Voice Evacuation Audibility in a High-Noise Machine Shop
You are testing the voice evacuation system in a 12,000 sq ft machine shop with high ambient noise. The shop has three CNC machining centers, two air compressors, and a dust collection system. With all equipment running, a sound level meter reads 92 dBA at the center of the shop floor. The fire alarm speakers produce 78 dBA at the same location when measured on the A-weighted scale. The design documents specify 'audible notification per NFPA 72,' and the contractor says the system passed 'audibility' because the alarm can be heard. The shop has no visual notification appliances except two small strobes near the office. You also note that the voice message is unintelligible near the compressors, and the shop employs two workers who are deaf.
1. Does 78 dBA meet the audibility requirement in a 92 dBA environment? Explain.
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No. For public-mode notification, the alarm must be at least 15 dBA above the ambient sound level, or 105 dBA where the ambient is high; 78 dBA is below the ambient itself, so it will be masked by machinery. The fact that someone can hear it in a quiet corner or during a shutdown is not the test condition. Measure with equipment running. Record the ambient and alarm levels at several locations. The system fails audibility in this area and needs redesign, not just a volume adjustment.
2. What correction options exist, and how do the deaf workers affect the solution?
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Options include adding speakers to increase coverage, relocating speakers closer to workstations, using higher-output appliances, and adding visible notification appliances in high-noise areas. A voice message must also be intelligible, not merely audible. Because two employees are deaf, visual notification is needed regardless of sound pressure; visible appliances should be placed where they will be seen. The redesign should be by a qualified person, and the results must be retested with the shop running.
3. What do you record and recommend?
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Record the sound level readings, locations, equipment operating conditions, the measured alarm output, the unintelligible voice areas, and the absence of visual appliances. Recommend a design review and additional notification appliances. Do not certify the voice system as compliant. Note that audibility and intelligibility are separate criteria and that the current installation fails both in the noisiest areas.