The professional contractor often greets the landing of a large project, such as a shopping mall, with enthusiasm; these projects offer both financial benefits to the firm and a long-term project for the contractor’s labor force. But, like any large project, it will only show a profit through careful planning and execution of the work required. The fire alarm system makes up a small percentage of the contractor’s overall work. However, failure to install the system properly could delay a certificate of occupancy.
What issues arise when installing a fire alarm system in a large structure? The mall normally consists of anchor stores—large multifloor buildings—and single-story stores. Additionally, a covered mall providing easy access to each of the stores becomes the assembly point for many shoppers. It can also serve as an area where the mall management provides exhibits, small vendor kiosks, entertainment venues or holiday-related photo sessions.
These retail or mercantile occupancies have a wide variety of goods on display and for sale. The stores typically display many combustible items throughout the available floor and counter space where the customer can readily view and purchase the items.
The occupants visiting theses stores and malls have little familiarity with the layout and exit arrangement of the mall and stores. Therefore, they may get turned around as to where they entered. Because of this possibility, the number of occupants within, and the size of the facilities, jurisdictional authorities have concern relating to adequate occupant warning. They also are concerned about directions that the occupants need to receive during an emergency.
Rise to the challenge
These factors alone prove challenging to fire alarm system communications. The mall essentially has a mercantile occupancy and an assembly occupancy that requires planning and separation for both detection and fire alarm notification. The challenges include the following:
Coordination of area notification
NFPA 72-2007, the National Fire Alarm Code, specifies two possible modes of notification appliance operation, public mode and private mode. Suffice to say here that we would use only the public mode requirements for a mall or mercantile occupancy.
The code requires that occupants clearly hear all public mode signals. To ensure this occurs, the code states: “[The signals] shall have a sound level at least 15 dB above the average ambient sound level or 5 dB above the maximum sound level having a duration of at least 60 seconds, whichever is greater, measured 1.5 m (5 ft) above the floor in the occupiable area, using the A-weighted scale (dBA).”
Many jurisdictions now require the fire alarm system design to include equipment to stop or reduce ambient noise, such as background music, recorded announcements or the sounds produced by entertainers. Additionally, Section 18.104.22.168.3 requires that “Relays, circuits, or interfaces necessary to stop or reduce ambient noise shall meet the requirements of Chapter 4 and Chapter 6.”
Designers must understand what ambient noise levels they will need to consider when laying out a notification appliance system for a large mercantile occupancy, such as a mall. The code advises that typical mall environment sound sources they should consider include air-handling equipment, large crowds with noises, background music and special presentations to name a few. Temporary or abnormal sound sources they can exclude from consideration include internal or external construction activities (e.g., construction equipment).
Many contractors do not know that audibility of the fire alarm signals is only one part of the equation in occupancies where they might use a fire alarm voice communication system. The voice announcements must provide an appropriate level of intelligibility any time a designer employs fire alarm/voice communication systems. The code states in section 22.214.171.124 that “Where required, emergency voice/alarm communications systems shall be capable of the reproduction of prerecorded, synthesized, or live (e.g., microphone, telephone handset, and radio) messages with voice intelligibility” [emphasis added].
NFPA 72-2007 defines voice intelligibility as “Audible voice information that is distinguishable and understandable.”
NFPA 72-2007 uses the words intelligibility and intelligible to describe those cases where voice communications systems reproduce human speech. As stated in Annex A of the code, “When a human being can clearly distinguish and understand human speech reproduced by such a system, the system is said to be intelligible. Satisfactory intelligibility requires adequate audibility and adequate clarity. Clarity is defined as freedom from distortion of all kinds.”
The code outlines three kinds of distortion that affect the reduction of speech clarity in a voice communication system:
“(1) Amplitude distortion, due to non-linearity in electronic equipment and transducers
“(2) Frequency distortion, due to non-uniform frequency response of transducers and selective absorption of various frequencies in acoustic transmission
“(3) Time domain distortion, due to reflections and reverberation in the acoustic domain
“Of these three kinds of distortion, frequency distortion is partially, and time domain distortion is totally, a function of the environment in which the system is installed (size, shape, and surface characteristics of walls, floors, and ceilings) and the character and placement of the loudspeakers.”
Typically malls have high ceilings, hard floors and walls with some sound-absorbing materials. These items will have an effect on the ability of mall occupants to clearly understand voice announcements from the fire alarm system.
If contractors understand the requirements for audibility, they will ensure that the notification appliances have a defined audible output (usually provided in dBA and shown on the product specification sheet), and will space them appropriately to achieve the 15 dBA above the ambient noise expected in the mall, generally somewhere between 55 and 60 dBA. The code annex provides typical average ambient sound levels for specific occupancies in Table A.7.4.2. However, this table only intends to offer design guidance.
To ensure that the design of the fire alarm system maintains intelligibility, one of the first steps will include providing more speakers evenly distributed throughout the mall and each retail store. The installer will need to set the output tap for each of these speakers at the appropriate wattage to ensure both the correct level of audibility and the correct intelligibility. However, as the code reminds us, “The numerical count of devices for a given design and protected space cannot, by itself, be used to determine the adequacy of the design. Sometimes, the acoustic problems of certain placement constraints can be satisfactorily overcome through the careful selection of loudspeakers with the requisite performance characteristics, rather than by increasing their number.”
Discussions with the owner as to what types of activities they will conduct in the open mall areas should assist in locating and distributing speakers to ensure intelligibility. The contractor will need to work closely with both the design engineer and the equipment supplier. The supplier can provide guidance as to how many amplifiers the contractor will need to distribute throughout the mall complex.
The code annex and the forthcoming 2007 edition of the National Fire Alarm Code Handbook provide much more detail regarding both audibility and intelligibility of fire alarm/voice communication systems.
The notification of occupants in a large retail or mercantile occupancy like a mall for example, presents no simple task. The professional contractor should be aware of the issues to understand the potential problems they may encounter.
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. and is located at the Warwick, R.I., office.