Emergency communications systems have amplified the need for effective voice communications systems. NFPA 72-2010 formally addresses measuring voice intelligibility to ensure what is said is heard properly.

Mass notification is a relatively new concept for the life safety community, which arose from the inability of emergency management personnel to communicate with and direct building occupants during emergencies. Since the publication of the Unified Facilities Criteria, a 2002 U.S. Department of Defense program outlining the design, operation and interfaces required for mass notification in military facilities (the final version for mass notification was approved in 2008), many U.S. military facilities throughout the world have installed mass notification systems (MNS). In the private sector, the demand for MNS has been rising steadily since Sept. 11, 2001. In response, the 2010 edition of NFPA 72 greatly improves design direction for the layout of intelligible voice systems.

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

Because the overall purpose of an ECS is to save lives and minimize injuries during emergencies, it is imperative for individuals to clearly understand voice messages delivered over facility-wide communications systems. As a result, the new ECS chapter includes intelligibility requirements for voice systems.

Intelligibility and Acoustically Distinguishable Spaces

What is intelligibility? Speech intelligibility is the measure of the effectiveness of speech. The measurement is usually expressed as a percentage of a message that is understood correctly. The 2010 edition of NFPA 72 defines intelligible (Section 3.3.126) 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 system to meet the day-to-day operational objectives. Chapter 24 of the 2010 code permits all 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 from 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 only require tone signaling, whereas other spaces may require no 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.

Designing an intelligible voice system does not lend itself to prescriptive design as visible notification appliances do. Speech intelligibility is not a physical quantity measured in feet, amperes, volts or even decibels. It is highly recommended that designers refer to annex D to plan, design, install and test 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 Certification in 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 system intelligibility before installation. These software programs model acoustic properties for 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 acoustical factors that influence intelligibility, such as the anticipated background noise level, occupancy type and architectural design of the space. The acoustical properties of the materials on the walls, floors and ceilings significantly impact the intelligibility of 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 intelligibility and audibility (decibel (dB) rating).

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

1) The speaker layout of the system shall be designed to ensure intelligibility and audibility.

2) Intelligibility shall first be determined by ensuring that all areas in the building have the required level of audibility.

3) 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 be considered to ensure speakers produce the correct levels of intelligibility and audibility. Ideally, 10-15 dBA above average ambient sound levels provide adequate intelligibility.

For the effects of speaker distance and wattage on audibility, see Figure 1.

Avoid installing wall-mounted speakers in large rooms with ceilings up to 15 feet in height as this contributes to more reverberation due to longer distances to opposing walls. 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. It is important to note that speech intelligibility testing is usually described as predictions, not measurements. Most instrument users, however, refer to the results as measurements. Because portable intelligibility meters are most commonly used for the accurate test results, the results 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 intelligibly is the Speech Transmission Index (STI) test protocol. STI is a quantitative methodology for 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 measurement locations 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 recorded voice message during an emergency is essential for the safety of a facility’s occupants, planning and testing is crucial. The best methodology to ensure a message is clear and intelligible in all situations is to measure intelligibility.

Tags: intelligibility, Mass Notification, Speakers and Strobes

Posted in Cover Features, Mass Notification, Summer 2010

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