October 31, 2013

Fire alarm notification appliances: why are they so loud?

At Loma Linda University Health, some buildings also have a voice evacuation system installed that can be used by personnel to give specific live information and/or instructions over the alarm system’s speakers using a built-in microphone. This provides a distinct advantage over horns or bells. Not all of our buildings have this feature yet, but I’m working on it.

I receive numerous complaints that the level of fire alarm speakers or horns is set too loud. Let me try to explain the reason the decibel level or noise level is set the way it is.

A fire alarm notification appliance is an active fire protection component. A notification appliance may use an audible or visible warning to alert the occupants of a fire or other emergency condition requiring action. In the United States, fire alarm evacuation signals generally consist of a standardized audible tone, with visual notification in all public and common use areas. Emergency signals are intended to be distinct and understandable to avoid confusion with other signals.

Alerting methods include:

  • Sound (audible signals) horns and speakers in the 65 decibel to 120 decibel range with a weighted rating at 10 feet from the device.
  • Light (visible signals) 15 to 1,000 candela at 1 flash per second strobe lights or white or red incandescent lights.

NFPA 72, Chapter 7, Notification Appliances, outlines requirements for mounting heights and loudness for fire alarm notification appliances. NFPA 72 states that audible fire alarm notification appliances used in the public mode must be a minimum of 15 dB (decibels) above average ambient sound levels. A typical office is between 50 and 60 decibels average ambient sound level.

A good rule of thumb is that up to six dB is lost every time you double the distance from the horn or speaker. Audible notification appliances are rated at 10 feet from the appliance. So, if you had a horn rated at 85 dBA at 10 feet and you measured the sound level at 20 feet, you could be as low as 79 dBA.

How much sound is actually lost is determined by the materials in the room. If the room has acoustical ceiling and carpets, more sound is lost than if the walls and ceiling are drywall and the floors are tile.

Another factor in determining sound levels in a given location is factoring for sound loss through doors. If a door is open, you will typically lose 4 dB to 12 dB. If you close the door, that loss will increase to 10 dB to 24 dB, depending on whether it is a hollow core door or a solid door. Sealing the openings around the door increases the sound loss to 22 dB to 34 dB. Unfortunately, not every LLUH room is equipped with an audible device, so the area in which the device is located must be adjusted to accommodate the other areas it serves.

Note: This post appears in “Speaking of Safety blog,” a two-way communication tool created to increase communication among LLUH safety coordinators and the department of environmental health and safety.