Day-night average sound level
A 24-hour noise metric with nighttime weighting for community impact.
The day-night average sound level (Ldn or DNL) measures the average noise over a full day. Readings taken between 10 p.m. and 7 a.m. are boosted by 10 decibels before averaging, to account for the typical 10-decibel drop in background noise during those hours. Although the calculation is the same, DNL usually refers to summing individual noise events like aircraft noise, while Ldn covers total noise from all sources.
A related metric, the day-evening-night average sound level (Lden or DENL), is common in other countries. California uses a version called Community Noise Exposure Level (CNEL). Both add an evening period from 7 p.m. to 10 p.m., where readings are increased by 5 decibels (or 4.77 dB for CNEL) to reflect the approximate drop in background noise then.
In the United States, the Federal Aviation Administration uses this measure for airport noise studies under Federal Aviation Regulation Part 150. The FAA considers a day-night average sound level above 65 decibels incompatible with residential areas. Communities in such zones may qualify for mitigation efforts, including soundproofing.
- Metric name
- Day-night average sound level (Ldn or DNL)
- Measurement period
- 24-hour period
- Nighttime period
- 22:00 to 07:00
- Nighttime boost
- 10 dB
- Incompatibility threshold
- 65 dB
- Regulatory context
- Federal Aviation Regulation Part 150
- Related metrics
- Lden, DENL, CNEL
Lore & Background
The day-night average sound level, abbreviated as Ldn or DNL, calculates the average noise level over a full day, but with a critical adjustment: noise measurements taken between 22:00 and 07:00 are artificially increased by 10 dB before averaging. This weighting accounts for the approximate 10 dB decrease in community background noise during nighttime hours, making people more sensitive to intrusive sounds. While the calculation method is identical, DNL specifically refers to the summation of single-source noise events such as aircraft noise, whereas Ldn denotes total noise exposure from all sources.
A similar metric, the day-evening-night average sound level (Lden or DENL), is commonly used in other countries, and the Community Noise Exposure Level (CNEL) is used in California legislation. These add an evening period from 19:00 to 22:00, during which noise measurements are boosted by 5 dB (or 4.77 dB in the case of CNEL) to reflect the approximate decrease in background community noise during that time.
The Federal Aviation Administration has established the day-night average sound level as a community noise exposure metric to aid airport noise analyses under Federal Aviation Regulation Part 150. The FAA states that a maximum DNL higher than 65 dB is incompatible with residential communities, and communities in affected areas may be eligible for mitigation measures such as soundproofing.
Reader's Guide
The day-night average sound level serves as a key regulatory tool in the United States for evaluating and managing community noise exposure, especially from aviation. Its significance lies in its ability to translate complex, time-varying noise into a single number that can be compared against a threshold—65 dB—beyond which residential areas are considered incompatible. This threshold triggers potential eligibility for mitigation programs like soundproofing, directly influencing land-use planning and airport operations. The metric's legacy is tied to its adoption by the Federal Aviation Administration under Part 150, making it a standard for airport noise analyses. However, the existence of alternative metrics such as Lden and CNEL, which incorporate an evening weighting, highlights ongoing debate about the most appropriate way to capture community annoyance. The distinction between DNL (single-source events) and Ldn (total exposure) further underscores the metric's specialized role in aviation contexts, where it remains a central reference for assessing and addressing noise impacts on surrounding communities.
Did You Know?
- The day-night average sound level artificially increases nighttime noise measurements (22:00–07:00) by 10 dB before averaging.
- The Federal Aviation Administration considers a DNL above 65 dB incompatible with residential communities.
- DNL refers to single-source noise events like aircraft noise, while Ldn refers to total noise exposure from all sources.
- The California Community Noise Exposure Level (CNEL) adds an evening period with a 4.77 dB boost.
Frequently Asked Questions
What is Day-night average sound level (Ldn/DNL)?
It is a 24-hour noise metric that averages sound levels across an entire day while applying an extra 10-decibel penalty to readings recorded between 10 p.m. and 7 a.m. Under Federal Aviation Regulation Part 150, it serves as the standard gauge for assessing how drone and aircraft operations affect surrounding communities.
Why does Ldn add 10 decibels to nighttime readings?
Ambient background noise naturally falls by roughly 10 dB during the 22:00–07:00 window, so the boost normalizes those hours against the louder daytime baseline. The adjustment ensures a sound that is equally loud at 2 a.m. as at 2 p.m. is treated as more disruptive in the final average.
What is the difference between Ldn and DNL?
The underlying calculation is identical in both cases; the distinction is one of scope. DNL typically refers to summing discrete noise events such as individual aircraft flyovers, while Ldn covers the total noise from every source in the environment.
What noise level triggers regulatory concern under Part 150?
A day-night average of 65 dB is the incompatibility threshold, meaning sustained community exposure at or above that level is deemed incompatible with residential land use. Regulators reference this benchmark when evaluating whether drone operations or flight paths are generating excessive noise.
How does Ldn compare to Lden or California's CNEL?
Lden (day-evening-night) divides the day into three weighted periods rather than just day and night, and California's Community Noise Exposure Level is a similar variant. All three share the core principle of weighting nighttime noise more heavily, but the exact time windows and boost values differ by jurisdiction.
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