Which DNL values were specified as levels to assess adverse impact in the 1992 act?

• A. 65, 60, and 55 dB
• B. 70, 65, and 60 dB
• C. 60, 55, and 50 dB
• D. 75, 70, and 65 dB

Answer: (A)

DNL combines all noise exposure into one number by weighting nighttime noise more heavily, so it reflects how people experience noise throughout a day. The 1992 act uses three specific DNL thresholds to classify the potential adverse impact and guide mitigation: 65 dB, 60 dB, and 55 dB. These levels give a graduated sense of severity—65 dB signals a likely significant impact that typically requires mitigation, 60 dB indicates a moderate impact where mitigation is usually considered, and 55 dB flags a lower threshold where even relatively smaller exposures are examined. This tiered approach provides a consistent framework for evaluating and comparing projects. The other sets of numbers don’t match the triplet established by the act, which is why this combination is the correct one.