For many years, the audibility and intelligibility of fire alarm signals were ignored. Traditionally, a contractor or designer would put one audible/visible appliance above each manual fire alarm box (pull station) and maybe one or two more in the hallway. It seemed the unwritten rule was if you could hear the alarm in the halls of residential or office buildings, you were OK.
Anyone familiar with the National Fire Alarm Code knows the requirements are much stricter today than they were 12 or 15 years ago. For contractors who design systems, it is imperative that they understand both the requirements of NFPA 72-2007 and the basics of sound transmission.
NFPA 72 allows two modes of signaling: public mode and private mode. The requirements for public mode are specified in Section 18.104.22.168. The code states, “To ensure that audible public mode signals are clearly heard, … they 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 area required to be served by the system using the A-weighted scale (dBA).”
NFPA 72-2007, Section 22.214.171.124 states, “To ensure that audible private mode signals are clearly heard, they shall have a sound level at least 10 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 area required to be served by the system using the A-weighted scale (dBA).”
The annex of the code provides some background. It states, “Audio levels are commonly measured using units of decibels, or 1⁄10 Bell, abbreviated dB. When measured using a sound level meter, the operator can select either an A-weighted, B-weighted, or C-weighted measurement. … The A-weighted measurement filters the input signal to reduce the measurement sensitivity for frequencies to which the human ear is less sensitive and is relatively flat from 600 Hz to 7000 Hz. This results in a measurement that is weighted to simulate the segment of the audio spectrum that provides the most significant intelligibility components heard by the human ear. The units used for measurement are still dB, but the shorthand for specifying use of the A-weighted filter is typically dBA. The difference between any two sound levels measured on the same scale is always expressed in units of dB, not dBA.”
Typically, the only time private mode operation is allowed is in a well-staffed environment, such as a hospital. Public mode signaling is normal for most building designs. However the point must be made that all systems installed in buildings must meet these audibility criteria. How can professional contractors ensure systems they design will be audible? The answer is simply to design and install more audible appliances throughout the enclosed spaces.
Audible appliances installed only in corridors will not meet the code requirements, regardless of how many or how loud they might be. Sound transmission losses through solid doors and walls can range from 20 to 30 dB. In addition, 6 dB is lost every time you double the distance from the sound source.
Based on this background, each apartment, motel room and enclosed office will require an audible appliance within the space. More audible appliances also will mean additional power requirements for the system.
Sleeping rooms have an additional requirement. Section 126.96.36.199 of the code requires that where “audible appliances are installed to provide signals for sleeping areas, they shall have a sound level of 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 or a sound level of at least 75 dBA, whichever is greater, measured at the pillow level in the area required to be served by the system using the A-weighted scale (dBA).”
In order to meet this requirement, an audible appliance will need to be placed in each bedroom of an apartment or hotel/motel room. If there is a barrier between the notification appliance and the pillow, the code also requires the sound pressure level be measured at the pillow with the barrier in place.
It should be obvious from all the requirements quoted above that the professional contractor also must own and be able to use a sound-level meter. NFPA 72-2007 requires that the meters used comply with ANSI S1.4a, Specifications for Sound Level Meters, Type 2 requirements. The code requires that sound levels throughout protected area be measured and recorded. In order to record the optimum readings, the sound level meter must be set in accordance with ANSI S3.41, American National Standard Audible Evacuation Signal, using the time-weighted characteristic F (FAST).
For some contractors, these requirements have already been learned the hard way. When the authority having jurisdiction arrives and asks for the sound pressure levels to be measured, make sure your design will pass. Adding audible notification appliances after the building is complete and the fire alarm system has already been installed will be a costly mistake. Understand these requirements to ensure audibility.
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.