Got Intelligibility?

Be heard and understood, because people’s safety depends on it

  • By Christa Poss
  • Sep 01, 2011

For the life safety community, mass notification is a relatively new concept.

It arose from emergency management personnel’s inability to communicate with and direct building occupants during emergencies. Since the publication of the Unified Facilities Criteria, many U.S. military facilities throughout the world have installed mass notification systems. The criteria outlined the design, operation and interfaces required for mass notification in military facilities, and the Defense Department approved the final version for mass notification in 2008.

In the private sector, the demand for MNSs has been rising steadily since theSept. 11 attacks. In response, the 2010 edition of NFPA 72 greatly improves designdirection for the layout of intelligible voice systems.

The National Fire Protection Association introduced MNS criteria in the annexof the 2007 edition of NFPA 72, where it was presented for explanatory purposesonly. After the 2007 edition of NFPA 72 was published, the NFPA StandardsCouncil created a technical committee to develop a new chapter for the 2010 edition. Released in October 2009, the 2010 edition of the National Fire Alarmand Signaling Code provides emergency communication system (ECS) requirements,which include MNSs, in chapter 24.

Because an ECS is designed to save lives and minimize injuries during emergencies, it is imperative for individuals to clearly understand voice messages deliveredover facility-wide communications systems. As a result, the new ECS chapter includesintelligibility requirements for voice systems.

Intelligibility and Acoustically Distinguishable Spaces
What is intelligibility? Speech intelligibility is the measure of the effectiveness ofspeech. The measurement is usually expressed as a percentage of a message thatis understood correctly. The 2010 edition of NFPA 72 defines intelligible (Section3.3.126) as being capable of being understood, comprehensible and clear. Intelligibility (Section 3.3.125) is the quality or condition of being intelligible.

The first step in designing an intelligible voice system is to determine what type of ECS the building owner desires. The voice communication system will often include in-building fire EVACS, in-building mass notification and a paging systemto meet the day-to-day operational objectives. Chapter 24 of the 2010 code permitsall three systems to be combined, resulting in an ECS.

Voice intelligibility requirements refer to acoustically distinguishable spaces (ADSs). This term, which is new to the 2010 edition of NFPA 72, originated with research conducted by the Fire Protection Research Foundation on how to design and measure intelligibility.

Section 3.3.2 defines an ADS as “distinguished from other spaces due to acoustical, environmental or use characteristics, such as reverberation time and ambient sound pressure level.” An ADS allows the building to be divided into definable spaces so the system designer can identify which spaces in a building may require voice intelligibility.

Not all areas of a building are required to have voice intelligibility. In fact, some building spaces may require only tone signaling, whereas other spaces may requireno occupant notification at all. Per Section 24.3.1, an ECS must be capable of reproducing prerecorded or live messages with voice intelligibility in accordance with Chapter 18. Section 18.4.10.1 requires the system designer to identify ADSs during the planning and design of the ECS, and according to Section 18.4.10, each ADS may or may not require voice intelligibility.

It is recommended that designers refer to annex D to plan, design, install andtest voice communication systems. The majority of the annex contains recommendations for testing voice system intelligibility. Designers who are new to voice systems may want to consult other sources, such as the National Institute for Certificationin Engineering Technologies (NICET) program for audio systems or the National Electrical Manufacturers (NEMA) Emergency Communications Audio Intelligibility Applications Guide. Due to the complexity of designing a voice system, it may also be useful to use a software design program to predict voice systemintelligibility before installation. These software programs model acoustic propertiesfor specific environments and speaker configurations.

Design Factors to Consider
Several factors to consider when designing a voice system are: signal-to-noise ratio,frequency response, harmonic distortion and reverberation. Therefore, properly designing an intelligible voice system requires knowledge of the acousticalfactors that influence intelligibility, such as the anticipated background noise level, occupancy type and architectural design of the space. The acoustical properties ofthe materials on the walls, floors and ceilings significantly affect the intelligibilityof the space. Achieving voice intelligibility may be difficult, or even impossible, depending on the architectural design.

An important step in designing a voice system is determining the effect of the environmental and acoustical properties on speaker placement. In the past, fire alarm voice systems typically had too few speakers. It is important for designers to require the right speaker quantity and placement to ensure proper intelligibilityand audibility (decibel (dB) rating).

Section 24.4.1.2.2.1 requires the following be met for layout and design:

  • 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.

A rule of thumb is to install speakers in rooms with 10- to 12-foot ceiling heights at intervals measuring twice the ceiling height and 1 Watt per 750 to 1,000 square feet. The ambient noise level of the space served by the speakers must beconsidered to ensure speakers produce the correct levels of intelligibility and audibility.Ideally, 10-15 dBA above average ambient sound levels provide adequate intelligibility.

Avoid installing wall-mounted speakers in large rooms with ceilings up to 15 feet in height because the longer the distances to opposing walls contributes tomore reverberation. Also avoid installing speakers on ceilings that are greater than 20 feet in height, especially in rooms with highly reflective walls.

Testing Methodologies
Following installation, the system must be tested for intelligibility. Because portableintelligibility meters are most commonly used for the accurate test results, theresults are usually referred to as measurements to avoid confusion.

In accordance with D.2.1.1.1 in the annex, the recommended method for measuring intelligibility is the Speech Transmission Index(STI) test protocol. STI is a quantitative methodologyfor measuring intelligibility. Another method ,the Common Intelligibility Scale (CIS), was created to map all methods to the same scale so that all different results could be compared. In accordance with section D.2.4.1, the intelligibility of an ECS is considered acceptable if at least 90 percent of the measurementlocations within each ADS have a measured STI of not less than 0.45 (0.65 CIS) and an average STI of not less than 0.50 STI (0.70 CIS).

Because clearly understanding a live or recordedvoice message during an emergency is essential forthe safety of a facility’s occupants, planning and testing is crucial. The best methodology to ensure amessage is clear and intelligible in all situations is to measure intelligibility.

This article originally appeared in the September 2011 issue of Security Today.

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