Understanding Audibility Requirements for Emergency Communication

Published: 2014-11-12

How many times are you at a venue and cannot understand the message that is being delivered over the public address system? On the other hand, you will be able to understand the spoken word at a theater performance, a hotel meeting room or at the ballpark.

The same intelligibility consideration is now being given to fire alarm and emergency communications systems when a voice message is required.

Voice intelligibility was added to the NFPA 72, 1999 edition. The definition in that edition was, “Audible voice information that is distinguishable and understandable.” It was in the 2007 edition of NFPA 72 that voice intelligibility began to take a more prominent position.

An annex to the chapter covering audibility provided information as to the types of distortion that could be responsible for the reduction of voice intelligibility: amplitude distortion; frequency distortion; and time domain distortion.

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The first type is a function of the system; the second and third are functions of the space that the speakers are to be installed within. These include size, shape, surface characteristics of the walls, floors and ceilings, and the character and placement of the speakers.

The following guidelines are presented:

“Because voice is composed of modulated tones, it is not valid to compare loudness measurements of tone signals with loudness measurements of voice signals. A voice signal that is subjectively judged to be equally as loud as a tone signal will actually produce a dB reading below that of the tone signal. The modulated tones of a voice signal may have the same or greater peak amplitude as that of a tone signal. However, because they are modulated meters with fast or slow time, constants will show a lower dB or dBA reading.

“A voice signal must have sufficient audibility to result in intelligible communication. Intelligibility modeling/measurements (subject based and instrument based) include audibility as well as many other factors when determining whether a voice signal is adequate or not adequate.

“Where a voice signal includes an audible alert or evacuation tone, the tone portion of the signal should meet the audible signal requirements listed in [Section] 7.4.2.”

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This was the first indication within NFPA 72 that voice intelligibility and the measurement of the same signal is not the same as for the production of sound pressure levels for audible tones that are produced by horns or the alert tone prior to an audible announcement. Voice intelligibility is not achieved by having the sound pressure level be 15dBA over the average sound pressure level of the space(s) that are to have coverage.

In the 2010 NFPA 72, the definition was changed to incorporate intelligibility and intelligible. Locations that may require voice intelligibility within their protected premises’ fire alarm and emergency communications systems include: schools, high-rise occupancies, theaters, places of worship, casinos, restaurants, transportation hubs or terminals, museums, malls and shopping centers, amusement park rides, etc.

Section 18.4.10 added the requirement that when designing a system in which voice intelligibility is required, Acoustically Distinguishable Spaces (ADS) must be included as a part of the design. This recognizes that not all areas within a premises may require voice intelligibility. Section Voice Evacuation Messages offers requirements for intelligibility within voice systems.

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