In order for any emergency communications system (ECS) to communicate information properly, it must reproduce the desired messages in a way that the intended listeners will both hear and understand. In trying to apply the require-ments of National Fire Protection Association (NFPA) 72, many designers, installers and authorities having jurisdiction (AHJ) have struggled with the concept of intelligible voice messages. To develop an understanding of these intelligibility requirements, they must first have a basic understanding of sound and communications principles.
First, designers and installers should understand the importance of having a good distribution of speakers, rather than trying to use a higher power output from a few speakers. A proper design will provide a distributed sound level with minimal sound-intensity variations to achieve an intelligible voice message. The secret: use more speakers with less sound intensity from each speaker.
Annex D of the recently adopted 2010 edition of NFPA 72 provides guidance for system designers. It states, “The designer and the authority having jurisdiction should be aware that the acoustic performance parameters of the chosen loudspeakers, as well as their place-ment in the structure, play a major role in determining how many appliances are necessary for adequate intelligibility. The numerical count of appliances for a given design and protected space cannot, by itself, be used to determine the adequacy of the design … .”
Installers, designers and AHJs can reference Annex D for a rule of thumb: “... for a standard building configuration with normal ceil-ing height (8 ft to 12 ft) (2.4 m to 3.7 m), normal ceiling construction (i.e. drop acoustical ceiling tiles), standard wall configurations and finishes and carpeted floors, the following should apply:
• “Ceiling-mounted speakers should be installed in all normally occupiable spaces and in corridors spaced at a maximum of twice the ceiling height
• “Or as determined by a commercially available computer acoustical/speaker modeling program.
“In general, low ceilings require more ceiling mounted speakers per square foot of area than high ceilings.
“Where wall-mounted speakers are used, manufacturer’s recommendations should be reviewed and/or computer modeling should be employed.
“There are other factors that affect the intelligibility of the messages that include background noise, the configuration of the space being addressed, the acoustical properties of the materials on the walls, floors, and ceilings, the distortion and bandwidth of the sound equip-ment, and the characteristics of the person speaking. (male/female, accent, microphone technique, etc.)”
The technical committee that developed the requirements for all emergency communications systems placed the responsibility for complying with the intelligibility and audibility requirements of the code squarely on the designer’s shoulders. Section 18.104.22.168.1 states: “The following requirements shall be met for layout and design:
“A. The speaker layout of the system shall be designed to ensure intelligibility and audibility.
“B. Intelligibility shall first be determined by ensuring that all areas in the building have the required level of audibility.
“C. The design shall incorporate speaker placement to provide intelligibility.”
And, to help ensure AHJs will uniformly enforce the requirements, the code provides a list of where designers need not provide a spe-cific determination of intelligibility: private bathrooms, shower rooms, saunas and similar rooms/areas; mechanical/electrical/elevator equipment rooms; elevator cars; individual offices; kitchens; storage rooms; closets; and rooms or other areas where intelligibility cannot reasonably be predicted.
Contractors should become aware of the common mistakes made when installing a communications system. For example, to prevent feedback when someone uses a microphone, don’t locate speakers near the emergency voice/alarm communications system control equipment. Keep this concept in mind when installing other similar systems, such as firefighter telephones.
Choose output settings on each speaker that will ensure audibility while maintaining intelligibility. This requires that the technician knows which tap to connect. Do not assume that all speakers will have the same setting. Connecting to a high setting to meet the audibility requirements of the code could distort the intelligibility of the signal.
Give special attention to acoustically challenging areas. Such areas typically incorporate appreciable hard surfaces (e.g., glass, marble, tile, metal, etc.) or appreciably high ceilings (e.g., atriums, multiple ceiling heights). These conditions will require more stringent design guide-lines to ensure intelligibility.
Contractors experience challenges when installing fire alarm/voice evacuation and mass notification systems. Using the information in the 2010 edition of the National Fire Alarm and Signaling Code will help ensure a code-compliant emergency communications system.
MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a co-editor of the current National Fire Alarm Code Handbook. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office.