DNL stands for Day-Night Average Sound Level.
In the context of acoustics and environmental noise measurement, DNL (sometimes written as Ldn) is a 24-hour average sound level, with a 10 dB penalty added to sound levels during nighttime hours (typically 10:00 PM to 7:00 AM). This penalty is applied because noise is generally considered more intrusive during the quieter nighttime hours when people are sleeping.
Here's a breakdown of what DNL entails:
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24-Hour Averaging: DNL takes into account all noise events occurring over a full 24-hour period. It calculates an equivalent continuous sound level (Leq) for the entire day.
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Nighttime Penalty: A 10 dB(A) adjustment is added to the sound levels recorded during the nighttime period (10 PM to 7 AM). This weighting reflects the increased sensitivity to noise disturbance during sleep hours. The "A" weighting filters the sound to mimic the human ear's sensitivity to different frequencies.
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Purpose: DNL is commonly used for assessing community noise exposure and for land use planning around airports, highways, and other major noise sources. It helps determine potential impacts on residential areas and informs noise mitigation strategies.
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Units: DNL is typically expressed in decibels (dB), often using the A-weighting scale (dBA).
Why is DNL important?
DNL is a valuable metric because it provides a single number representing the cumulative noise exposure over an entire day, taking into account the increased annoyance of nighttime noise. It simplifies noise assessments and allows for easier comparison of noise levels in different locations or under different scenarios. It helps regulatory agencies and planners set noise standards and implement measures to protect public health and welfare from noise pollution.
