Horn dBA rating for Fire Alarm notification

[Akten]

Hi, I would like to ask how to calculate required # of Fire Alarm Horn notification appliance in a building? I believe NFPA 72 provides ambient Sound level based on location, and the Horn dBA should be 15 dB above the ambient sound level of a location.

• Is there a standard Horn dBA value that people use for design for example, can we assuming a horn would be rated for 85dBA (or a different number) at every 10ft?
• I was looking at a Horn product from Microm and based on the data sheet, there are different dBA values given in the table. I believe US installation should be based on UL464 table, but I am not sure which sound pattern and volume I should be looking at to determine the rating. (https://mircom.com/wp-content/uploads/product_documents/CAT-5360_FH-400_Wall-Ceiling_Mount_Horns.pdf)

 

[Caddman99]

You are correct, NFPA 72 states that you should measure the ambient sound level for an area when designing, but offers a guide in the form of a general table in section A18.4.4 (2025 Edition) to give average Sound Pressure Levels for typical occupancies.
Also correct: 15 dBA above ambient for public mode and 10 dBA above ambient for private mode.
If you are working with a standard fire alarm system with a temporal 3 evacuation tone, use MGC's UL464 table, focusing on the Temporal row and Reg. 24 VDC column. This allows you to select between the High and Low settings for the device producing either 84.9 dBA for the high setting or 80.5 dBA for the low setting (measured at 10'). Keep in mind there is a formula for how sound pressure levels diminish as you get farther from the device. I can't recall the exact rule of thumb for SPL drop, but it is something like; for each 6' distance from the device, the SPL is cut in half.

 

[Caddman99]

You are correct, NFPA 72 states that you should measure the ambient sound level for an area when designing, but offers a guide in the form of a general table in section A18.4.4 (2025 Edition) to give average Sound Pressure Levels for typical occupancies.
Also correct: 15 dBA above ambient for public mode and 10 dBA above ambient for private mode.
If you are working with a standard fire alarm system with a temporal 3 evacuation tone, use MGC's UL464 table, focusing on the Temporal row and Reg. 24 VDC column. This allows you to select between the High and Low settings for the device producing either 84.9 dBA for the high setting or 80.5 dBA for the low setting (measured at 10'). Keep in mind there is a formula for how sound pressure levels diminish as you get farther from the device. I can't recall the exact rule of thumb for SPL drop, but it is something like; for each 6' distance from the device, the SPL is cut in half.

Sorry, a correction to the SPL rule of thumb: Every time the distance from the source doubles, the sound level decreases by about 6 decibels (dB).