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Aircraft Noise Background |
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Noise has historically been the principal environmental issue for aviation. It remains high on the agenda of public concern. In addition to aircraft in the air and on the ground, noise can arise from ground transport and other operational sources. This page however, focuses on aircraft generated noise.
Noise disturbance is a difficult issue to evaluate as it is open to subjective reactions. Its impact is not a lasting one on the actual environment, but it can have significant adverse effects on people living close to an airport, including: interference with communication, sleep disturbance, annoyance responses, learning acquisition, performance effects and cardiovascular and psycho-physiological effects. Doubling the distance between yourself and the source of a noise effectively cuts the intensity of the sound by 6 dBA; i.e. the noise will only sound about 25% as loud.
Annoyance is the most widespread problem caused by environmental noise. Annoyance reflects the way that noise affects daily activities. People's social circumstances, their culture and the environment in which they live can all determine the degree of perceived annoyance for a given noise level. However, quality of life will suffer when people are disturbed or annoyed by noise. Sleep disturbance caused by environmental noise can also affect the quality and quantity of sleep, leading to drowsiness and poor performance the following day.
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Unless there are very many aircraft following a route, it is widely recognised that aircraft flying at a height of at least 10,000ft above the ground do not usually produce “significant” noise impact. But because of the subjective nature of disturbance and the wide variance of local factors, this is not an absolute rule. It is normal for aircraft noise to be associated with airports, because of the low height involved.
In fact, the level of noise and the metrics used to describe noise significance vary from country to country and from airport to airport. The European Commission will require all major airports in the European Union to use the Lden/Lnight noise metrics for noise mapping and specifies the 65 Lden and 55 Lnight levels as being of interest for mapping. These broadly equate to noise levels and metrics generally regarded as marking the onset of significant disturbance around airports. Values similar to these are used for planning restrictions on residential development. This does not mean that lower noise levels are unimportant, but below these levels, scientific evidence is that the majority of people are not unacceptably disturbed.
It is not just the ambient noise level itself, but change in noise level or tonality that is important. So, when any one of aircraft, operating procedures or flight routes are modified this can result in significant perceived change even when average ‘significant’ noise levels have not been exceeded. Whilst most people may not notice a 3 dB change, most will detect a 3-6 dB change as significant; above 6 dB this change may be perceived as severe.
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How do aircraft affect the noise climate? |
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Noise is generated by the engine (e.g. fan (front) and exhaust (rear)) and the airframe (e.g. body, wings, flaps, speed brakes and landing gear). On departure, high levels of thrust are used and so most of the noise is from an aircraft’s engines and this is typically the noisiest aircraft operation. Because the climb angle is usually steep, however, aircraft quickly reach a height above the ground where noise impact is less significant.
On arrival, aircraft use much less thrust, but because they are using flaps and landing gear, airframe noise is equal to or greater than that of the engines. Although arriving aircraft are less noisy than on departure, they descend towards the runway on a shallow "glide slope" of typically 3 degrees (ICAO standard) and so are closer to the ground for a much greater distance from the airport.
Aircraft also create noise on the ground when: taxiing, queuing, landing (especially when using reverse thrust), testing engines and when using the auxiliary power unit.
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How can noise affect aviation? |
There is a strong international, national and local policy response to aircraft noise impact. These can include:
- stringent aircraft standards;
- noisy aircraft phase out;
- non optimal runway configuration or routes;
- curfews;
- limits and caps on airport capacity or noise contours;
- mitigation costs (e.g. monitoring systems, sound insulation and engine pens);
- community relations programmes;
- delay/refusal in planning permission.
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How do we quantify aircraft noise? |
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Above certain noise levels (if it is too quiet measurement can become inaccurate), the noise levels on the ground from aircraft operations can be measured using carefully sited, purpose designed microphones. This is particularly useful to measure how an aircraft is being flown. Because of the effects of factors such as ambient noise and weather, individual measurements only give a snapshot assessment and do not always reflect the average situation. For this reason measurement is most often used in assessing operational performance rather than overall noise impact.
To reach an understanding of average noise levels, noise is usually modelled using computer programmes that simulate aircraft “virtually” following an airports operating procedures, but with suitable variability such as track dispersion to make it more realistic. These models, such as the widely used “International Noise Model”, produce aircraft noise footprints for the number of and type of aircraft using an airport in order to calculate the extent of particular noise levels around the airport. This will assume average weather conditions. These noise “contours” can then be placed on a map to see which communities are subjected to different degrees of average noise levels. But it should be remembered that, as average conditions rarely occur, the noise contours are only indicative of typical noise impact.
Land use planning restrictions, noise insulation schemes and airport development decisions are often made based on noise contours. Some account may be taken of modelled peak noise for the noisiest aircraft and to a lesser degree historical measured noise levels from microphones.
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What are the different units used to measure noise levels? |
The most widely used unit for measuring noise levels is dB(A) - the A-weighted scale in decibels. This unit attempts to reflect human reaction to "loudness". The human ear's response to sound relates to sound pressure in a way that is approximately logarithmic. This means that a significant reduction in noise activity results in a comparatively small reduction in the noise heard by the human ear.
Other dB based measurement units are uniquely related to aircraft.
The perceived noise (PNdB) and effective perceived noise (EPNdB) scales incorporate the different frequencies and duration of noise patterns, resulting from various speeds and modes of operation of aircraft. There is no agreement, even amongst the experts, on which measurement is the most representative, or the most relevant in a particular situation. However, the International Civil Aviation Organisation (ICAO) uses EPNdB for expressing its noise certification standards; Maximum Sound Level (LAmax) is the maximum instantaneous value recorded; Equivalent continuous sound pressure (Leq) is a measure of the average sound pressure level during a period of time.
The European Community proposes “Lden” as the common unit for measuring transport noise. Day-evening-night level (Lden) is based on Leq over a whole day with a penalty of 10 dB(A) for night time noise (22.00-7.00) and an additional penalty of 5 dB(A) for evening noise (i.e. 19.00-23.00).
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How loud are different noises? |
A guide to noise level is:
- normal conversation 50 - 60 dB(A)
- a loud radio 65 - 75 dB(A)
- a busy street 78 - 85 dB(A)
- a heavy lorry about 7 metres away 95 - 100 dB(A)
- a pighouse at feeding time 110 dB(A)
- a chain saw 115 - 120 dB(A)
- a jet aircraft taking off 25 metres away 140 dB(A) (unlikely to impact the general public!)
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Aircraft Noise Management |
All commercial aircraft must meet the International Civil Aviation Organization's (ICAO's) noise certification standards. These apply to aircraft designs and types when they are first approved for operational use, and they have been progressively tightened since the initial Chapter 2 standard was adopted in 1971. This was for aircraft designed prior to 1977. Since 1977, any new aircraft designs have been required to meet stricter (Chapter 3) standards. On April 1st 2002 most aircraft not meeting Chapter 3 standards were phased out by international agreement. From 1 January 2006, a more stringent standard (Chapter 4) applied for new aircraft designs. This standard will be one third quieter than the existing Chapter 3 standard.
The 33rd ICAO Assembly adopted Resolution A33/7 introducing the concept of a ‘balanced approach’ to noise management, thereby establishing a policy approach to address aircraft noise, including international guidance for the introduction of operating restrictions on an airport-by-airport basis. The ‘balanced approach’ concept of aircraft noise management comprises four principal elements and requires careful assessment of all different options to mitigate noise, including:
- reduction of aircraft noise at source;
- land-use planning and management measures;
- noise abatement operational procedures; and,
- operating restrictions.
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The Balanced Approach has since been incorporated into European Community legislation as Directive EC/2002/30).
At a local level, some airports operate a noise quota system over and above the noise certification standard. In these cases, the number of movements of aircraft are regulated by a combination of a limit on the number of movements and a quota or "noise budget". The noise budget represents the total sum of noise over a specific period.
Other commonly applied noise management measures include:
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- establishing a technical partnership between airport air traffic control, aircraft operators and the airport operator to jointly progress aircraft noise management;
- depicting preferred noise routes on a map that avoid residential areas as far as possible, monitoring track adherence accuracy and seeking to improve track-keeping;
- avoiding over-flying sensitive sites such as hospitals and schools;
- ensuring that the optimum runway(s) and routes are used as far as conditions allow;
- using continuous descent approaches and departure noise abatement techniques;
- avoiding unnecessary use of auxiliary power units by aircraft on-stand;
- building barriers and engine test-pens to contain and deflect noise;
- towing aircraft instead of using jet engines to taxi;
- limiting night operations;
- limiting the number of operations or the extent of a critical noise contour;
- providing noise insulation for the most severely affected houses;
- applying different operational charges based on the noisiness of the aircraft;
- monitoring individual noise levels and penalising any breach.
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Last validation: 11/07/2007
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