Audibility, Continued
High Background Noise
Large Areas
To meet the 15 dBA requirement, there are cases where high levels of background noise require extremely high levels of emergency annunciation to overcome the noise. When background noise exceeds 105 dBA or when the SPL calculations require greater than 110 dBA, the use of visual notification appliances is warranted.
There are cases where the background noise can be reduced or eliminated by allowing the emergency voice/alarm communications system to remove the source of the noise. For example, background music can be disabled during an alarm condition.
Very large areas such as arenas pose a particular challenge to meeting NFPA 72 requirements. These areas have sound systems installed that produce tens of thousands watts of power, however these systems are not typically designed to meet NFPA 72 requirements. If the systems are engineered to provide adequate SPL and intelligibility then it would make sense from a performance perspective to use the systems for emergency evacuation. By working closely with the AHJ and the building’s owner, it may be possible to allow the use of the existing PA system. However, some modifications may be required.
