Demystifying Emergency Communications System Message Delivery
Proper design of an emergency communications system requiring voice intelligibility depends on factors such as room acoustics, speaker layout and even the manner in which the person is announcing a live instruction.
The majority of voice systems that are installed as a part of a fire alarm system are not installed in such large venues, but in smaller applications such as schools, assembly spaces or floors within a high-rise building. In these cases, Annex D, Speech Intelligibility, within the 2013 edition of NFPA 72 should be used. This annex provides information on four topics: fundamentals of test protocol; pre-planning; test equipment calibration; and post-test procedures.
Measuring Acoustics & Loudspeaker Coverage
In this article I will only be addressing preplanning, or the design of the system. A key concept is the determination of acoustically distinguishable spaces (ADS). An area of the premises that has been designated as an ADS would be required to meet the intelligibility requirements. Those areas that are not within an ADS do not. The final acceptance of a premises’ ADS will be left with the authority having jurisdiction (AHJ) during the plan review.
Within Chapter 18, Notification Appliances, A.220.127.116.11.1 there a list of areas within an occupancy that may not require intelligibility:
- Private bathrooms
- Mechanical, electrical and elevator equipment rooms, or similar areas
- Elevator cars
- Individual offices
- Storage rooms
- Rooms or areas where intelligibility cannot reasonably predicted
The last entry is fairly broad in its coverage; a reasonable approach should be taken. If I were to design a system for an elementary school, for example, I would include within the ADS the classrooms, interior hallways and assembly areas. For a house of worship, the assembly areas and the education spaces. The fact remains that not all spaces or areas within the occupancy will require intelligibility.
Once the ADS and non-ADS areas have been determined, the spaces that are to have intelligibility need the appropriate number of speakers installed to meet required coverage. This can be a bit of trial and error.
Several test products on the market will produce a signal that when ported through a speaker or speaker and with the appropriate receiving equipment can show if intelligibility is being achieved. Even prior to a system being installed, this equipment can be used for “spot checks” to determine the number and location of speakers that may be required within a space. Please note that in the 2013 edition of NFPA 72, qualitative test methods are no longer required, but still may be used.
If the space has not been created yet, there are software or mathematical methods that may be employed. Be familiar with their use before performing a design with these methods. Section D.3.4 within Annex D describes one mathematical method.
The placement and overlap of speakers will affect the coverage of the system and the overall intelligibility. As the height of the installed speaker is increased, the coverage is extended. However, more power will be required to produce the same sound pressure level to the listener’s position. Depending on the space’s acoustics and the level of intelligibility that is to be achieved, the more the number of speakers may be required. Be sure to evaluate loudspeaker performance, too, as they produce varying coverage areas. In some cases, overlap of the speakers’ coverage areas may be required to prevent any “dead” spots.
The installing of speakers for voice intelligibility is not the same as the placing of horns, or even speakers in the past. The intent is not to blast the message into a space, but to provide adequate coverage so that the message can be clearly understood. For this to occur, you may need more speakers than what may have been previously required. If you can understand the principles behind intelligibility, you’ll be able to design systems that meet the requirements of NFPA 72 for the majority of venues that you may be called upon to install an ECS voice solution.
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