Environment
Noise
1. Noise. What is it?
Noise is an audible manifestation of mechanical vibrations in a physical, elastic medium, like air. The vibrations heard by the human ear are a sound signal with different physical parameters, the main ones being the intensity and frequency of the sound.
2. How is noise measured?
Sound levels are measured in decibels (dB), on a logarithmic scale, where 0 dB is considered to be the "threshold of hearing" (the minimum noise a human being is capable of hearing) and 120 dB is the "threshold of pain" (the level from which the human ear begins to suffer physical damage).
3. What is a decibel?
The decibel, dB(A), or dBA, is the level of sound pressure (intensity) at which human beings can actually hear a certain noise.
4. What equipment do we use to measure noise levels?
All ANA-VINCI airports have noise monitoring systems, consisting of monitoring stations (microphone, sound level meters and weather stations), distributed along the runway approach areas.
The measurement of noise levels follows strict procedures, and the acoustic testing is accredited according to the legal regulations in force.
Through this system, it is possible to distinguish total environmental noise (sum of all sound sources) from a particular noise (in this case, associated with planes), and residual noise (environmental noise minus the particular noise).
5. Can I accurately measure noise levels using mobile phone apps?
No. No mobile phone is properly calibrated for this, so the results will almost always be inaccurate. In addition, there are specific measuring procedures which are in accordance with legal (and physical) requirements to ensure their accuracy. It should be added that it is also essential to be able to distinguish particular noise – the noise associated with aircraft movements, in order to be able to effectively measure its contribution in relation to total environmental noise (otherwise, we would simply be measuring the sum of all noise sources without understanding the specific contribution of the planes).
In the same way, when using mobile phone apps, the noise that is normally measured is what is called instantaneous noise, where, in terms of the applicable legal requirements, what is used is an integration of specific noise indicators for a certain time. Therefore, comparison exercises are not possible.
6. How are legal noise limits defined?
According to the General Noise Regulation, legal noise limits are established for two different indicators – the entire day and the night-time period.
In either case, they refer to total environmental noise (sum of all sound sources). Therefore, the maximum noise limits set by law do not directly correspond to a level of particular noise, such as from an aircraft, but rather a set of different noises in the city.
7. What causes the noise from planes?
Aircraft noise is produced during ground and air operations. It is formed by two main sources - the frame and the engine.
The noise from the frame is generated by the passage of air over the aircraft frame. The amount of noise depends on the type of aircraft, the speed it's travelling at and its flight configuration. For example, when approaching the runway, the aircraft produces more air resistance caused by drag when the landing gear is lowered, creating more noise than if the landing gear was up and housed in the fuselage.
In the same way, the use of flaps (a system of movable panels on the aircraft wings which allows lift to be increased at lower speeds) when landing, increases the aerodynamic noise because it generates more of a drag effect on the air mass.
Engine noise is the main source of the noise generated by aircraft. It is more intense at take-off given that more acceleration is needed to enable take-off.
8. Why do I hear aircraft nose?
Normally, you hear aircraft noise because:
_You live or work near an airport.
_You live or work near a take-off or landing route.
_You live or work near an air traffic holding area.
9. Do aircraft have acoustic certification?
Yes. All aircraft are certified according to the noise levels they emit, bearing in mind the certification standard determined by the International Civil Aviation Organisation (ICAO). In the airports managed by ANA – Aeroportos de Portugal, this certification determines the time slots when these aircraft can operate.
10. Why do some planes seem to be noisier than others?
In airports, particularly international ones, aircraft with a wide range of noise levels operate, from smaller aircraft, many of them propeller-driven, called “general aviation”, to airliners, in a variety of sizes. The impact of aircraft noise depends mainly on the type of engine used, the size of the aircraft and whether they are landing or taking off.
11. Why do planes seem to make more noise at night than during the day?
This is mainly due to the fact that there is less environmental noise in the area around the airport at night. Environmental noise levels during the day tend to be louder than at night (more road and rail traffic, construction and industry noise, for example). In contrast, aircraft noise seems to be more intense in the relative silence of night.
12. Why do planes sometimes take off from one runway and sometimes from another one?
The choice of aircraft take-off direction depends on the weather conditions at the time, particularly the direction of the wind. For greater safety, all aircraft should take off and land into the wind. So, you can say that the choice of runway is always “where the wind takes you”.
Each runway ends up having a dual function, as if it were two. Let's take the example of Runway 03/21 in Lisbon (03 – North and 21 - South), where the prevailing wind is from the north. In order to land into the wind, you have to land from south to north. If the wind changes direction, the runway changes too (it becomes 21/03) and the aircraft make their approach and land from north to south. Stormy weather conditions or low visibility (LVO – Low Visibility Operations) also determine the approach route to the runway and landing, due to the location of the ILS (Instrument Landing System) systems required for operations such as these. Generally speaking, as take-off is the operation that emits higher levels of noises, residents in the areas around the north and south of the city will notice these differences in the intensity of the sound emitted.
13. Why can't planes fly over other areas?
In order to ensure aviation safety, the airspace around airports is strictly regulated. Airspace has countless “invisible roads” where the aircraft can operate, air traffic control areas, in the form of a corridor and routes, where the aircraft travel as if they were following the lanes on a motorway. They are defined corridors that link a certain place to another, at each specific altitude. Portuguese airspace has countless transit routes, which aircraft merely flying along to other countries, without constituting sound that is considered noise. In the area immediately around airports, due to the short distance and low altitude, aircraft must always be positioned in alignment with the runway and they cannot travel outside that alignment. This is why airports with a lot of air traffic often have an aircraft spending a few minutes circling at high altitude, in “holding corridors”, until it's their turn to approach the runway and land.
14. What kind of measures can be put in place for noise management?
The strategies for minimising the impact of airport infrastructures, particularly in terms of noise, can take on different forms and use different solutions. Noise can be reduced by acting on three areas:
a) the noise source;
b) the places where it's felt;
c) and the sound propagation routes (sometimes “diverting” the noise so that it doesn't get to areas of higher population density, for example).
It is very rare for there to be only one solution. The optimisation of solutions almost always implies the best combination of the largest number of possible alternatives, effectively minimising the effects of the noise on the neighbouring communities, safeguarding operating and safety conditions, assuring affordable costs through a rational use of resources and always bearing in mind the “balanced approach” principle, which is widely recommended in the European Union.
In this context, Action Plans have been defined for the Reduction of Noise from Large Air Transport Infrastructures located near the largest urban centres in the country, namely:
_Lisbon Airport Noise Reduction Action Plan (in portuguese)
_Porto Airport Noise Reduction Action Plan (in portuguese)
15. Can planes operate at night?
The airports that can operate at night, according to operational restrictions, are identified in specific legislation. There are operating restrictions in terms of the type of aircraft and maximum number of flights permitted for the period between midnight and 6 am for Humberto Delgado Airport - Lisbon, Francisco Sá Carneiro Airport - Porto, Madeira Airport, Porto Santo Airport and João Paulo II Airport – São Miguel, Azores.
16. How many flights can operate at night (midnight to 6 am) at Humberto Delgado Airport - Lisbon?
In the specific case of Lisbon, up to 91 flights are permitted per week. However, there may not be more than 26 flights per night. This scope does not include, for example, State, military, emergency or turboprop flights, given that these flights are of maximum necessity and/or minimum disturbance, according to the legislation in force.
17. Are there any exceptions to the night-time operating restrictions?
Yes. Bearing in mind specific and exceptional conditions, deemed to be force majeure, there are no restrictions on flights under the following general conditions:
a) Aircraft carrying out humanitarian missions, for medical emergencies or evacuations;
b) Aircraft in emergency situations, due to weather conditions, technical failure or flight safety;
c) Flights previously and exceptionally approved by the National Civil Aviation Authority (ANAC), bearing in mind reasons of recognised public interest;
d) Flights where there has been an unforeseen schedule change caused by abnormal disruption in air traffic control;
e) Flights up to 1 am scheduled for periods up to midnight, due to delays not the fault of the airport management body or the carrier;
f) Flights to and from the Autonomous Regions of Madeira and the Azores, due to weather conditions;
g) Landings made during the period between 5 am and 6 am, due to weather conditions, provided the arrival had been scheduled for after 6 am.
Adaptations of a local nature are defined in the applicable legislation, particularly for the airports in Madeira, Porto Santo and João Paulo II, São Miguel – Azores.
18. Why is Humberto Delgado Airport in Lisbon practically in the middle of the city?
Lisbon Airport was built in 1942, at a time when the entire area, including the surrounding land, was 100% rural. Lisbon City, as it was at the time, was far away from the airport. There are records that the discussions around its location, in 1929, had supporters in favour of Portela, Sacavém and Campo Grande, on the city limits at the time. With the growth in population and urban expansion, new residential neighbourhoods came closer and closer to the airport, finally surrounding it completely.
