To Measure, or Not to Measure?
Rethinking the need for intelligibility testing of fire alarm systems

NFPA Journal®, January/February 2011

In "Can You Hear (and Understand) Me Now?" in the November/December issue, Bob Schifiliti looked at voice intelligibility issues related to emergency communications systems that NFPA 72®, National Fire Alarm and Signaling Code®, does not directly address. One had to do with measuring intelligibility, which was defined as a message that is not only audible but that can be clearly understood. The question before the fire alarm community is whether the code should require the measurement of intelligibility.

For years, NFPA 72 has required that the sound output of all fire alarm systems be measured. It makes sense that the fire alarm system installer should place notification appliances where they will provide sufficient sound output to gain occupants’ attention to warn them of a fire. Designers, installers, and authorities having jurisdiction (AHJs) have long had to consider code requirements for fire alarm system audibility.

By contrast, NFPA 72 did not refer to intelligible voice directions for building fire emergency voice/alarm communications systems until the 1999 edition. And the concept of voice intelligibility measurement did not appear in the Annex A material for Chapter 7, "Notification Appliances," until the 2007 edition. The NFPA technical committee placed the material in the annex rather than the body of the code because measuring intelligibility depends on a number of items that often cannot be controlled when a fire alarm system is tested before a building is occupied. While the 2010 edition of NFPA 72 does not require the measurement of intelligibility, it does present recommendations for intelligibility testing in a new Annex D. As Schifiliti pointed out, "The test protocols in the annex are not required; the code permits them to be used, but also allows a simple ‘listen’ test."

Even so, design specifications now frequently require that all voice communications systems be measured for intelligibility. Obviously, any fire alarm system using voice communications to provide direction to building occupants during an emergency must provide intelligible communications. Whether or not an installer measures intelligibility, occupants must always clearly understand the message, and building fire emergency voice/alarm communications systems must be designed and installed accordingly.

As for requiring measurement of intelligibility in the code, are we placing the cart before the horse?

The audio communications industry has been able to design for intelligibility for many years, and it has spent at least the last decade developing performance standards for the equipment it produces. By contrast, those in the fire alarm system community remains in catch-up mode. Historically, the design of building fire emergency voice/alarm communications systems was patterned after the design of nonvoice systems, using just a few speakers installed only in common areas. AHJs accepted this design concept largely because they had no information as to what constituted an intelligible voice communications system. 

Should we require measurement before we also require the fire alarm amplifiers and speakers currently available to meet the same acoustic performance standards that basic professional sound and communications equipment provide? Should we require measurement without making certain that designers, installers, and AHJs have the opportunity to learn about the basic sound reproduction and communications concepts?

It is my opinion that before we rush to measure intelligibility, designers, manufacturers, installers, and AHJs should understand what it takes for both the equipment and design to ensure intelligibility. After all, during a fire or other emergency, no one has the luxury of asking, "Can you hear (and understand) me now?"