Voice Evacuation Audibility After Tenant Fit-Out
An 18-story office tower has a 2012-installed emergency voice alarm system. A tenant fit-out on floors 9 through 12 added private offices and one conference room. Three ceiling speakers were temporarily removed for drywall work and reconnected by the tenant's electrical contractor. The base-building fire alarm contractor performed a post-construction audibility test. In a corner office the average ambient level was 52 dBA and the maximum level lasting at least 60 seconds was 58 dBA; the alarm reading was 62 dBA. In the open office area the average ambient was 60 dBA, the 60-second maximum was 66 dBA, and the alarm reading was 70 dBA. In the conference room the average ambient was 48 dBA, the 60-second maximum was 52 dBA, and the alarm reading was 66 dBA. The contractor reports that every reading is at least 10 dBA above ambient and requests sign-off. The AHJ also requires voice intelligibility verification.
1. Evaluate each of the three spaces for compliance with NFPA 72 public-mode audible notification criteria. Show the calculation for each space, not just pass/fail.
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Public-mode audible notification is not a fixed 10 dBA above ambient. The NFPA 72 criterion is the greater of (a) 15 dBA above the average ambient sound level or (b) 5 dBA above the maximum sound level lasting at least 60 seconds, measured at 5 ft above the floor.
- Corner office: 52 + 15 = 67 dBA; 58 + 5 = 63 dBA. Required = 67 dBA. Measured 62 dBA, so it fails by 5 dB.
- Open office: 60 + 15 = 75 dBA; 66 + 5 = 71 dBA. Required = 75 dBA. Measured 70 dBA, so it fails by 5 dB.
- Conference room: 48 + 15 = 63 dBA; 52 + 5 = 57 dBA. Required = 63 dBA. Measured 66 dBA, so it passes.
The contractor applied a simplified 10 dB rule, which is not the public-mode criterion. The system must be corrected in the two failing spaces, and the AHJ's intelligibility requirement must be tested separately. A space can pass sound pressure and still fail intelligibility.
2. The contractor proposes to increase the tap setting on the speakers serving the failing areas from 1 W to 4 W. Evaluate this proposal and describe at least two checks that must be made before accepting it.
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Increasing the tap from 1 W to 4 W can add about 6 dB because each doubling adds roughly 3 dB. On paper that would bring 62 dBA to about 68 dBA and 70 dBA to about 76 dBA. The proposal is not automatically acceptable.
First, louder is not necessarily more intelligible. Higher output in a reverberant office can reduce the direct-to-reverberant ratio and worsen STI or CIS. Adding speakers at lower taps usually improves intelligibility better than driving existing speakers harder.
Second, verify the supporting design: amplifier channel capacity, notification appliance circuit voltage drop, battery calculation, and the manufacturer's listed tap settings. Recheck the three reconnected speakers for polarity, secure connections, and supervision. Reverse polarity on one speaker in a circuit can cause cancellation and low output.
Accept the change only after recalculation, an intelligibility test, and a repeat of the sound pressure measurements in the failing spaces. Update the as-built tap schedule and test report.
3. List the documentation and acceptance test steps that should be completed before the AHJ accepts the voice system.
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Documentation should include:
- As-built drawings showing speaker locations, circuit numbers, tap settings, and zone assignments.
- Amplifier and battery calculations, plus voltage-drop calculations for each speaker circuit.
- The sequence of operations for floor selection and voice message prioritization.
- An intelligibility test plan and results using an approved method, such as STI or CIS, in representative occupied spaces including the two failing rooms.
- Recorded ambient and alarm sound pressure levels at the required measurement height.
- Functional test of every speaker circuit and every reconnection, including the three tenant-modified speakers.
- A signed acceptance test report and the AHJ witness record.
The AHJ should not sign off on the contractor's 10 dB statement. The test report must show the 15/5 calculation and the intelligibility result.