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Audibility vs. Intelligibility of Fire Alarm Systems
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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.

AUDIBILITY
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 engineering community.

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 field.

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


INTELLIGIBILITY

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 both audibility 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 distinguishable spaces).

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 notification appliances.

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

  1. NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 2010.
  2. 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.
  3. IEC 60268-16, Sound System Equipment - Part 16: Objective Rating of Speech Intelligibility By Speech Transmission Index, International Electrotechnical Commission, Geneva, Switzerland, 2011.
  4. 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.
  5. NEMA SB 50, Emergency Communications Audio Intelligibility Applications Guide, National Electrical Manufacturer's Association, Rosslyn, VA, 2008.

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