The FAA currently defines a significant change in noise exposure as an increase in DNL of ______ dB if the new DNL is above 65 dB?

• A. 0.5
• B. 1.5
• C. 2.0
• D. 1.0

Answer: (B)

DNL, the day–night average sound level, is the metric the FAA uses to gauge how many people are exposed to aviation noise and how loud that exposure is over a 24-hour period. When the new DNL ends up above 65 dB, the FAA considers a change in exposure to be significant if the increase from the existing DNL is at least 1.5 dB. This threshold is chosen because it reflects a noticeable rise in the number of people exposed to higher noise levels without flagging every small fluctuation, while still being sensitive enough to detect meaningful changes in community impact.

So if the current DNL is above 65 dB and it increases by 1.5 dB or more, that change is regarded as significant. For example, going from 67 dB to 68.5 dB would be significant, whereas a smaller 1.0 dB rise would not meet the threshold.