Code Speaks Louder About Intelligibility

Large occupancies in which high-quality sound is imperative, such as stadiums and performance theatres, invest big dollars in audio design, testing and equipment systems. Less attention has been paid to the quality of sound emitted by emergency notification devices in large structures; NFPA 72-2010 is making engineers and contractors take notice, however.

The NFPA 72-2010 code focuses on intelligibility and the need for voice evacuation systems to provide alerts with information that is audible and understandable. It defines intelligibility as the quality or condition of being intelligible (3.3.124) and intelligible as capable of being understood, comprehensible, clear (3.3.126). The code also adds the key term Acoustically Distinguishable Space (ADS) to help clarify intelligibility requirements.

ADS is an emergency communication system (ECS) notification zone, or subdivision thereof. This may 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 (3.3.2).

Establishing ADSs is foundational to planning an intelligible system. An ADS is any space that can or cannot have intelligibility. The ADS needs to be determined at the beginning of the project.

Chapter 18 – Notification Appliances, NFPA 72-2010 states that within the ADS, where intelligibility is required, voice systems shall reproduce prerecorded, synthesized, or live messages with voice intelligibility (18.4.10). In each of these spaces, measuring for intelligibility may or may not be required.

ADSs shall be determined by the designer during the planning and design of all ECSs ( Each ADS shall be identified as requiring or not requiring intelligibility ( Where an ADS is required by the authority having jurisdiction (AHJ), ADS assignments shall be submitted for review and approval (

Chapter 24 – Emergency Communication Systems provides requirements for designing an intelligible voice evacuation system for an ECS. 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; and the design shall incorporate speaker placement to provide intelligibility (

To meet NFPA requirements, the following is needed: the average ambient background noise level of the area; room characteristics such as length, width, and height of the ceiling and reflectivity of the surfaces in the room; and the coverage angle or polar plot of the speaker.

Annex D provides guidance on the planning, design, installation, and testing of voice systems. The annex also contains recommendations for testing intelligibility methods and requirements for testing.

When testing intelligibility, Annex D.2.4.1 recommends that 90% of all measurements in an ADS meet required intelligibility scores to be considered acceptable. These scores fall on the lower end of the intelligibility scale: a measured Speech Transmission Index scale (STI) of not less than 0.45 (0.65 CIS) or an average STI of not less than 0.50 (0.70 CIS). CIS stands for Common Intelligibility Scale.

The many factors involved can make designing a system that meets current intelligibility requirements challenging. However, the NFPA code has been designed to limit the complexity of these systems by minimizing the potential for overdesign. Therefore, the best approach is to be familiar with NFPA requirements and definitions before attempting to design a voice evacuation system for intelligibility.

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