Listen Up
The expanding world of emergency communications systems.
NFPA Journal®, July/August 2009
With the recent advent of terrorist activities, shootings on college and high school campuses, and extreme weather, public strategies are being refined to include more communication of actionable emergency information in real time. This shift has led to the development of robust emergency communications systems (ECS) both inside and outside buildings. Such events have also helped prompt the development of more comprehensive code requirements for those systems.
Almost 30 years ago, NFPA began work on NFPA 72F, Installation, Maintenance and Use of Emergency Voice/Alarm Communication Systems. Finally published in 1985, the document contained slightly more than two pages of requirements for voice/alarm signaling service and two-way telephone communication service.
By contrast, Chapter 24 of the 2010 edition of NFPA 72®, National Fire Alarm and Signaling Code, provides 20 pages, including annex material, of requirements for emergency communications systems, including requirements that extend beyond the walls of a building to the outdoor environment.
As you peruse this new chapter, you may be surprised to find that it requires a properly designed ECS to have more than double the number of audible notification appliances than previous editions of the code required. Why? Because a major operational goal of any effective communications system is to ensure that the messages the ECS distributes remain intelligible to those receiving them.
According to the Emergency Communications Audio Intelligibility Applications Guide published by the National Electrical Manufacturers Association in 2008, a number of factors affect the intelligibility of messages presented over public address systems. Among them are background noise; the configuration of the space in which the systems are installed; the acoustic properties of the materials covering the walls, floors, and ceilings; the distortion and bandwidth of the sound equipment; and the characteristics of the person speaking.
ECS designers cannot control all of these factors. Rather, they must design the system to compensate for those factors not under their control. To compensate for possible distortion introduced by the amplifier and signal processing equipment, for example, ECS designers and installers should provide a good distribution of a larger number of lower-powered speakers, rather than a fewer number of higher-powered speakers. Equipment quality also plays an important part in the delivery of an intelligible message to the listener.
When the 2010 edition of NFPA 72 is approved for publication, designers, installers, and authorities having jurisdiction will need a better background in communications systems design and operation to meet its requirements, as well as a basic understanding of sound and communications principles. Fortunately, the new edition of the code and its annex material should provide some much-improved design guidance for the layout of effective audible and intelligible communications systems.
Wayne D. Moore, P.E., FSFPE, is a principal with Hughes Associates and immediate past chair of the NFPA 72 Technical Correlating Committee.