Audibility, From Nfpa 72, 2002 Edition

Audibility

Tones and SPL

For emergency messages to be heard, NFPA 72 suggests that the sound level of the emergency evacuation tone to be measured at 5-feet. This is the average “ear level” of someone standing. The messages must be 15 dBA above normal ambient sound or 5 dBA above sounds lasting longer than 60 seconds. When performing a site investigation for speaker placement and power settings, a survey of the area with a dBA reading sound level meter is used to determine the proper sound level required. In sleeping areas, the audibility requirements are the same except measurements are taken at the normal ear level for someone sleeping (at pillow level).

From NFPA 72, 2002 Edition:

“7.4.2.1* To ensure that audible public mode signals are clearly heard, unless otherwise permitted by 7.4.2.2 through 7.4.2.5, 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 occupiable area, using the A-weighted scale (dBA).”

When measuring tones, the SPL must be maintained in all parts of the building, however several cases receive special consideration. Elevators (7.4.2.2) and restrooms (7.4.2.5) are cited.

When determining the level of background noise, it is not necessary to take into consideration sources such as construction equipment or other sources that would not normally be present. It is however required to take into consideration normal noise sources that last longer than 60 seconds; vacuum cleaners are cited as an example in Annex A.7.4.1.3.

When designing for new construction where a survey is not possible, Annex A of NFPA 72 provides guidance of the anticipated ambient sound level.

From NFPA 72, 2002 Edition:

A.7.4.2 The typical average ambient sound level for the occupancies specified in Table A.7.4.2 are intended only for design guidance purposes. The typical average ambient sound levels specified should not be used in lieu of actual sound level measurements.

Table A.7.4.2 Average Ambient Sound Level According to Location

Location Average Ambient	Sound Level (dBA)
Business occupancies	55
Educational occupancies	45
Industrial occupancies	80
Institutional occupancies	50
Mercantile occupancies	40
Mechanical rooms	85
Piers and water-surrounded structures	40
Places of assembly	55
Residential occupancies	35
Storage occupancies	30
Thoroughfares, high density urban	70
Thoroughfares, medium density urban	55
Thoroughfares, rural and suburban	40
Tower occupancies	35
Underground structures and windowless buildings	40
Vehicles and vessels	50

Note: Audibility must be verified at the time of system commissioning and periodically verified in accordance with the requirements stated in the “Inspection, Testing, and Maintenance” chapter of NFPA 72.

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