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
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
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
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 18.104.22.168 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 22.214.171.124.2.4 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
NFPA 72 has added a significant amount of annex material to assist designers in the design of intelligible voice systems.
2nd Quarter 2011 – Signaling Strategies for Means of Egress – Robert P. Schifiliti, P.E., FSFPE, R.P. Schifiliti Associates, Inc. on behalf of NEMA
This article addresses some of the more interesting and controversial
means of egress issues in the Life Safety Code, the IBC, and the IFC
over the last 10 years, as well as some of the changes that NFPA and ICC
will be considering in the near future. Subjects covered include
minimum stair width, additional exit stairways in high-rise buildings,
non break-a-way sliding doors, luminous egress path markings, egress
capacity, dead-end corridor limitations, and elevators. READ MORE
4th Quarter 2007 – It’s Not Your Father’s Fire Alarm Code Anymore – NEMA
The National Fire Alarm Code has evolved greatly since its origination
back in the late 1800s and early 1900s. The 2007 edition has branched
out to address risks and solutions for more than just fire. This article
– part one of a two-part series – examines the evolution. Part two will
discuss how the evolution might continue during the next 10 years. READ MORE
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