You are designing an audible alarm notification system for an office environment with an ambient noise level of 55 dBA. What dBA level should the horn be set at?

40 dBA is 15 dBA below the ambient noise level, meaning the alarm would be completely masked and inaudible in the 55 dBA environment.

50 dBA is 5 dBA below ambient and is still masked by background noise, failing the NFPA 72 requirement of being at least 15 dBA above ambient.

55 dBA equals the ambient level and provides zero audible margin above background noise, falling far short of the 15 dBA above ambient requirement.

65 dBA is only 10 dBA above the 55 dBA ambient level, which is insufficient and does not meet the NFPA 72 minimum margin of 15 dBA above ambient.

Per NFPA 72, audible notification appliances must emit a sound pressure level of at least 15 dBA above the average ambient sound level measured in the occupiable area. Adding 15 dBA to the 55 dBA ambient level yields the required minimum of 70 dBA at the device. This margin ensures the alarm signal remains clearly distinguishable above background noise and audible to occupants throughout the space.