Is the fire alarm too loud? If you’ve ever been asked this question, you may have wondered whether the questioner had a point or not.
First I reviewed the Underwriters Laboratories (UL) listed horn available on the market today. Currently, the standard commercial horn sounds at a measured decibel output (by UL) in the ranges of 87 dBA to 95 dBA measured at 10 feet from the sound source in the UL anechoic chamber (no reverberation). All of these appliances sound at a frequency of approximately 3,000 Hertz (Hz).
Based on research over the last 10 years, the very young and the very old have difficulty hearing that frequency level. That research also showed that hearing a fire alarm signal at 520 Hz was equally heard by the very young, the very old and those who needed awakening at night. Based on that research, the code changed the requirements for audible alarm appliances in all sleeping rooms to be at that frequency.
Keep in mind the requirement for audibility in these areas remains at 75 dBA measured at the pillow level in the bedroom. To awaken occupants the audible appliances must produce a low frequency alarm signal that complies with the 520 Hz requirement. The Annex A information also provides guidance for areas “intended for sleeping and in areas that might reasonably be used for sleeping. For example, this section requires a low frequency audible signal in a bedroom of an apartment and also in the living room area of an apartment as it might have sleeping occupants.”
The annex does advise us that it is not required to use the low frequency signal in the hallways, lobby and other tenantless spaces. For example, in hotels, the guestrooms would require use of the low-frequency signals, but other spaces that might require audible signals could use any listed audible appliances regardless of the frequency content of the signal.
The code requires a minimum of 15 dBA above the ambient sound levels present in the space. The bottom line is the audible signals are not too loud but must be loud enough to move the occupants to take action.
Still you may come across customers who are concerned that the volume of an alarm could negatively affect their hearing. For example, I have been asked by a principal of a K-12 school what her response should be to parents who are worried the alarm signal is so loud it will injure children’s hearing.
To answer the principal’s questions, first I turned to the information provided in the annex of NFPA 72-2019, National Fire Alarm and Signaling Code. Annex A for Section 18.104.22.168 discusses the maximum sound pressure level of 110 decibels (dBA) permitted in a space. In the 2007 edition, this was reduced from 120 dBA in previous editions.
According to NFPA 72, “the change from 120 dBA to 110 dBA was made to coordinate with other laws, codes, and standards. In addition to the danger of exposure to a high sound level, long-term exposure to lower levels could also be a problem.”
For example, when occupants must traverse long egress paths to exit or technicians test large systems over extended time, the exposure to loud alarm signals could cause damage to their hearing.
The issue is the code is silent on how long a person can or should be exposed to an audible notification system. The limit of 110 dBA has been set as a reasonable upper limit by the code for the optimal performance of a system.
The annex goes on to say, “For workers who could be exposed to high sound levels over the course of a 40-year employment history, OSHA has established a maximum permitted dose before a hearing conservation program must be implemented. A worker exposed to 120 dBA for 7.5 minutes a day for 40 years might be in danger of suffering a hearing impairment. The OSHA regulation includes a formula to calculate a dose for situations where a person is exposed to different sound levels for different periods of time. The maximum permitted by the regulation is an 8-hour equivalent dose of 90 dBA. It is possible to calculate the dose a person experiences when traversing an egress path where the sound pressure level varies as he/she passes close to, then away from, audible appliances. Table A.22.214.171.124 depicts OSHA permissible noise exposures.”
I used the above information to assure the principal that the short duration of the students’ exposure to the alarm signal was not going to cause hearing damage. In addition, the sound pressure levels in the hallways of her school did not come close the maximum of 110 dBA allowed by the code.
All in all, you can tell your customers that alarm systems are loud by design. The uncomfortable volume during egress should be short-lived and well worth it to help as many occupants as possible be spared more serious injury or harm.