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Intelligibility Requirements of NFPA 72 – Dispelling the Myths
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Issue 94: Intelligibility Requirements of NFPA 72 – Dispelling the Myths

By Ray Grill, P.E., FSFPE, Principal, ARUP

Intelligibility of voice alarm systems became a regular topic of discussion and debate in the industry after the development of quantifiable testing equipment in the late 1990’s.  The debate calmed until NFPA 72 began incorporating criteria allowing the use of a fire alarm system to provide warning of other than fire emergencies. The incorporation of mass notification and the creation of a new Chapter 24 in the 2010 Edition of NFPA 72 also reinvigorated the debate.  This article reviews some of the history of the requirement for intelligible voice communication and tries to clarify some of the misconceptions that the author often hears expressed by engineers, contractors and authorities having jurisdiction.


Myth: The requirement for intelligible voice is new in NFPA 72

Believe it or not, the first edition of NFPA 72 that actually had mandatory requirements for audible notification appliances was the 1993 Edition. Prior to that edition, criteria for audibility (loudness) was provided in the Annex to the code.  The 1993 Edition of NFPA 72 also mandated that speakers be able to reproduce normal voice frequencies.


The 1993 Edition was also the first edition of NFPA 72 to have mandatory testing and inspection requirements for fire alarm systems.  Prior to that edition, NFPA 72H provided guidance on testing fire alarm systems.


The 1993 Edition of NFPA 72 noted that the clarity of voice produced by speakers was to be verified as part of the testing.  This is the first edition where testing of voice intelligibility became a requirement.


The 1999 Edition enhanced this requirement and required that voice messages be intelligible and also incorporated a definition for intelligibility. Voice intelligibility was defined as audible voice information that is distinguishable and understandable.


The requirement for intelligible voice has been in NFPA 72 as long as the requirement for fire alarm testing. It has been in the code for more than 20 years.


Myth: NFPA 72 requires quantifiable measurement of intelligibility

There has never been a requirement in NFPA 72 that voice systems be quantifiably measured for intelligibility.  For those of you who may not be familiar with quantifiable measurement of voice, there are various ways to do this and these ways are identified in Annex D of NFPA 72.  The most common way to perform quantifiable measurement of voice in the fire alarm industry is to utilize an intelligibility test kit. The kit includes a "talk box” and an intelligibility meter.  The microphone of the system is set up in the talk box.  The talk box plays a calibrated sound into the microphone.  In the field, the intelligibility meter is used to measure the intelligibility performance of the system.  The measurement is either on the Common Intelligibility Scale (CIS) or the Speech Transmission Index (STI).  The calibrated sound is perceived by a listener (at least this listener) as an undulating rasping sound.  The meter takes into consideration acoustical characteristics of the space and the noise and distortion that may be inherent in the system.  A score of 0.5 on the STI scale and 0.7 on the CIS scale are considered to be intelligible.


Currently, the only organization that requires quantifiable measurement of intelligibility of voice for emergency communications is the Department of Defense.


Myth: Voice messages are required to meet audibility requirements of NFPA 72

Some people believe that a voice system is required to be enhanced after installation because the voice wasn’t loud enough when measured using an audibility meter.  Measuring the voice message with an audibility meter is not required by NFPA 72. Section of the 2013 Edition of NFPA 72 specifically indicates that voice messages shall not be required to meet the audibility requirements of the code.  This doesn’t mean that we don’t test the system for audibility. It means that the alert tone that is required to be played prior to the message is what is used to measure audibility.  Voice messages consist of a broad range of sound pressure levels. Audibility meters typically average the sound pressures and will inevitably show a lower sound pressure level than that which is being produced by the alert tones.


Myth: If a system is designed for intelligibility, the designer does not have to worry about audibility

This approach could be a disaster waiting to happen.  As noted above, when implementing voice messaging, we are not required to measure the auditability of the voice, but we are required to design our system to produce tone signals at the appropriate audibility levels based on the requirements of NFPA 72.  NFPA 72 requires 15 dB above ambient unless private mode signaling or other special conditions such as sleeping areas come into play.

Myth: Quantifiable measurement of intelligibility is easy and produces repeatable results

The test equipment that is now available for performing quantifiable measurement is relatively easy to use.  However, there are many variables in the setup of the equipment and the environment that can affect the results. During the code development cycle for the 2013 Edition of NFPA 72, there was a research project conducted through the Fire Protection Research Foundation. The voice alarm system in three different types of facilities were tested for intelligibility using multiple intelligibility measurement kits.


There was significant debate during the setup of the talk box.  The instructions for different systems on volume adjustment and distances between the speaker of the talk box and microphone were different. It was also noted that not only are the acoustical characteristics of the area where the speakers are located important, but the acoustical characteristics of the room in which the talk box was located could have a significant impact on the intelligibility measurement.  Those conditions could easily be changed over time to impact intelligibility of the measurement.


Buildings that were tested had voice systems that performed at an acceptable level based on subjective testing (being able to understand the messages).  During quantifiable testing, much of the area did not come close to meeting the quantifiable level of intelligibility that would be considered acceptable.


These tests where enlightening to many of the participants who had not previously been exposed to quantifiable intelligibility testing.


Myth:  If a building is required to have voice messaging, intelligibility is required throughout the building

NFPA 72 in Section specifically exempts certain areas from having to have intelligible voice.  These areas include, private bathrooms, shower rooms, saunas, mechanical and electrical rooms, elevator cars, individual offices, kitchens, storage rooms, closets and rooms/areas where intelligibility cannot reasonable be achieved.


When this allowance is identified, skeptics often ask "how will people know there is a fire or other emergency?”  First, as noted in the research and from listening to numerous fire alarm tests, the voice is often understandable even though the measurement may be below what would be considered quantifiably adequate. Also, the voice message is only one aspect of the system.  The system is first required to produce two cycles of an alert tone to get the attention of the occupants.  NFPA 72 also recommends that the voice message be repeated at least 3 times.  Repeating the message allows people to better understand the message or to move toward a speaker to get better intelligibility.



The design of emergency voice communication systems can be complicated and challenging.  Designers often don’t know what the actual building conditions will be at the dime the design is being prepared.  Engineers need to ask more questions in the case of designing systems for new buildings and take into consideration existing acoustical characteristics of buildings when voice is being added to existing structures.

NFPA 72 has added a significant amount of annex material to assist designers in the design of intelligible voice systems.

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