Intelligibility And Mass Notification Systems
This article has been contributed by Richard Closs, director of the systems group at Acentech, Inc., a multi-disciplinary acoustics, audiovisual systems design, and vibration consulting firm.
A mass notification system (MNS) is designed to clearly communicate with people during a catastrophic disaster or threat, steering them away from danger and toward safety. There are two classifications of the MNS: wide-area, providing real-time information to outdoor areas, and in-building, providing information and instruction to people inside a building by way of intelligible voice communications and often, visible signals, text, and graphics.
This article focuses on a specific aspect of the in-building MNS—the voice communications of an emergency communication system (ECS) that provides fire alarm, public address, firefighter, area of refuge, and elevator communications during an emergency. The National Fire Protection Agency (NFPA) provides code requirements for a MNS and ECS used for these purposes as well as weather emergencies; terrorist events; biological, chemical, and nuclear emergencies; and other threats. The NFPA code referred to in this article is the NFPA 72® National Fire Alarm Code® 2010 and 2013 editions.
NFPA Code Requirements
In 2010, the NFPA added two new terms relating to building communications to its National Fire Alarm Code® revision: acoustically distinguishable space (ADS) and speech intelligibility. An ADS is a space that is acoustically different from adjacent spaces and therefore requires a different approach to the design of the adjacent spaces: translated into code-speak, this space is “an emergency communications system notification zone, or subdivision thereof, that might be an enclosed or otherwise physically defined space, or that might be distinguished from other spaces because of different acoustical, environmental, or use characteristics, such as reverberation time and ambient sound pressure level.”
An example of an ADS could be a multi-story atrium adjacent to low ceiling concourses. Speech intelligibility, as the name implies, is a measure of the actual understandability of speech when all factors (acoustics of the space, background noise levels, and sound system design) are considered. These terms have greatly changed the design of communication systems for buildings.
Prior to the National Fire Alarm Code® 2010 and 2013 edition changes, fire protection engineers were required to design systems to be audible, which meant announcements were louder than the ambient noise in the space by 15 dBA (the logarithmic scale used to measure levels of sound). This design approach applied to all types of acoustic spaces and did not address the complexities of room acoustics, ambient noise control, and transducer (loudspeaker) selection, often resulting in audible announcements that were not intelligible.
Millions of people visit large, complex buildings such as transportation hubs, healthcare facilities, convention centers, and casinos every year. Emergency communications in these facilities must be clear to prevent panic, injury, and loss of life, particularly because the visitors to these types of buildings generally are not familiar with the facility and have difficulty finding a safe exit path without explicit instruction. To illustrate this point, a hypothetical airport is used as an example to describe what people might experience in such a space.
Case Study: International airport
Travelers at this international airport may arrive at an expansive four- or five-story open space in the main terminal that houses ticketing; the security, international arrivals, and baggage claim areas may be in separate areas with lower ceilings and completely different architectural finishes. People may move from the open main terminal ticketing area through the noisy tunnel-like security area to another open area of the main terminal lined with restaurants and retail shops. Airline concourses connect at what seem to be random points and angles to the main terminal. The linear concourses, often lined with retail concessions, lead travelers down to individual departure and arrival gates. The departure gates are attached to the concourse and provide a place to wait to board the plane.
In this hypothetical example of an international airport, each of the airport areas (ticketing, security, concourse, and gate) that the traveler moves through is an ADS. These areas may also be subdivided or zoned into a number of individual ADSs depending on the architecture of the area. The reverberation time and background noise level (ambient sound pressure level) are factors that must be considered by acousticians and designers during the design of the ECS in order to provide the specified level of intelligibility, a threshold determined by the Speech Transmission Index (STI) set forth by the International Electrotechnical Commission (IEC).
In our example, the traveler arrives to acoustic bedlam in the main terminal due to the non-absorptive finishes throughout the terminal. The finishes cause a buildup of reverberant sound, making it almost impossible to understand a talker only a few feet away from a listener. The paging system is operational, but only adds to the background noise. The traveler cannot understand the announcements due to the high reverberation time and the poor location of loudspeakers. The high noise levels and inability to understand courtesy and information announcements only add to the travelers’ anxiety. If a catastrophic event occurs, the traveler will not be able to understand the announcements.
Contrast that to a traveler arriving at the same main terminal that is just as active but with generous absorptive finishes that provide a relaxing atmosphere, allowing the traveler to easily orient to his surroundings. The paging system has been designed to provide courtesy and information announcements clearly and at a conversational level. Emergency announcements are clear and concise, providing straightforward direction and guidance to the travelers.
The magnitude of reverberation, or acoustical liveness, of a space is referred to as the reverberation time and is indicated as RT60—the time it takes for sound produced in a room to decay 60 decibels (dB) after the source has been silenced. RT60 is directly proportional to the volume of a room and inversely proportional to the amount of absorptive materials in the room. A required reverberation time is not defined by the NFPA, but rather by the STI.
Spaces with a short RT60 of 0.2 seconds to 1.5 seconds are considered “dead” and those with 1.6 seconds of RT60 and higher are considered “live.” The RT60 for large areas such as the open, high-ceiling ticketing area described above can be well over 2.0 seconds. With this RT60, people would have the feeling of energy and excitement, but providing a sound system with an adequate STI becomes challenging.
The sound level in a space is commonly rated by a noise criterion (NC) value. The NC levels are a family of sound pressure level curves that take into account the spectral response of human hearing to provide a single number rating of the sound levels across the frequency spectrum. The NC is commonly used to define the allowable noise from mechanical systems that serve a space; however, it can also apply to other noise sources such as the lighting fixtures and the activity noise generated by occupants. Required NC levels are not specified in the NFPA code but must be considered in designing an intelligible ECS.
Speech intelligibility outlined in the National Fire Alarm Code 2013 edition, while used for years by acoustical consultants and sound system designers, is a new concept for fire protection engineers in designing systems. Indeed, until relatively recently, it was very difficult and/or expensive to measure intelligibility and impossible to predict accurately the intelligibility of a sound system during design. However, the development of objective intelligibility algorithms have spurred the evolution of relatively inexpensive measurement instruments and software design tools that can predict the intelligibility of a system under design.
The National Fire Alarm Code® 2013 edition uses the IEC 60268 standard for the measure of the Speech Transmission Index (STI) of an ECS. The ideal intelligibility transmission path is a talker and listener that effectively speak the same native language facing each other in a quiet room with few or no reflective surfaces. In this instance, the talker and listener will clearly understand each other’s communication and the STI will be 1.0. Non-ideal conditions, such as reverberation, greater distances between listener and talker (or loudspeaker), and higher levels of ambient noise, will result in lower STI values.
The STI scale and the associated subjective intelligibility are below. The NFPA requires that a minimum of .5 be achieved. To put this in non-technical terms, an STI of .3 or less is the inability of a listener to hear a talker that is very close to the listener. To achieve a 1.00, there must be little noise and reverberation, and the effective distance between the talker and listener is very close.
STI Scale/Associated Subjective Intelligibility
Sound Transmission Index is 1.00-0.80 — Subjective Intelligibility is Excellent.
Sounds Transmission Index is 0.75-0.65 — Subjective Intelligibility is Good.
Sound Transmission Index is 0.60-0.50 — Subjective Intelligibility is Fair.
Sound Transmission Index is 0.45-0.35 — Subjective Intelligibility is Poor.
Sound Transmission Index is 0.30-0.00 — Subjective Intelligibility is Bad.
Bringing It All Together
Since the changes to the NFPA code initiated in 2010, acousticians and sound system designers have become engaged to provide ECS design services that address the acoustic issues in order to meet the STI specifications. Computer based tools are now available that use sophisticated prediction and analytic models to address the acoustic characteristics of the space and optimize the selection and placement of ECS devices (loudspeakers and sensors) to provide intelligible communications. This integration of acoustics into the system design is necessary to provide intelligible communications for the completed building. Once the building has been constructed, the acoustician and sound system engineer use instrumentation to verify that the intelligibility goals have been achieved, verifying compliance with the NFPA requirements.
The changes in the NFPA-72® National Fire Alarm Code® 2010 and 2013 editions finally bring intelligibility to the forefront in the design of emergency communication systems. Designers and engineers working with the facility owners and architect can optimize the acoustics and systems in each acoustically distinguishable space to provide clear, intelligible communications, often without increasing the cost of the systems (and hence the building) significantly. The end result is a facility with intelligible communication systems that may reduce panic, injury, and loss of life during a crisis.
(Photos courtesy of Acentech, Inc.)
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