One Horn/Strobe for a 40 ft Room
A 40 ft by 32 ft training room in a new office building has a 10 ft finished ceiling, carpet, and a measured 60 dBA ambient with the projector and HVAC running. The design shows a single wall-mounted combination horn/strobe at 7 ft AFF centered on one 32 ft wall. The appliance is listed at 90 dBA at 10 ft on axis. Its published candela coverage table lists 60 cd for a maximum room dimension of 30 ft and 110 cd for 40 ft. The farthest corner of the room is roughly 40 ft from the appliance in a straight line, off axis. The owner's program requires public-mode notification. The installing contractor says "one device per room is our standard" and the 60 cd units are already on the truck.
1. Is the 60 cd strobe adequate for this room? Show your reasoning.
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No.
- Public-mode visible notification is selected from the appliance's listed coverage, not from a rule of thumb. The manufacturer's table ties candela to the maximum room dimension the appliance can cover at the mounted height.
- This room's controlling dimension is 40 ft. The table gives only 30 ft of coverage at 60 cd, so 60 cd is undersized regardless of square footage - a 40 × 32 room is not "about the same" as a 30 × 30 room on this table.
- Moving up the table to the candela whose listed maximum dimension is 40 ft (here 110 cd) is the minimum compliant selection, and the mounting height must match the height at which the table was developed.
- Alternative worth pricing: two lower-candela appliances properly placed may be cheaper than one 110 cd unit and will usually produce better light distribution around furniture and the far corner - but the design must be re-run against the manufacturer's layout method, not improvised in the field.
- The correct action is a design revision, not installation of the on-site 60 cd units.
2. Estimate the audibility at the far corner and decide whether the audible portion is acceptable.
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Rough free-field estimate (6 dB loss per doubling of distance):
| Distance | Level |
|---|---|
| 10 ft (rated) | 90 dBA |
| 20 ft | 84 dBA |
| 40 ft | 78 dBA |
Public-mode criterion: 15 dBA above average ambient, i.e. 60 + 15 = 75 dBA at the listener's location.
Result: 78 dBA vs. 75 dBA required - a 3 dB margin that exists only in theory.
Why the margin is not acceptable as-is:
- 90 dBA at 10 ft is an on-axis rating; the far corner is off axis and the published polar data will show several dB less than the on-axis number.
- The 6 dB/doubling model assumes no absorption advantage, no partial-height partitions, and no furniture; the room has carpet (helps) but also an HVAC system whose noise is already in the ambient measurement.
- A 3 dB calculated margin is inside measurement and appliance-to-appliance tolerance, and the projector/HVAC ambient can rise when the room is full of people.
Recommendation: add a second appliance (or a higher-output device) so the calculated level at the farthest point carries a real margin, then verify by measurement with the room in its normal configuration and with all notification appliances operating.
3. The contractor's crew is ready to install today and the ceiling grid closes Friday. What do you do?
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Stop the installation on this circuit and get it corrected before the grid closes. Actions, in order:
- Issue a written field direction that the 60 cd devices are not to be installed and that the room is to be held until a revised notification layout is approved. Once the grid is closed and devices are trimmed out, correcting candela and adding a second appliance becomes a rework cost and a schedule problem.
- Send the revised design (correct candela from the listed coverage table, appliance locations, and the audibility calculation showing the margin at the farthest point) as a bulletin or revised drawing.
- Handle the cost properly. The specification required public-mode notification to the listed criteria; the contractor's "one device per room" practice was never a compliant basis, so the added device/candela is not automatically a change order. Review the original design first - if the design drawings showed 60 cd, the correction is a design error and the owner pays; if the drawings showed 110 cd and the contractor substituted, the contractor pays.
- Verify at acceptance by measurement, and record the results on the inspection form rather than relying on the calculation.