To obtain a daily DNL reading, noise is measured each ________ and recorded for a 24 hour period. For the hours 10 pm to 7 am, ______ decibels are added to the recorded measurement.

• A. Measured each second; 10 decibels are added
• B. Measured each minute; 5 decibels are added
• C. Measured each hour; 15 decibels are added
• D. Measured each day; 20 decibels are added

Answer: (A)

DNL combines a 24-hour sound level with a nighttime penalty to reflect greater disturbance at night. To get an accurate 24-hour average, you record noise continuously with high time resolution, typically capturing instantaneous levels every second so the daytime and nighttime fluctuations are preserved. Then, for the hours from 10 pm to 7 am, you add a fixed 10 dB penalty to those measurements before forming the 24-hour average. This nighttime adjustment is what distinguishes DNL from a simple 24-hour Leq and is why measuring each second and adding 10 decibels during the night is the correct approach.