[The CAA and Heathrow are measuring the noise using different metrics].
The CAA use the 57 LAeq noise contour.
Heathrow use the 55 Lden contour.
“The CAA is using the Government’s preferred noise measurement known as the 57 LAeq contour. This looks at the number of planes, and the noise of each plane, flying over an area over a 16 hour day. The noise is then averaged out over the day. If the average is 57 decibels or more, that area is considered to be affected by the noise.
This 57 decibel cut off point has for years been widely criticized. Places like Fulham and Putney, clearly impacted by aircraft noise, lie outside the contour. Most European countries use a different measurement and the recent Airports Commission Report downplayed it.
But the CAA is correct that the numbers within the 57LAeq contour have risen. This is thought to be down to people moving into new homes and properties becoming households of multiple occupancy. [ie. higher population density. AW note]. This increased population has off-set any benefits from less noisy aircraft and improved operational practices.
So where did Heathrow get its figures? It used the measurement known as Lden recommended by the European Commission. This averages out noise over a 12 hour day, then separately over a four hour evening and an eight hour night. It adds 5 decibels to the evening measurement and 10 decibels to the night one to allow for lower background noise levels at these times.
It is regarded as more accurate than the 57LAeq contour. It certainly tallies more closely with the actual areas where noise is problematic. In London is includes area such as Clapham. The total numbers affected has been considerably over 700,000.
Heathrow, however, has brought the total number down to 702,000. It’s is good PR for the company. Whilst their new figure is technically accurate, the sleight of hand they have used to get it would have wide-boys of the world purring with pleasure.
The reason why the overall numbers is down is simply because of a reduction in night noise. Because 10 decibels are added to night flight noise to account for the lower background levels, Heathrow has only to introduce a relatively small number of less noisy planes at night to make a disproportional impact to overall noise levels. That is what has happened.
[So the reduction in noise is more a statistical artefact than a real improvement for residents. AW note]