Sleeping Room Audibility and Low-Frequency Signaling in a New Hotel Wing
A new 40-room hotel wing is undergoing acceptance testing. Each guest room has a photoelectric smoke detector on the ceiling and a corridor horn/strobe outside the door. The designer did not provide an in-room sounder, citing the corridor appliance as sufficient. With the guest room door closed, ambient sound in the room is 44 dBA. You measure 63 dBA at the pillow with the corridor horn/strobe operating. The horn is a standard 3,000 Hz electronic horn. The owner's representative says the measurement exceeds ambient by 19 dBA, so it passes. The corridor appliances are synchronized, and the panel is programmed for alarm verification on the smoke detectors. The building is new construction, and the local AHJ has adopted the current edition of NFPA 72. The contractor has not provided documentation for low-frequency signaling.
1. Does the measured 63 dBA meet the audibility requirement for a sleeping area? Explain.
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No. For sleeping areas, the standard requires a minimum of 75 dBA at the pillow. The 63 dBA measurement is 19 dBA above the 44 dBA ambient, but the ambient-plus criterion does not replace the 75 dBA pillow minimum. The measured level is 12 dBA short. The corridor horn outside the closed door cannot produce the required level at the pillow. The system fails the audibility test, and the contractor must add an in-room sounder or otherwise achieve 75 dBA at the pillow with the door closed.
2. What about the 520 Hz low-frequency requirement?
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The corridor horn is a 3,000 Hz device. For new construction, the standard requires a low-frequency 520 Hz signal in sleeping areas because it is more effective at awakening occupants. A corridor horn outside the room does not satisfy that requirement for the sleeping area. The design needs an in-room 520 Hz sounder or a 520 Hz appliance in the sleeping room. The contractor's lack of documentation is itself a deficiency. Verify the installed appliance's listed frequency and measure the sound level with the door closed.
3. The contractor proposes to increase the corridor horn's volume to meet 75 dBA at the pillow. What are the problems?
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Increasing a 3,000 Hz horn may raise the measured dBA, but it does not provide the required low-frequency signal and may not reliably awaken occupants. It may also produce excessive sound levels in the corridor and adjacent rooms, and it may still not achieve 75 dBA at the pillow with the door closed. The correct solution is an in-room 520 Hz sounder, not a louder corridor horn. If the corridor level is increased, recheck the maximum allowable sound level and the audibility in all affected spaces.