The FAA defines a significant change in noise exposure as an increase in DNL of how many dB, given the new DNL is above 65 dB?

• A. 0.5 dB
• B. 1.0 dB
• C. 1.5 dB
• D. 2.0 dB

Answer: (C)

DNL, or Day-Night Average Sound Level, is the 24-hour noise metric that weights nighttime hours more heavily to reflect potential sleep disturbance. When evaluating how a new airport-related noise scenario changes exposure, the FAA uses a threshold that depends on whether the resulting DNL exceeds 65 dB. If the new DNL ends up above 65 dB, an increase of 1.5 dB is considered a significant change. This threshold helps ensure that meaningful, perceptible shifts in a already noisy environment are identified without overreacting to small fluctuations. So, the significant increase is 1.5 dB. Smaller increments like 0.5 or 1.0 dB would not meet this criterion in areas over 65 dB, while larger increases exceed the threshold but are not the defined standard.