Picture a house in Hounslow, under the final approach to Heathrow. A plane passes every 90 seconds for half the day, low enough to read the airline's name. Its sound lands on this house and not on one three miles north. Carbon spreads over everyone; noise lands on an address.
Measure the sound→Draw the 55-decibel line→Count the people inside→2024: 561,600
Noise is measured in decibels, but the thing Heathrow counts is people. It draws a line on the map around everywhere the average noise over a day is above 55 decibels. That line is called a contour. Then it counts the people living inside it. In 2019: 664,300 people. In 2024: 561,600.
Inside the linecounted
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Outside the linenot counted
What does Heathrow count to measure its noise?
Complaints received.
Not yet. Complaints count people who wrote in, not people who hear the planes. The lesson comes back to why those two numbers are very different.
People living inside a line on the map where the day's noise averages above 55 decibels.
Right. A decibel is a loudness; a contour turns loudness into an area; the count is who lives in it. The number that matters is people, 561,600 of them in 2024.
Decibels at the runway.
Not yet. Loudness at the runway is high and tells you nothing about who hears it. The count follows the sound out to the homes.
Move the control through the years. Each frame is the number of people inside Heathrow's two contours: the day line at 55 decibels and the night line at 50.
Inside the day line664.3 thousand people
Inside the night line228.5 thousand people
Year: 2019
2019: 664,300 people inside the day line, 228,500 inside the night line. The baseline year.
Inside the day line508.3 thousand people
Inside the night line202.1 thousand people
Year: 2023
2023: 508,300 and 202,100. Flights were still below 2019, and quieter planes had arrived.
Inside the day line561.6 thousand people
Inside the night line219.9 thousand people
Year: 2024
2024: 561,600 and 219,900. Up on 2023 as flights came back, still below 2019. Heathrow's target is 10 in 100 fewer people highly annoyed or sleep-disturbed by 2030.
The count rose from 2023 to 2024. What is the most likely reason?
More people moved under the flight path.
Not yet. Heathrow's count uses the latest population map, so new homes do count. It also prints the change on a fixed 2019 map, and that version shows a bigger fall. Either way, the 2024 rise came from the flights.
More flights. Planes came back after Covid faster than quieter planes replaced older ones.
Right. The 2023 line was drawn around fewer flights. In 2024 flights rose, and the line grew with them, even though the share of quieter planes also rose.
Planes got louder.
Not yet. The opposite: the share of the quietest planes rose from 82 to 86 in 100. More flights outweighed quieter flights, which is the growth story from lesson 8.
Move the control.
2019119 nights with no flights, 23:30 to 04:30
2023144 nights with no flights, 23:30 to 04:30
2024189 nights with no flights, 23:30 to 04:30
2025207 nights with no flights, 23:30 to 04:30
The night line is drawn lower, at 50 decibels, because sleep is broken by quieter sounds. Heathrow set itself at least 150 nights a year with no flights after 23:30, and counts the late-running departures after that time. In 2025 there were 207 nights with no flights between 23:30 and 04:30, up from 119 in 2019.
Tap the year with the fewest quiet nights.
Tap the part of the picture that answers it.
2019119 nights with no flights, 23:30 to 04:30
2023144 nights with no flights, 23:30 to 04:30
2024189 nights with no flights, 23:30 to 04:30
2025207 nights with no flights, 23:30 to 04:30
2019
Right. 119 nights, the baseline. Heathrow puts the rise since then down to working with airlines on late-running departures: 257 in 2019, 123 in 2025.
2023
Not yet. 144. Already better than 2019, but a long way from the 207 of 2025.
2024
Not yet. 189. The deal was working; this is the second-best year.
2025
Not yet. 207 nights, the most. The target was at least 150.
71,540Complaints
++
1,424People complaining
++
561,600People inside the line
Now the number that is not a measure. In 2025 Heathrow received 71,540 noise complaints. They came from 1,424 people, about fifty complaints each on average, and a small number of people wrote in many times. So complaints measure how many people write, not how many people hear. The contour count does the second job.
Each is a number about Heathrow's noise. Does it count people who hear the planes, or something else?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
561,600 inside the 55-decibel day line
71,540 complaints in a year
219,900 inside the 50-decibel night line
87 in 100 of planes meet the quietest standard
207 nights with no flights after 23:30
17 in 100 fewer people highly annoyed than in 2019
561,600 inside the 55-decibel day line → People who hear it
Not yet. People who hear it. The line is drawn around the sound, and the count is who lives inside it.
71,540 complaints in a year → Counts something else
Not yet. Something else: how many messages were sent. One person writing every day sends 365 of them.
219,900 inside the 50-decibel night line → People who hear it
Not yet. People who hear it, at night. The night line is drawn lower, at 50 decibels, because sleep is more easily broken.
87 in 100 of planes meet the quietest standard → Counts something else
Not yet. Something else: a fact about the fleet. It helps explain the people count but is not one.
207 nights with no flights after 23:30 → Counts something else
Not yet. Something else: a count of nights. It is a cause of less disturbance, not a measurement of who was disturbed.
17 in 100 fewer people highly annoyed than in 2019 → People who hear it
Not yet. People who hear it. It is the contour count turned into an estimate of how many inside the line are badly affected.
3People who hear it
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3Something else
Right. Three count people under the sound. Three count messages, planes or nights. All six are real; only the first pile measures the cost next door.
Complaints down3,000 to 2,000
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People inside the linenot counted
A town measures a new road's noise. The council reports 2,000 complaints, down from 3,000 last year, and calls it progress. What is missing?
A count of people inside a noise line. Fewer complaints may mean people gave up writing.
Right. Complaints count writers, and writers tire. A contour count of homes above a noise level would say whether fewer people hear the road, which is the thing that was promised.
Nothing. Fewer complaints is less noise.
Not yet. A thousand fewer letters can mean a thousand people who stopped bothering. The road's sound was not measured at all in that report.
The decibel reading at the roadside.
Not yet. Closer, but a roadside reading says how loud the road is, not how many people live within its sound. The missing number is people inside a line.
Lesson complete
Noise is measured in people inside a line on a map, not in complaints.