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Anyone routinely involved with the installation, maintenance and testing of fire alarm systems is aware of the need to ensure the audibility of fire alarm system notification appliances.
NFPA 72 2010, the National Fire Alarm and Signaling Code, requires that occupants of a building must clearly hear audible public mode signals.
According to guidance provided in Annex A of the code, “Audio levels are commonly measured using units of decibels, or one-tenth of a 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 C-weighted measurement is nominally flat from 70 Hz to 4000 Hz, and the B-weighted measurement is nominally flat from 300 Hz to 4000 Hz. 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.”
The code specifies the audible public mode appliances must “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 5 ft. (1.5 m) above the floor in the area required to be served by the system using the A-weighted scale (dBA).”
The code also discusses the ambient noise level of a space and the need to estimate or measure the ambient noise levels of the protected areas. The code prohibits the total sound pressure level produced by combining the ambient sound pressure level with all audible notification appliances operating from exceeding dBA at the minimum hearing distance.
In addition, when dealing with sleeping areas, the audible notification appliances must “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).” And further, the code requires that the installer must take these specific measurements at the pillow level with any barrier in place, such as a door, curtain or retractable partition, located between the notification appliance and the pillow.
The code requires that initial and reacceptance testing of notification appliances and sound pressure levels of a fire alarm system be measured “with a sound level meter meeting ANSI S1.4a, Specifications for Sound Level Meters, Type 2 requirements. Sound pressure levels throughout the protected area shall be measured to confirm that they are in compliance with Chapter 18. The sound level meter shall be set in accordance with ANSI S3.41, American National Standard Audible Evacuation Signal, using the time-weighted characteristic F (FAST).”
Equally important when measuring sound pressure levels, you must ensure you have calibrated the meter you will use. Of course, first, you must purchase a meter that you can calibrate, and you need to own two meters to have one available to use while you send the other one out for calibration.
As important as it is for you to have a sound pressure level meter, it is equally important for you to know how to properly use it and to understand where to take the base line (initial) measurements. You also must understand that the code provides a maximum level of sound pressure allowed in any area within the building.
NFPA 72 2010, section 220.127.116.11 states that the “total sound pressure level produced by combining the ambient sound pressure level with all audible notification appliances operating shall not exceed 110 dBA at the minimum hearing distance.” This measurement includes ambient sound from normal or permanent sources, having durations greater than 60 seconds, but it does not include sound from temporary or abnormal sources of sound in a facility.
A professional fire alarm contractor must own and know how to operate a sound level meter. And when you begin measuring the audible sound pressure levels, you must ensure that you document the readings throughout each floor and throughout the facility.
Ensuring that your fire alarm system audibility complies with the code becomes an integral part of your installation completion. Measuring the sound pressure levels throughout the facility also will ensure that you have no noncompliant areas or areas where the sound pressure levels may exceed the maximums allowed by the code. Having the right tools and knowing how to correctly use them will help ensure a code-compliant and efficient installation process.
MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a past chair of the NFPA 72 Technical Correlating Committee. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office. He can be reached at [email protected].