Thursday, August 3, 2017

Low Frequency Sounders for Sleeping Areas

What are Low Frequency Sounders for Sleeping Areas?

Anyone in the fire alarm design and installation industry has more than likely heard the term low frequency sounders.  These newer sounders first appeared in the NFPA 72 2010 edition under section and stated an effective date of January 1, 2014.  In short these are re-designed audible appliances for sleeping areas only!  Multiple studies have been performed on sleeping parties to see how they react in different scenarios.  Some of the scenarios involved older individuals who have a difficulty with their hearing,  others were individuals who are under the influence of narcotics.  The original thought was that these people would have a difficult time awaking from a sleeping state via the standard 3 Hz sound put out by a typical mini-horn commonly found in apartment and hotel sleeping areas.  The newer low frequency sounders have a square wave signal with 520 Hz plus or minus 10 percent.  Below are two links to show you the difference between the two sound outputs:

Listen to a 520HZ Low Frequency Tone

Listen to a 3 KHz Standard Tone 

Now that it is 2017, most of the country is currently enforcing the NFPA 72 2013 edition.  If your AHJ is enforcing the 2013 version of this standard, you have been required to provide these low frequency appliances in all sleeping areas for nearly 4 years.

Are you Installing Low Frequency Sounders Correctly?  

Now the same installation factors found in the NFPA 72 standard apply to low frequency sounders.  They shall have a sound level of at least 15 dB over average ambient, 5 dB or maximum sound level (duration of at least 60 seconds) or 75 dB whichever is highest.  Now seeing that NFPA 72 2013 has been nice enough to provide us with a chart depicting average ambient sound levels for different types of occupancies, this cannot be used in lieu of actual readings taken at the site.  However we can take away from this chart that a residential occupancy has an average ambient sound level of 35 dB.  If we use this as an example and tack on the additional 15 dB per NFPA 72 2013 18..4.5.1, we only come up with 50 dB.  This is 25 dB lower than the required 75 dB per the same standard section.  An example of the only time you would be going higher than 75 db is a sleeping area with an average ambient sound level of 61 dB.  61 dB plus 15 dB = 76 dB.  In short, 99 percent of the time, a designer will be aiming to achieve 75 dB.

Occupancy Average Ambient Sound Level NFPA 72
NFPA 72 Occupancy Average Ambient Sound Levels
Tips to keep in mind:

  • If you double the power to the sounder, you will gain 3 dB
  • The dB from the appliance is measure at 10' away.  Every time you double the distance from the appliance, you loose 6 dB.  Read more here.

What Exactly is a Sleeping Area?

Seems like a pretty easy question to answer right?  Well I'll have you know this comes up a lot and more often than not, designers get it wrong!  A sleeping area is obviously associated with a bedroom and a bedroom is not classified as such unless it has a closet.  However, as defined by NFPA 72 2013 A., low frequency sounders are required for use in areas intended for sleeping (bedrooms) as well as areas that might be reasonably used for sleeping (Living Rooms).  Most apartments or hotel rooms are outfitted with a comfy couch in the living room.  It's not uncommon to find yourself falling asleep in this area.  This is what has prompted the requirement for low frequency sounders to be installed in not only the bedrooms but the living rooms as well. 

How Does this Affect my Design?  

It is pretty common knowledge that these 520 Hz low frequency sounders draw a lot more current than the standard 3 Hz mini-horns.  You will need to take this into effect when calculation voltage drop and battery calculations for the remote power supplies serving the areas in question.  Depending on the size of the project, this may mean you need additional notification appliance circuits and remote power supplies. 

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