Use Your Inside Voice: Intelligibility requirements for fire alarm systems

Published On
Sep 11, 2020

Your life could depend on being able to hear and understand emergency messages broadcast from an emergency voice alarm system. Originally, NFPA 72, National Fire Alarm and Signaling Code, only stated that the message had to be “distinguishable and understandable.” This language was subject to interpretation and test methods varied from one facility to another.

The concept of intelligibility testing was added to the 2007 edition of NFPA 72 with the intent to provide a quantitative method of testing. The intent was to test voice messages with an intelligibility meter to ensure certain minimum qualities were met. It was also intended to improve consistency in testing. The test methods allowed in the testing table of Chapter 10, “Inspection, Testing and Maintenance,” included use of a subject-based test method, use of methods and instruments that measure certain physical parameters and provide a common intelligibility scale (CIS) score, or use of test methods acceptable to the authority having jurisdiction.

As you can see, there was still a wide range of test methods. I believe the intent was to have a requirement for testing using instruments to provide a CIS score. This has never happened to date. The technical committees still feel that the increase in the cost of that level of testing does not justify the use of such methods for all voice communications systems, so any requirements for intelligibility testing for CIS scores is usually part of the design specifications.

The 2007 edition was the first time a new annex was added to describe the use of mass notification systems. As part of the restructuring of NFPA 72 in the 2010 edition, that annex became part of the new Chapter 24, “Emergency Communications Systems.” A new Annex D was added to primarily provide recommendations for intelligibility testing of voice systems. If you are installing a fire alarm or mass notification voice systems, review information in Annex D prior to installation. In the 2010 edition, a new definition for acoustically distinguishable space (ADS) was added to define spaces with different audible and environmental characteristics.

From here on, this article references the 2019 edition of NFPA 72. Currently, NFPA 72 Section states that when we use voice for notification, we are not required to meet the audibility requirements of Chapter 18, but we shall meet intelligibility requirements where intelligibility is required. The alert tone is the only thing that must have its audibility level tested. Section 18.4.11, “Voice Intelligibility,” includes useful information.

It is the responsibility of the design professional to determine the ADSs and which ones will or will not require voice intelligibility. Voice intelligibility requirements must be specifically required by a law, code, standard or other part of NFPA 72. That would likely come from Chapter 24, “Emergency Communication Systems.” Section 18.4.11 also states that quantitative measures are not required. That means if you are required to conduct intelligibility tests, you would not be required to use the instruments that provide a CIS score or speech transmission index; however, these methods are allowed.

Chapter 24, Section 24.3.1 “Intelligible Voice Messages” states that, if listed speakers do not achieve the intelligibility requirements of the code, you can use nonlisted speakers to meet the requirements. It is important that the speaker layout be designed to ensure intelligibility and audibility and that intelligibility is first determined by ensuring audibility levels are met.

Many manufacturers provide online speaker layout tools you can use to determine how many speakers will be needed based on room dimensions, ceiling height, acoustic characteristics and audibility levels. This is necessary to provide clearer, more understandable messages in buildings.

To the best of my knowledge, no national model code requires voice intelligibility to be tested. This may be a requirement of the design specifications. Obviously, it is more critical to verify intelligibility is some facilities than in others. The only written requirement I have seen is in the Unified Facilities Criteria for “Design and O&M: Mass Notification Systems,” which is used by the Army Corps of Engineers.

I hope this helps provide a better understanding of the requirements—or lack thereof—when installing emergency voice alarm communications systems or mass notification systems. I tried to provide enough detail about which chapter and section provides information so you can find it a little more easily the next time you are preparing to install one of these systems.

About the Author
Tom Hammerberg

Thomas P. Hammerberg

Life Safety Columnist

Thomas P. Hammerberg, SET, CFPS is an independent fire alarm presenter and consultant in The Villages, Fla. He can be reached at

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