International Panel Discussion on Metrics
Webinar No 16
Do we get more disturbed by aircraft noise than the official figures show? Is there a right metric to measure noise annoyance from aircraft?
The way aircraft noise is measured is complex. Metrics are ways of measuring noise. Different metrics measure different things. We outline the key metrics used below. Decibels indicates how loud a noise is. All noise, not just aircraft noise, is measured in decibels. You will also come across the phrase ‘noise contours’. As we explain below, a contour indicates the level of noise in a certain area.
We explained more in the 3 June 2024 webinar. In brief, the most common metrics are:
The LAeq Metric
The number of aircraft passing over an area during a 16 hour day is counted. The noise of each plane is measured in decibels (db). The noise is then averaged out for the day. This is then turned into an annual average.
The Lden Metric
Lden averages the noise out over an 8 hour day, a 4 hour evening and an 8 hour night, with 5 and 10 decibels added to the evening and night figures respectively to account for generally lower background levels at those times. Used by The World Health Organisation (WHO) and the European Commission, as they believe it is more meaningful than LAeq. Lnight averages the noise only during the night period and is also used by WHO.
The DNL Metric
The DNL Metric averages the noise over a 24-hour period but adds 10 decibels of the night period to allow for the lower background noise levels, expressed as the noise level for the average day of the year on the basis of annual aircraft operations. This is used in America and some other countries.
The ‘N’ Metric
This measures the number of planes going over a house which are above a given decibel level. So, for example, N60 would show the number of planes over 60 decibels which fly over a home during a given period.
What is a noise contour?
Most airports produce noise contours. They usually are annual contours. They show the level of aircraft noise in an area averaged out over a year. So, for example, 65dbLAeq contour indicates the area when the noise averages out at 65 decibels or more over the course of the year.
What is a safe level of annoyance?
The World Health Organisation says that the daytime safe level for aircraft is 45Lden and at night 40Lnight. The European Union says that for most people noise annoyance starts at 55Lden. America puts it much higher at 65DNL. The UK goes lower: 51LAeq.
Low Frequency Noise
There is an argument that none of the metrics used measure the low-frequency noise from aircraft. Low frequency noise is a different type of noise. There is a lot of low frequency noise in, for example, a sound system. As well has hearing it the body can ‘feel’ it. Aircraft noise also has a lot of low frequency. There are good arguments to say that all the above metrics don’t fully capture the low-frequency noise because they use ‘A’ weighting. ‘C, ‘D’ or ‘Z’ weighting are a lot better at capturing low-frequency noise. In 1999 the World Health Organisation acknowledged the importance of low frequency noise:
Special attention should also be given to: noise sources in an environment with low background sound levels; combinations of noise and vibrations; and to noises with low-frequency components. Guidelines for Community Noise, Exec Summary 3.10 – World Health Organisation [WHO]
