Fire alarm systems perform many functions, but one of the most important reasons to install the system is to notify the occupants. Determining the proper audibility level is key. If you can’t hear the alarm, you can’t respond to it.
Unfortunately, NFPA 72 requirements for audibility are rather vague. They state that it must be a minimum of 15 dBA (decibels measured using the “A” scale on a sound level meter) over average ambient sound levels. Over the years, I have asked many fire marshals and plan reviewers if fire alarm designers ever discuss with them acceptable sound levels before the installation. If this isn’t done and it’s installed with a sound level lower than the fire inspector’s expected level, it will be a problem. It is not easy to go in after all the walls and ceilings are installed to add more audible devices.
NFPA 72 requirements
NFPA 72 now requires the designer to provide the design audibility level as part of the submittals. That is a good start.
So how do you know how loud it needs to be? What is the ambient sound level going to be in the building if it is not built yet?
In the 2019 edition of NFPA 72, Table A.18.4.4, Average Ambient Sound Levels According to Location, gives some typical averages. In my opinion, however, they are not all accurate. For example, it shows an average of 54 dBA for business occupancies. I agree with that. However, it shows an average of 45 dBA for education occupancies, which I strongly disagree with.
Average ambient sound level is measured over the time the building is occupied. During class time, it is probably right. But it will be considerably higher during class change. If the system only provided for 15 dBA over average ambient sound level, it would only need to be 60 dBA or higher. In my opinion, that is too low. This is a good reason to discuss the expected sound level with the fire inspectors beforehand.
Steps before installation
The best way to determine what to expect is to use a sound level meter and take readings in an existing building with similar occupancy. If your company provides fire alarm testing services, that’s a great opportunity to take readings when doing the tests. Keep a record of these. If you don’t do fire alarm testing, you can still go into many public buildings and take readings, for the most part, although schools typically restrict entry due to safety concerns.
Once the average ambient sound level is established, determine how loud the fire alarm audible sounders will be. There are some simple facts to help decide that. It is not an exact science, but it will certainly be better that just installing horns and hoping it is loud enough.
One simple rule is that every time the distance from the audible device is doubled, up to 6 decibels (dB) is lost. That number will vary based on the building materials used. Hard surfaces reflect more sound than soft surfaces. I like to plan for the worst case, so I would always use the maximum of 6 dBA loss. So, if the horn is rated at 95 dBA at 10 feet, it will be 89 dBA at 20 feet from the device, 83 dBA at 40 feet, etc. This is in open areas such as hallways or rooms.
If the sound has to go through a door, window or wall, factor in the increased loss in sound. The average loss through an open doorway is 8 dBA, with a range of 4–12 dBA based on whether it is a single or double-wide door. Closing the door increases the average sound loss to 17 dBA, with a range of 10–24 dBA based on the type of door (hollow core, solid, etc.). I have found that for solid doors such as in hotel rooms and schools, it will be close to the 24 dBA loss. Again, I would use that number to determine worst case. You can find much of this information online.
Also, don’t forget to deduct for distance from the device from the other side of the door. For example, if 83 dBA is measured on the corridor side of the door, and it’s a solid- core door, there would only be about 59 dBA on the room side of the door. Once in the room, choose the worst-case location and deduct a few more dBA. If the sound level is not high enough to meet code, it may be necessary to install a horn in each room.