Issue 69: Audibility vs. Intelligibility of Fire Alarm Systems
By Ernesto Vega Janica
Regulatory and measurement requirements for audible notification
appliances related to fire alarm systems have evolved during recent
years. Today, accurate measurements and computer simulations are often
used to demonstrate compliance with National Fire Alarm Code1 requirements pertinent to the design, commissioning, maintenance and testing of audible notification appliances.
This article reviews audibility and intelligibility concepts; their
measurement, including affecting factors; and acceptance criteria to
demonstrate compliance. For illustration purposes, multiple scenarios
for application of audible appliances are presented and discussed.
Prior to the 1990 edition of the National Fire Alarm Code, fire alarm systems were only required to "be heard clearly regardless of the maximum noise level.”2 Presently, after a constant evolution of the National Fire Alarm Code,
these requirements are more quantitative. Important definitions as well
as measuring standards have become available to the fire protection
Audibility requirements now depend on the occupancy, operating mode
(e.g., public, private or sleeping area - Figure 1) and average ambient
noise levels (Figure 2). For example, if the NFPA 72 Annex values are
used, fire alarm speakers installed in an office building corridor would
be designed to provide 70 dBA (55 dBA average ambient noise level to
account for business occupancy, plus 15 dBA for public mode). For the
same corridor located in an area requiring a private mode of operation,
such as, a correctional facility, the fire alarm speakers would be
designed to provide 60 dBA (50 dBA to account for institutional
occupancy, plus 10 dBA for private mode). Special attention is to be
given to hotel guest rooms and similar sleeping rooms where a minimum of
75 dBA sound level needs to be provided at the pillow level. As noted
in the Annex, the values are examples for different occupancies and
should not be used in lieu of actual ambient values obtained in the
Figure 1: Operating Mode
Figure 2: Average Ambient Noise Levels
Assuming the fire alarm audible notification design complies with
audibility requirements, an important question arises: will occupants
with normal hearing capabilities understand the message? Compliance with
audibility requirements ensures only that the message is loud enough to
be recognized by the occupants among other expected sounds.
MEASURING AMBIENT NOISE LEVELS (LA.eq. vs. LA.eq.24)
NFPA 72 prescribes the measurement of ambient noise levels in terms of
the equivalent sound level taken over the period of occupancy (LA.eq.t), or over a 24 hour period (LA.eq.24)
if the protected premises are occupied 24 hours a day. Ambient noise
levels are required to include all normal sound sources (e.g., air
handler units, background music, cleaning equipment, occupant noise,
etc.) but should exclude temporary or abnormal sound sources (e.g.,
construction equipment or office rearrangements). Although LA.eq.t is more accurate, in many cases LA.eq.24 is used due to its convenience and simplicity. However, LA.eq.24
readings can be misapplied where the ambient noise greatly varies
during a 24 hour period (Figure 3) and could provide a false low reading
relative to LA.eq.t (Figure 4). Therefore, it is prudent to
consider the occupancy period that provides the most accurate portrayal
of the ambient noise level when performing LA.eq.t measurements.
Figure 3: LA.eq.24 = 38.4 dBA
Figure 4: LA.eq.t (8AM-4PM) = 42.6 dBA
The definition of "voice intelligibility"1 has been modified
several times over the past years but, in essence, the goal still
addresses the same question: Can the occupants really understand the
message being transmitted? To help answer this question, NFPA 72
includes an annex on speech intelligibility (Annex D) which discusses
pertinent concepts and provides guidance regarding the terminology,
design, acceptance testing, limitations and requirements of
intelligibility. Among the concepts discussed are speech transmission
index (STI), common intelligibility scale (CIS), and acoustically
distinguishable spaces (ADS).
Can you hear me now? = Audibility But, do you really understand me? = Intelligibility
One of the most important facts to consider is that bothaudibility
and intelligibility are required by NFPA 72. But, only audibility
measurements are enforceable since they are part of code sections
(Figure 1). Intelligibility measurement guidelines are still located in
the annex sections (Figure 5). Therefore, they are not enforceable; they
are simply a good engineering practice recommended by the standard. In
other words, the audibility requirements must be met when designing fire
alarm systems, but intelligibility scores using STI or CIS methods are
not required unless they are directly referenced in other enforceable
codes or standards. For example, Department of Defense projects require
that intelligibility scores be measured.
MEASURING INTELLIGIBILITY (CIS or STI)
How intelligibility can be achieved and which elements must be
considered when designing fire alarm systems? The typical standard is to
provide 0.70 CIS (0.50 STI) throughout the protected premises in order
to meet the guidelines identified in NFPA 72 (Figure 5).
To accomplish this standard, engineers can make use of reference materials such as Annex D of NFPA 72, IEC 60268-16,3 ISO 7240-19,4 NEMA SB 50-20085
and measuring tool manufacturers' recommendations. Generally speaking,
if a design achieves acceptable intelligibility scores in accordance
with these references, audibility should also be accomplished. In other
words, an intelligible system should be audible, but an audible system
will not necessarily be intelligible.
Figure 5: Intelligibility Scores as per NFPA 72
Consideration should be given to factors affecting intelligibility
such as, background noise, reverberation, room dimensions, and ceiling
heights. While satisfying audibility requirements will help to overcome
background noise, reverberation can be reduced using new construction
and finishing materials designed for acoustical applications. As shown
in Figures 6 and 7, proper consideration of room dimensions and ceiling
heights can be accomplished by using the inverse square law and other
empirical assumptions (e.g., a 20% change on ceiling elevation is
generally considered as a separation between acoustically
Figure 6: Inverse square law and Signal to Noise ratio
Figure 7: Other factors affecting Intelligibility
Current measurements of intelligibility make use of many tools,
including new technologies incorporating software simulation
capabilities and more convenient field testing devices. The versatility
of these measuring tools allows engineers to evaluate a diversity of
real case scenarios even within the same facility. For example, an
office building with open cubicle areas, conference rooms and elevator
lobbies (Figure 8) may not represent a significant challenge when
pursuing intelligibility, but mechanical rooms, bathrooms and,
peripheral offices (Figure 9) might require additional calculations and
Figure 8: dBA and CIS measurements in cubicle areas, conference rooms and elevator lobbies
Figure 9: dBA and CIS measurements in mechanical rooms, bathrooms and peripheral offices
Ernesto Vega Janica is with Rolf Jensen & Associates in New York City
NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 2010.
NFPA 72A, Standard for the Installation, Maintenance, and Use of Local Protective Signaling Systems for Guard's Tour, Fire Alarm, and Supervisory Service, National Fire Protection Association, Quincy, MA, 1987.
IEC 60268-16, Sound System Equipment - Part 16: Objective Rating of Speech Intelligibility By Speech Transmission Index, International Electrotechnical Commission, Geneva, Switzerland, 2011.
ISO 7240-19,. Fire Detection And Alarm Systems -- Part 19: Design,
Installation, Commissioning and Service of Sound Systems for Emergency
Purposes, International Organization for Standardization, Geneva, Switzerland, 2007.
NEMA SB 50, Emergency Communications Audio Intelligibility Applications Guide, National Electrical Manufacturer's Association, Rosslyn, VA, 2008.
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This article discusses the three distinct parts of egress along with
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in each of the parts. Differing code requirements are also covered. READ MORE
Summer 2008 -- Mass Notification Systems: Design Challenges for the FPE – Wayne D. Moore, P.E., FSFPE
This article provides guidance on how to incorporate a mass
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owner expects from the MNS and presents considerations such as how to
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Winter 2007 -- Response to Fire Alarms – Guylene Proulx, Ph.D.
The way people respond to a fire alarm depends on a number of factors.
This article will explore some of these factors, including the type of
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their role in the building, training and past experiences, audibility
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