How to use the audibility calculator
- Pick a distance unit. Use the same unit for the rating distance and the distance to the spot; only their ratio matters.
- Enter the average ambient sound level at the listening spot in dBA, from a survey or the design basis.
- For each horn or speaker, enter the rated output in dBA and the distance that rating is given at, both from the data sheet, then the distance to the spot. Add a wall or door loss in dB if one is in the path, from the manufacturer or the designer.
- To compare with a requirement, enter the margin over ambient that your code edition and AHJ require. Leave it blank to see the levels only.
How the estimate works
- Distance. In open space, sound pressure falls in proportion to 1 ÷ distance, so the level drops by 20 × log10(r2 ÷ r1) dB. Twice the distance is 6.02 dB less and ten times the distance is 20 dB less.
- Walls and doors. The tool subtracts the loss you enter as it stands. It has no built-in figure for any wall or door.
- Several appliances. Unrelated sources add by energy: L = 10 × log10(10^(L1 ÷ 10) + 10^(L2 ÷ 10) + …). Two equal appliances give 3.01 dB more than one, never double the decibels.
- The meter reading. A meter in the room hears the signal and the ambient together, added the same way, so it reads a little more than the signal alone. The calculator shows both.
- Reach. With a margin entered, each appliance's reach is the distance at which it alone falls to ambient + margin: rating distance × 10^((rated − loss − target) ÷ 20).
Both formulas are in Sound pressure: Distance and Sound pressure: Multiple sources. The same article notes that reflections in a closed room change what a meter reads, which is why this is an estimate and a survey is the test.
What the code asks for
NFPA 72 Chapter 18 sets the audible requirements for notification appliances, including how far a signal must sit above the average ambient and how the level is measured. The figures can change between editions, so read Chapter 18 in the edition your AHJ enforces. NFPA offers free read-only access to NFPA 72. If you sit the NICET exams with the code on screen, using NFPA 72 at the test center covers how to find a chapter quickly.
Worked example 1: one horn across a room
Invented numbers. A horn is rated 84 dBA at 10 ft and the listening spot is 40 ft away with a clear path. Distance change = 20 × log10(40 ÷ 10) = 20 × 0.60206 = 12.04 dB, so the level at the spot is 84 − 12.04 = 71.96 dBA. With a 55 dBA ambient that is 16.96 dB above ambient. Against a 12 dB margin, an invented figure for this example, the target is 55 + 12 = 67 dBA and the horn has 4.96 dB to spare. On its own it stays at or above 67 dBA out to 10 × 10^((84 − 67) ÷ 20) = 70.8 ft.
Worked example 2: a second horn behind a door
Invented numbers, and the calculator's starting inputs. Add horn B, also 84 dBA at 10 ft, 20 ft from the same spot with a door the designer puts at 10 dB. Horn B gives 84 − 20 × log10(20 ÷ 10) − 10 = 84 − 6.02 − 10 = 67.98 dBA. Together: 10 × log10(10^7.196 + 10^6.798) = 73.42 dBA, which is 18.42 dB above the 55 dBA ambient. A meter would read 10 × log10(10^7.342 + 10^5.5) = 73.48 dBA with the ambient present. Horn B adds only 1.46 dB because it is 4 dB quieter at the spot than horn A.
These are original arithmetic examples, not exam questions.
Common mistakes
- Adding decibels as plain numbers. Two 70 dBA horns give about 73 dBA, not 140 dBA.
- Taking 6 dB off for every fixed step of distance. The 6 dB is per doubling: 10 to 20 ft, then 20 to 40 ft.
- Comparing with the wrong ambient. Use the average ambient measured while the space is in normal use, not a quiet reading taken after hours.
- Mixing up the rating distance. A rated output means nothing without the distance it was measured at; take both from the same data sheet line.
- Forgetting the path. A closed door or wall between the horn and the listener can take more off than the distance does.
Ambient noise and audibility are part of the site survey in the Level II submittal and layout domain; see submittals and layout and the NICET Fire Alarm Level II exam guide. For the visible side, read candela rating, and for the evacuation tone, temporal-3 pattern. Once the appliances are placed, the fire alarm voltage drop calculator checks the circuit that feeds them.
Fire alarm audibility questions
How much quieter is a horn at twice the distance?
About 6 dB in open space. Sound level falls by 20 × log10 of the distance ratio, and 20 × log10(2) = 6.02 dB, so a horn rated 84 dBA at 10 ft gives about 78 dBA at 20 ft and 72 dBA at 40 ft. Reflections in a real room usually hold the level up a little, and walls and doors take it down.
How do two horns add up?
As unrelated sources, not as plain numbers. Convert each level with 10^(L ÷ 10), add those, then take 10 × log10 of the sum. Two equal levels give 3.01 dB more than one, and a second horn 10 dB quieter than the first adds only about 0.4 dB.
Can I use this to approve a notification layout?
No. Results are for study and planning practice only. Real notification layouts follow the code edition the AHJ enforces, the appliance data sheet and a sound level survey of the finished space. The required margin over ambient and the other audible limits are in NFPA 72 Chapter 18 for the edition your AHJ enforces; this page carries none of those values.