Lden, DNL and Lnight explained: the numbers on official noise maps

Official noise maps do not show how loud each plane is. They show an average: the steady level that would carry the same total sound energy as all the flights over a day, a summer or a year. The names differ from country to country, and so do the hours and the extra weight given to the night.

The measures at a glance

The Lden and Lnight hours are the EU defaults; each country may move them, or shorten the evening by an hour or two.

Leq: the average behind all of them

Every measure in the table is an equivalent continuous level, or Leq: the steady sound that would deliver the same energy as the real, varying noise over the period. For aircraft it is almost always A-weighted, hence LAeq; dB vs dBA explains the A.

It is built by adding up the sound energy of every flight and spreading it over the hours. Because decibels are logarithmic, the loudest flights dominate, and doubling the number of flights adds only 3 dB. The FAA gives an example: one flight a day with a sound exposure level of 114.4 dB, ten flights at 104.4 dB or a hundred at 94.4 dB all produce the same 65 dB DNL.

Lden and Lnight: Europe's noise maps

The EU's Environmental Noise Directive defines Lden as the average over a year of a 12-hour day, a 4-hour evening and an 8-hour night, with 5 dB added to the evening and 10 dB to the night. In plain terms, each evening flight counts about three times over and each night flight ten times. The year is meant to be a typical one for weather. The level is worked out 4 m above the ground at the most exposed wall of a building, leaving out the sound reflected off that wall.

Lnight is simply the average over the same 8-hour night, every night of the year, with no weighting.

Every five years (2017, 2022, next in 2027), EU countries must map airports with more than 50,000 flights a year in both measures, counting people from 55 dB Lden and 50 dB Lnight. In the 2022 round, 2.4 million people in Europe were above 55 dB Lden from aircraft. The World Health Organization's guidelines use the same measures and recommend keeping aircraft noise below 45 dB Lden and 40 dB Lnight; the WHO guidelines explained covers them. Airports in England covered by the Environmental Noise (England) Regulations 2006 still produce Lden and Lnight maps.

LAeq,16h and LAeq,8h: the UK's summer averages

UK policy uses its own averages: 07:00 to 23:00 and 23:00 to 07:00, over the 92 days from 16 June to 15 September. At Heathrow in 2025, an average summer day had about 1,344 flights, against 1,315 across the whole year.

Under the Air Navigation Guidance 2026, in force from 1 October 2026, new flight paths are assessed on these two measures, and adverse effects are taken to start at 51 dB by day and 45 dB at night. The government has accepted since 2017 that significant community annoyance starts at 54 dB LAeq,16h, and a CAA survey in 2023-24 found 10% of people highly annoyed at about 43 dB.

DNL and CNEL: the US measures

In the US, the FAA uses DNL, the day-night average sound level, for all its studies of aircraft noise. It is a 24-hour average over a year, with 10 dB added to noise between 22:00 and 07:00 and no evening weighting. 65 dB DNL is the FAA's threshold for significant exposure and for whether homes are considered compatible with an airport. Its review of noise policy has not changed that, although the FAA's own national survey, published in 2021, found 60-71% of people highly annoyed at 65 dB DNL, where the older curve behind the threshold predicted 12%.

California uses CNEL, which also weights the evening: noise from 19:00 to 22:00 counts three times over and night noise from 22:00 to 07:00 ten times.

Other countries, other averages

• Australia plans land use around the ANEF, an annual-average-day measure with two periods, 07:00-19:00 and 19:00-07:00, and a 6 dB penalty on the second. Under the Australian standard, houses are acceptable below 20 ANEF, acceptable with sound insulation between 20 and 25, and unacceptable above 25. Australia also counts loud events alongside it.
• Germany draws its noise protection zones from an LAeq by day (06:00-22:00) and by night (22:00-06:00) over the six busiest months, not the year. It also adds extra flights to each runway direction to allow for years when the wind sends more that way. The night zone is set both by the average and by how many times a night the peak level is exceeded.

Why the same place gets different numbers

The same flights can produce quite different figures for one address:

• The hours and weights differ. A plane at 22:30 counts once in the UK's LAeq,16h, about three times over in Lden and ten times in DNL. A plane at 06:30 counts ten times in both Lden and DNL, falls in the UK's night figure rather than the day one, and in Germany's day figure.
• The period differs. A summer average, a whole-year average and the six busiest months give different results for the same airport.
• The runway split differs. Maps can use the split between runway directions of one year or a standard long-term split. Heathrow's report publishes both, plus maps for all-westerly and all-easterly operation.
• Modelled or measured. Almost all noise maps are calculated with a noise model rather than measured. A measurement next to a wall picks up the reflection the model leaves out, which the EU directive puts at about 3 dB.

Heathrow's official 2025 maps show the effect. The 54 dB LAeq,16h summer contour covered 124.5 km² with 387,200 people; the 55 dB Lden contour covered 142.4 km² with 508,100 people, bigger even though its threshold is a decibel higher, because evening and night flights carry extra weight in it. The 50 dB Lnight contour held 219,200 people. Official airport noise maps explained shows how to read the maps themselves.

What an average cannot tell you

An average is good at comparing airports or years, and it tracks how annoyed a whole community is. It is poor at describing what a day at one address is like.

• The same number hides very different days. An average over a year blends the days when the wind sends every plane over a street with the days when none come.
• Big changes look small. Doubling the flights adds 3 dB, about the smallest change most people notice in a single sound, even though there are now twice as many planes.
• Nobody hears the average. In the Australian government's assessment for Western Sydney's new airport, a sequence of flights whose loudest peaked at 75 dB worked out at an average of 55 dB. People hear the peaks, not the 55.
• The contour lines sit where many people are already bothered. Europe reports from 55 dB Lden, 10 dB above the WHO's recommendation, and the UK's 54 dB contour lies well above the 43 dB at which the CAA found one person in ten highly annoyed.

For those reasons, some airports and regulators, including Heathrow and Australia's, publish counts of loud planes alongside the averages. Why counting loud planes works better explains how they are used.