Understanding Audibility Requirements for Emergency Communications Systems

Learn where and when you may need to heed audibility requirements of fire alarm and emergency communications systems based on how NFPA 72 has addressed the intelligibility issue.

It was in the 2007 edition of NFPA 72 that voice intelligibility began to take a more prominent position. The definition did not change, but the requirements did undergo a slight modification:* Voice messages shall not be required to meet the audibility requirements of 7.4.2 (Public Mode Audible Requirements), 7.4.3 (Private Mode Audible Requirements), 7.4.4 (Sleeping Area Requirements) or 7.4.5 (Narrow Band Tone Signaling for Exceeding Masked Thresholds), but shall meet the intelligibility requirements of where voice intelligibility is required.

Important Influences on Intelligibility

The annex to 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.

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 annex to provided the following guidelines:

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 7.4.2.

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. As I will discuss next month, 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 edition of 72, the definition was changed to incorporate intelligibility and intelligible.

3.3.125 Intelligibility. The quality or condition of being intelligible.

3.3.126 Intelligible. Capable of being understood; comprehensible; clear.

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