 dBA or 15 dBA above the average ambient, whichever is greater, and also 5 dBA above the maximum ambient for 60 seconds. Average ambient gives 60 + 15 = 75 dBA; maximum ambient gives 68 + 5 = 73 dBA. The governing requirement is 75 dBA. The farthest listener is a corner opposite the horn. The horizontal distance from the centered horn to a far corner is √(15² + 40²) = 42.7 ft; with a 5 ft vertical difference between the 10 ft horn and a 5 ft listener, use about 43 ft. Inverse-square attenuation is 20 log(43/10) = 12.7 dB. The horn yields 85 − 12.7 = 72.3 dBA, below 75 dBA. It also fails the 73 dBA maximum-ambient check. One horn is not adequate; add a second horn/strobe or use a higher-output appliance and recalculate.
2. Select the minimum strobe candela for a single wall-mounted strobe and state whether the 60 cd device is adequate.
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Using the 10 ft ceiling wall-mounted strobe table, a 60 cd appliance covers about 30 ft × 30 ft. The proposed single strobe is centered on the 30 ft wall, so it covers only 15 ft to each side but the room is 40 ft deep. The 40 ft dimension exceeds the 30 ft coverage of a 60 cd device. A single 110 cd strobe is the minimum listed size that covers 40 ft × 40 ft from a wall location at 10 ft. Alternatively, two 60 cd strobes could be used if their coverage areas overlap and each covers no more than 30 ft in both directions, but that is more appliances than the owner wants. The 60 cd single-strobe proposal is not adequate. Check that the selected strobe is listed for the mounting height and that the candela rating is not reduced by a lens or lens cover.
3. If two synchronized horn/strobes are added, what installation and circuit checks must be made?
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With two horn/strobes, verify that both appliances are listed for synchronization and are controlled by the same manufacturer's sync module or panel sync protocol. Confirm the maximum number of appliances per sync circuit and the maximum NAC current; adding a device may push the circuit over its rating. Check audibility again at the worst-case point, because two devices at lower output can create phase and coverage issues. Mount strobes 80–96 in. above finished floor to the bottom of the lens, or per the accessibility standard, and keep them visible from all seating positions. Verify that the projector screen does not block the strobe and that the horn is not aimed into a wall. Finally, update the battery calculation for the added alarm current and the shop drawings.