Understanding Voice Intelligibility Loud and Clear

Audibility requirements of fire alarm and emergency communications systems have become more prominent in recent years. Learn where and when you may need to heed such standards based on how NFPA 72 has addressed the intelligibility issue.

Recently I was asked a question about voice intelligibility relating to an emergency communications system that was being installed within a casino. The general concern was regarding the volume of the sound and whether it would achieve the required intelligibility readings required by NFPA 72, Standard for Fire Alarm and Signaling Systems.

So let’s delve into the topic of intelligibility. This month I’ll focus on the progression of the intelligibility requirements within NFPA 72. Next month, I’ll address the designing of a system to meet these requirements.

Determining Which Areas Face Requirements

How many times are you at a venue and cannot understand the message that is being delivered over the public address system? You may notice this at a mall, subway platform, airport departure lounge, etc. 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 provided for that edition was, “Audible voice information that is distinguishable and understandable.” The first requirements were found within Chapter 4, Notification Appliances for Fire Alarm Systems.

4-3.1.5* Emergency voice/alarm communications systems shall be capable of the reproduction of prerecorded, synthesized or live (for example, microphone, telephone handset and radio) messages with voice intelligibility.

The annex to this paragraph provided reference to several International Electrotechnical Commission (IEC) documents and discussed that a system should exceed the equivalent of a Common Intelligibility Scale (CIS) score of 0.70. But no guidance was provided within this edition on how to achieve this score with a system.

In the 2002 edition, the requirements now in Chapter 7 did not change, but text was added to the annex to provide a level of guidance that was not present in the 1999 edition. The annex stated, in part:

The designer and the authority having jurisdiction (AHJ) should both be aware that the acoustic performance parameters of the chosen loudspeakers, as well as their placement in the structure, play a major role in determining how many devices are necessary for adequate intelligibility.

The annex also discussed the idea that not all areas within a protected premises would require intelligibility, noting:

There might be applications where not all spaces will require intelligible voice signaling. For example, in a residential occupancy such as an apartment, the authority having jurisdiction and the designer might agree to a system that achieves the required audibility throughout, but does not result in intelligible voice signaling in the bedrooms. The system would be sufficient to awaken and alert. However, intelligibility might not be achieved in the bedrooms with the doors closed and the sounder in the adjacent hallway or room.

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About the Author


Shane Clary, Ph.D., is Security Sales & Integration’s “Fire Side Chat” columnist. He has more than 37 years of security and fire alarm industry experience. He serves on a number of NFPA technical committees, and is vice president of Codes and Standards Compliance for Pancheco, Calif.-based Bay Alarm Co.

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