Quest for Clarity
Why it's important to understand MNS intelligibility
BY WAYNE D. MOORE
THE PROLIFERATION OF installed mass notification systems (MNS) and the building code requirements for voice fire alarm systems in occupancies such as new K–12 schools have raised ongoing questions regarding the intelligibility requirements of the 2016 edition of NFPA 72®, National Fire Alarm and Signaling Code. The code specifies voice intelligibility for all voice systems, but it does not require the evaluation of intelligibility “through quantitative measurements.” The code does not say why measurements are not required, but an understanding of the factors that affect intelligibility will give users and enforcers of the code a better understanding of the technical committee's intent.
The majority of newly installed fire alarm systems must pass a final acceptance test for the building to receive its occupancy permit. The contractor conducts this test before the building is occupied, often before the installation of carpeting and furniture. An empty building has different acoustic properties than a building fully occupied with all furniture and interior finishes in place, however, and early in the process of intelligibility measuring it became apparent that many buildings would not pass the measurement requirements, as defined in Annex D, until they became fully occupied.
In fairness to most fire alarm system designers, they have not received much education in the area of sound and communications. To that end, the code and the handbook have tried to include additional guidance for the placement of speakers to help ensure the achievement of intelligibility in new system installations. For example, the annex to 184.108.40.206.2 states that, generally speaking, “in a standard building configuration with normal ceiling height (8 to 12 feet), normal ceiling construction (e.g., drop acoustical ceiling tiles), standard wall configurations, and finishes and carpeted floors, ceiling-mounted speakers should be installed in all normally occupiable spaces and in corridors spaced at a maximum of twice the ceiling height or as determined by a commercially available computer acoustical/speaker modeling program.”
Annex guidelines provide one key takeaway, namely that a one-for-one replacement of existing notification appliances, regardless of type (horns, bells, or speakers), will not meet the goal of intelligibility. Acoustic properties of the space involved, such as ceiling height and reverberant characteristics, will play an important part in the decision on speaker type and placement. In response, the 2016 edition of the code allows the use of non-listed speakers, which would include the use of their associated non-listed amplifiers, to interface with the fire alarm system to meet the requirement for intelligibility in acoustically challenging spaces. The code states that “where no listed loudspeaker exists to achieve the intelligibility requirements of the code for a notification zone, non-listed loudspeakers shall be permitted to be installed to achieve the intelligibility for that notification zone.”
The technical committee explains in the annex that in some acoustically challenging areas, listed fire alarm speakers might not be capable of producing an intelligible message and that non–fire alarm listed speakers are permitted to be installed in these limited areas. “A failure of a non-listed speaker should not disrupt the operation of listed fire alarm speakers and operation of the fire alarm or mass notification control equipment,” the annex states. “Typically, a dedicated speaker circuit and other audio components such as amplifiers could be necessary to meet this functionality.”
Building owners, system designers, installers, and authorities having jurisdiction must remember that effective communication cannot take place without intelligibility, and that without intelligibility occupants cannot understand the message—or take appropriate action.