The FAA currently defines significant change in noise exposure as an increase in DNL of 1.5 dB when the new DNL is above what value?

• A. 60 dB
• B. 65 dB
• C. 70 dB
• D. 75 dB

Answer: (B)

DNL is the day-night average sound level used in FAA noise analyses, with nighttime noise weighted more heavily to reflect potential sleep disturbance. The significant change criterion uses two conditions together: the DNL must increase by at least 1.5 dB, and the new DNL must exceed 65 dB. This threshold ensures that only changes with a meaningful impact on exposure levels are treated as significant. For example, a rise from 63 dB to 64.5 dB is a 1.5 dB increase but does not exceed 65 dB, so it wouldn’t be considered significant. A rise to 65.2 dB meets both conditions and would be considered significant.