An Intelligent Approach to Intelligibility

NFPA 72-2010 includes a section outlining important new requirements for voice evacuation systems. Better understand the concept of intelligibility, learn about the code changes and how they apply to notification systems.

<p>When installing speakers, each time the power output or number of installed speakers is doubled, the sound output increases by 3dB. Each time the distance between the listener and sound source is doubled, there is a 6dB loss in loudness.</p>Reverberation — This is the persistence of a sound through echoes and reflections after the initial sound source is removed. These echoes and reflections are the main reason why it is often difficult, sometimes impossible, to achieve a required intelligibility score in certain environments, like large, open areas with very reflective surfaces. If echoes are spaced too closely together, the sound cannot be clearly distinguished by the listener, so minimizing reverberation is key to achieving the intelligibility score desired.

Some of the main factors that affect reverberation include room size, the reflective properties of the surfaces in the space, the orientation of the speaker and the sound output of the speaker.

Talker and Listener Abilities — There is great variation in the human element relating to intelligibility, specifically around the abilities of the talker and listeners. Talker abilities that could affect intelligibility would include accents, dialects, diction, frequency of voice, etc. Listener ability is the sensitivity of a listener’s hearing.

Cracking Open the Code

While previously located in the Annex of NFPA 72-2007, intelligibility requirements have been written into the body of the code for NFPA 72-2010 (Chapters 18 and 24). For intelligibility, Chapter 24 states that the system should be capable of reproducing intelligible messages in accordance with Chapter 18.

As noted earlier, ADS is an important new term added to the NFPA 72-2010 code. An ADS is “an emergency communication system notification zone, or subdivision thereof, that might be an enclosed or otherwise physically defined space, or that might be distinguished from other spaces due to acoustical, environmental, or use characteristics, such as reverberation time and ambient sound pressure level” (section 3.3.2). Some factors that might acoustically distinguish spaces within a given area include floor, wall, and ceiling materials; ceiling heights; and ambient noise conditions.

According to the NFPA code (, each ADS is determined by the system designer during the planning and design phase of any system. In addition, each ADS shall be identified as requiring or not requiring voice intelligibility ( Finally, when intelligibility is required by the authority having jurisdiction (AHJ), ADS assignments have to be submitted for review and approval (

One of the main goals of the new wording around ADSs and the intelligibility requirement in NFPA 72-2010 is to help avoid over-designing and testing systems to meet intelligibility requirements in areas deemed “impossible” based on criteria identified in the ADS assignation phase.

Laying Out the System

Additional information in Chapter 24 provides requirements for laying out an intelligible voice evacuation system for an ECS. Section requires:

  • The speaker layout of the system shall be designed to ensure intelligibility and audibility
  • Intelligibility shall first be determined by ensuring that all areas in the building have the required level of audibility
  • The design shall incorporate speaker placement to provide intelligibility

In order to lay out speakers for a voice evacuation system, there are several things one needs to know about the speakers and the environment in which they will be placed. The following information is needed to meet NFPA requirements:

  • The average ambient background noise level of the area
  • Room characteristics, i.e., length, width, and height of the ceiling and reflectivity of the surfaces in the room
  • The coverage angle or polar plot of the speaker

Commercially available software programs can be used to simplify the design of intelligible voice evacuation systems. One such application is the EASE Evac software (, which is used by sound designers to model acoustical properties for specific environments and speaker configurations.

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