Unlike most other forms of pollution, noise pollution depends not just on the physical aspects of sound itself, but more importantly the human reaction to it. There are no international or national limits which must be adhered to: instead, the issue is the effect on people's health and quality of life. This makes measuring the impact of noise pollution a complex process.
The loudness of sound is generally measured in terms of decibels (dB). On the decibel scale, a doubling of noise energy results in an increase of 3dB. A change of 3dB has been defined as the minimum that is perceptible under normal conditions, while a change of 10dB corresponds to roughly a doubling or halving of loudness. 'A-weighted decibels' (dBA) are often used in measurements of aviation noise. This adjusts decibel values to take account of the frequencies that people are most sensitive to, as the human ear is less sensitive to sounds at low and high frequencies.
Some examples of typical loudness are: near total silence (0dB); normal conversation (60dB); a heavy lorry passing 15m away (80dB); a jet aircraft taking off at a distance of 300m (100dB).
The CAA has sound level meters and sound level analysers to record sound levels at different points as an aircraft takes off. Many airports, including Heathrow, Gatwick and Stansted, have their own equipment for the same purpose. In some cases, this is a legal requirement.
The information taken by this equipment is correlated with details of the aircraft and its flight path using a Noise Track Keeping (NTK) system. This gives a detailed insight into noise patterns and allows analysis by aircraft type, location, altitude, runway and even weather conditions.
Noise pollution is represented by the relation between noise levels and impact on health and quality of life. Some impacts can be measured objectively (e.g. speech disturbance or hearing loss), while most are highly subjective (e.g. annoyance).
To overcome subjectivity, noise pollution is generally measured by using the maximum level and occurrence of noise. Both directly affect annoyance and other disturbances. For some, an occasional loud noise is most disruptive, while constant background noise is more disruptive for others.
Two basic measures for assessing noise impact are:
- Leq which means the 'equivalent continuous sound level'. This is the average sound level for a specific location over a defined measurement period. The UK Government considers a Leq of over 57dBA to represent the noise level for the onset of significant community annoyance.
- Lden which uses an annual average of the Leq but also takes into account the additional annoyance/disturbance of noise generated in the evening and at night. 5dB is added to noise made in the evening (7:00pm-11:00pm) and 10dB for the night (11:00pm-7:00am).
To assess the impact of noise, analysts identify how many homes and residents are located in areas where the Leq is over 57 dBA or the Lden is over 55dBA. These can be shown on maps known as noise contour maps.
Types of noise
LAeq is used to measure continuous noise (see above).
Occasional loud noise
Occasional loud noise is measured in the UK by Sound Exposure Level (SEL). Studies have found that SEL above 90dBA generally leads to sleep disturbance. SEL footprints can be used to work out the areas where take-off creates an SEL over 90dBA to inform decisions about whether or not a particular type of aircraft should be permitted to operate at night, or to influence airport construction or extension in populated areas.
An SEL footprint shows the geographical area in which a particular SEL is reached from a single noise event (e.g. a plane taking off). Below is an example of an SEL footprint:
Helicopter noise is far more complex to measure, predict and assess than fixed-wing aircraft noise. Helicopters do not have to follow predefined routes; may hover over a specific area (increasing the impact of the noise in that location); and create asymmetric noise distribution because of the presence of a tail rotor and the direction of rotation of the main rotor.
In the UK, average summer day LAeq noise contour maps are typically produced annually from measurements taken between 16 June and 15 September, 7:00am to 11:00pm. Contours are normally plotted from 51 dBA to 72 dB LAeq,16h at 3dB intervals. Some airports also produce average summer night LAeq,8h contours between 11:00pm and 7:00am. The annual noise contours for Heathrow, Gatwick and Stansted from 1997 to 2015 are available from the Department for Transport website. Since 2016, noise contours maps have been published on each airport’s website.
Noise contour maps make it possible to identify how many people live in areas where there is significant annoyance from noise. They are used to measure progress against the Government's stated aim to limit and where possible reduce the number of people who are affected by noise. They also allow planners to consider noise or projected noise when working within affected areas, though they must balance economic, environmental and social factors when making decisions.
The Environmental Research and Consultancy Department (ERCD) at the CAA develops and maintains the UK civil aircraft noise contour model ANCON on behalf of the Department for Transport, which calculates contours from aircraft movement, route, noise generation and sound propagation data.
ANCON is used to generate the annual noise exposure contours for Heathrow, Gatwick and Stansted. Other major airports also use the model. As well as producing historical noise contour maps, it is also used to generate forecast noise contours for airport development and airspace change proposals.