Aircraft noise

Aircraft noise is noise pollution produced by aircraft during the various phases of a flight. The term is mostly used for external noise from the planes. A jet engine is among the most noisy man-made objects that exist, and the aircraft noise can be so violent that even a few seconds’ stay near a plane, especially during departure, can lead to hearing loss. The sound pressure 25 m from a jet plane that takes off is approx. 150 dBA (decibel -A), enough to burst the drums. In addition to the engine noise, shock waves will come in the form of violent clues if a plane is holding an overflow rate, which is not applicable to today’s civilian aircraft. But a plane gives significant aerodynamic noise long before it reachesaudio speed. Also internal noise and vibration in airplanes and helicopters are often annoying and in some cases so strong that it can cause hearing loss.

Sound production is divided into three categories:

Mechanical noise—rotation of the engine parts, most noticeable when fan blades reach supersonic speeds.
Aerodynamic noise—from the airflow around the surfaces of the aircraft, especially when flying low at high speeds.
Noise from aircraft systems—cockpit and cabin pressurization and conditioning systems, and Auxiliary Power units.
Health consequences include sleep disturbance, hearing impairment and heart disease, as well as workplace accidents caused by stress. Memory and recall can also be affected. Governments have enacted extensive controls that apply to aircraft designers, manufacturers, and operators, resulting in improved procedures and cuts in pollution.

Mechanisms of sound production
Aircraft noise is noise pollution produced by an aircraft or its components, whether on the ground while parked such as auxiliary power units, while taxiing, on run-up from propeller and jet exhaust, during take off, underneath and lateral to departure and arrival paths, over-flying while en route, or during landing. A moving aircraft including the jet engine or propeller causes compression and rarefaction of the air, producing motion of air molecules. This movement propagates through the air as pressure waves. If these pressure waves are strong enough and within the audible frequency spectrum, a sensation of hearing is produced. Different aircraft types have different noise levels and frequencies. The noise originates from three main sources:

Engine and other mechanical noise
Aerodynamic noise
Noise from aircraft systems
Engine and other mechanical noise
Much of the noise in propeller aircraft comes equally from the propellers and aerodynamics. Helicopter noise is aerodynamically induced noise from the main and tail rotors and mechanically induced noise from the main gearbox and various transmission chains. The mechanical sources produce narrow band high intensity peaks relating to the rotational speed and movement of the moving parts. In computer modelling terms noise from a moving aircraft can be treated as a line source.

Aircraft noise from jet engines
Aircraft gas turbine engines are responsible for much of the aircraft noise during takeoff and climb, such as the buzzsaw noise generated when the tips of the fan blades reach supersonic speeds. However, with advances in noise reduction technologies—the airframe is typically more noisy during landing.

The majority of engine noise is due to jet noise—although high bypass-ratio turbofans do have considerable fan noise. The high velocity jet leaving the back of the engine has an inherent shear layer instability (if not thick enough) and rolls up into ring vortices. This later breaks down into turbulence. The SPL associated with engine noise is proportional to the jet speed (to a high power). Therefore, even modest reductions in exhaust velocity will produce a large reduction in Jet Noise.

The sound generation during operation of a jet engine is primarily due to the flow around blades, the combustion in the combustion chamber and by friction of the mechanical parts; In addition, the sound emission comes from the generated turbulent flows behind the engines. The fan, the compressor and the turbine are paddle wheels, wherein in particular the compressor and the turbine are usually designed in multiple stages and thus have a variety of paddle wheels. The basic theory of flow-field sound generation was developed in 1952 by the British mathematician Michael James Lighthill, who transformed the Navier-Stokes equations into a wave equation. The solution of this equation, which can be written in the form of a retarded potential, describes the radiated sound of a paddle wheel in theoretical form. Aeroacoustics deals with the complex formation of noises caused by air currents in the engine.

Sonic blast
If an aircraft flies supersonically, a shock wave will be created on the fuselage and stern of the aircraft. These shockwaves spread in the shape of the Mach cone and arrive shortly after flying over an observer at this. For small aircraft and higher altitudes, these shock waves are perceived by one person as a bang, on larger aircraft or at low altitudes, as two immediately consecutive bangs. Contrary to popular belief, the sonic boom not only occurs at the moment when the sound barrier is breached, but it occurs permanently and is exposed to all over-flown at supersonic speeds places. The supersonic blast of a supersonically-flying aircraft at a height of one hundred meters can produce a sound pressure level of up to 130 dB (A), which is about as loud as gunfire fired from close quarters.

Aircraft noise due to air flow outside the engines
When starting an aircraft, the engines work under full load and emit high sound pressure levels; the sound emission of other components is marginal in relation to it. When approaching an aircraft (and in new flight strategies in certain phases of the launch, see below), however, the engines are operated at partial load; Here, the sound emission by other factors has a fairly high share of the total emissions. Main factors are the flow noise of high-lift propulsion (especially slats and flaps) and chassis.

At an opening below the airfoil, the Airbus A320 family’s tank pressure equalization port creates a high sound when air overflows (similar to blowing over a glass bottle). A metal plate can divert the air and attenuate the phenomenon by 4 dB.

Noise emission due to engine noise
Smaller sized aircraft, such as light aircraft, do not have engines, but usually propel their propellers with a piston engine. Due to the significantly lower maximum speeds and geometric dimensions that such aircraft have, the noise emissions from air currents are usually negligible. When the engine is switched off and in the air (as in gliders), these types of aircraft cause hardly any sound perceptible to the ground – in contrast to line and military aircraft, which emit loud noises even when the engines are switched off theoretically. The sometimes considerable sound pressure levels, which are generated by small aircraft, are thus due solely to the engine noise and the air flows caused by the propeller.

Aerodynamic noise
Aerodynamic noise arises from the airflow around the aircraft fuselage and control surfaces. This type of noise increases with aircraft speed and also at low altitudes due to the density of the air. Jet-powered aircraft create intense noise from aerodynamics. Low-flying, high-speed military aircraft produce especially loud aerodynamic noise.

The shape of the nose, windshield or canopy of an aircraft affects the sound produced. Much of the noise of a propeller aircraft is of aerodynamic origin due to the flow of air around the blades. The helicopter main and tail rotors also give rise to aerodynamic noise. This type of aerodynamic noise is mostly low frequency determined by the rotor speed.

Typically noise is generated when flow passes an object on the aircraft, for example, the wings or landing gear. There are broadly two main types of airframe noise:

Bluff Body Noise – the alternating vortex shedding from either side of a bluff body, creates low-pressure regions (at the core of the shed vortices) which manifest themselves as pressure waves (or sound). The separated flow around the bluff body is quite unstable, and the flow “rolls up” into ring vortices—which later break down into turbulence.

Edge Noise – when turbulent flow passes the end of an object or gaps in a structure (high lift device clearance gaps) the associated fluctuations in pressure are heard as the sound propagates from the edge of the object (radially downwards).

Noise from aircraft systems
Cockpit and cabin pressurization and conditioning systems are often a major contributor within cabins of both civilian and military aircraft. However, one of the most significant sources of cabin noise from commercial jet aircraft, other than the engines, is the Auxiliary Power Unit (APU), an on‑board generator used in aircraft to start the main engines, usually with compressed air, and to provide electrical power while the aircraft is on the ground. Other internal aircraft systems can also contribute, such as specialized electronic equipment in some military aircraft.

Health effects
Aircraft engines are the major source of noise and can exceed 140 decibels (dB) during takeoff. While airborne, the main sources of noise are the engines and the high speed turbulence over the fuselage.

There are health consequences of elevated sound levels. Elevated workplace or other noise can cause hearing impairment, hypertension, ischemic heart disease, annoyance, sleep disturbance, and decreased school performance. Although some hearing loss occurs naturally with age, in many developed nations the impact of noise is sufficient to impair hearing over the course of a lifetime. Elevated noise levels can create stress, increase workplace accident rates, and stimulate aggression and other anti-social behaviors. Airport noise has been linked to high blood pressure.

Cardiovascular Diseases
Aircraft noise has effects on the cardiovascular system and manifests itself in diseases of the system. The relationship between aircraft noise and cardiovascular disease has been demonstrated in several case studies.

According to a World Health Organization health report, 1.8% of heart attacks in Europe are caused by traffic noise in excess of 60 dB. The share of aircraft noise in this traffic noise remains open. In another study, the relationship between aircraft noise and high blood pressure in 2,693 subjects in the greater Stockholm area was examined and came to the conclusion that from a continuous sound level of 55 dB (A) and a maximum level of 72 dB (A) Significantly higher risk of disease is present. In the context of this study, the authors were also able to demonstrate that blood pressure increases even during sleep with increased noise levels without the people accustomed to the aircraft noise awakening.

Mental disorders
Occurring mental disorders can have different causes, some of which are not explored. Significant contributors to the occurrence of such disorders, which include subjective tinnitus (persistent ear noise), hyperacusis (a pathological hypersensitivity to sound), and, more rarely, phonophobia (a phobic disorder involving sound or specific sounds), are stress responses. This stress can certainly be triggered by long-lasting aircraft noise. In Germany alone, about one in ten people reports symptoms of tinnitus and 500,000 people suffer from hyperacusis.

German environmental study
A large-scale statistical analysis of the health effects of aircraft noise was undertaken in the late 2000s by Bernhard Greiser for the Umweltbundesamt, Germany’s central environmental office. The health data of over one million residents around the Cologne airport were analysed for health effects correlating with aircraft noise. The results were then corrected for other noise influences in the residential areas, and for socioeconomic factors, to reduce possible skewing of the data.

The German study concluded that aircraft noise clearly and significantly impairs health. For example, a day-time average sound pressure level of 60 decibels increasing coronary heart disease by 61% in men and 80% in women. As another indicator, a night-time average sound pressure level of 55 decibels increased the risk of heart attacks by 66% in men and 139% in women. Statistically significant health effects did however start as early as from an average sound pressure level of 40 decibels.

FAA advice
The Federal Aviation Administration (FAA) regulates the maximum noise level that individual civil aircraft can emit through requiring aircraft to meet certain noise certification standards. These standards designate changes in maximum noise level requirements by “stage” designation. The U.S. noise standards are defined in the Code of Federal Regulations (CFR) Title 14 Part 36 – Noise Standards: Aircraft Type and Airworthiness Certification (14 CFR Part 36). The FAA says that a maximum day-night average sound level of 65 dB is incompatible with residential communities. Communities in affected areas may be eligible for mitigation such as soundproofing.

Cabin noise
Aircraft noise also affects people within the aircraft: crew and passengers. Cabin noise can be studied to address the occupational exposure and the health and safety of pilots and flight attendants. In 1998, 64 commercial airline pilots were surveyed regarding hearing loss and tinnitus. In 1999, the NIOSH conducted several noise surveys and health hazard evaluations, and found noise levels exceeding its recommended exposure limit of 85 A-weighted decibels as an 8-hr TWA. In 2006, the noise levels inside an Airbus A321 during cruise have been reported as approximately 78 dB(A) and during taxi when the aircraft engines are producing minimal thrust, noise levels in the cabin have been recorded at 65 dB(A). In 2008, a study of Swedish airlines cabin crews found average sound levels between 78–84 dB(A) with maximum A-weighted exposure of 114 dB but found no major hearing threshold shifts. In 2018, a study of sound levels measured on 200 flights representing six aircraft groups found media noise level of 83.5 db(A) with levels reaching 110 dB(A) on certain flights, but only 4.5% exceeded the NIOSH recommended 8-hr TWA of 85 dB(A).

Cognitive effects
Simulated aircraft noise at 65 dB(A) has been shown to negatively affect individuals’ memory and recall of auditory information. In one study comparing the effect of aircraft noise to the effect of alcohol on cognitive performance, it was found that simulated aircraft noise at 65 dB(A) had the same effect on individuals’ ability to recall auditory information as being intoxicated with a Blood Alcohol Concentration (BAC) level of at 0.10. A BAC of 0.10 is double the legal limit required to operate a motor vehicle in many developed countries such as Australia.

Air travel and wildlife
Airplane noise can be annoying and harmful to the wildlife too. For example, fur breeders have experienced that the animals have eaten newborn puppies whose planes or helicopters have passed during the puppying. The problem has also been relevant in connection with military exercises with low-flying over national parks or nature reserves during the breeding and breeding season in spring.

Measures to reduce aircraft noise
Various measures have been taken to reduce aircraft noise. The procedures are generally subdivided into emission-reducing and immission-reducing measures (often also into active and passive noise control). While emission reduction measures aim to reduce noise directly at the source, ie aircraft or helicopters, the aim of immissions reducing methods is to minimize the impact on the population, animals or the environment. The latter can be achieved by various measures such as sound insulation or increasing the distance to aircraft.

Emission reduction measures
Through various design measures, noise emissions from engines, propellers and rotors have been significantly reduced over the past few decades. In jet engines this is done in addition to other changes mainly by turning away from Einstrom- and thus the increased use of turbofan engines; With propeller aircraft and helicopters, lower sound pressure levels can be achieved by changing the blade geometry, which enables low rotors speeds. By levying charges and banning particularly high-noise aircraft, as implemented in the US and the European Union, airlines and thus indirectly the aircraft and turbine manufacturers to the development and use of quieter aircraft models are urged.

Development in jet engines
Advances in the development of jet engines have, in particular, significantly reduced the noise emitted by civil aviation engines compared to the engines used since the 1950s.

A significant part of the lower noise emission has the implementation of the secondary flow in jet engines, ie the development of jet engines from single- jet engines to turbofan engines. While in the first generations of engines no or only a very small sidestream was used, modern engines produce a large part of up to 80% of the total thrust by the sidestream, the mass distribution of air in the sidestream to such in the main stream (“by-pass ratio”) partially in the ratio of 12: 1. The PW1124G engine, which will be installed in the Airbus 320neo, among other things, reduces the sound pressure level by 15 dB (A) according to the manufacturer, and the PW1521G engine developed by Bombardier even by 20 dB (A).

For some engines, it is possible to install silencers. Older aircrafts with a lower bypass ratio can – often only later – be fitted with hush kits, which among other things reduce the speed differences between the fast main flow and the ambient air. Disadvantage of the hush kits are power losses of the engine. The “Chevron Nozzles” built into the engines of the Boeing 787 follow a similar principle: a zigzag-shaped trailing edge of the engine is intended to better mix the secondary flow with the ambient air, thereby reducing noise emissions.

Another constructive measure is the use of new exhaust nozzles, which mixes the exhaust gas in some way with the ambient air, so that the noise emission is reduced. Even in modern engines enlarged distance between the stator and impeller of the compressor leads to a reduction of the sound. Other ways to reduce the noise emission are changed geometry of the paddle wheels in the engine or the use of noise-absorbing material at the engine air intakes.

Another way to reduce the noise emission of the engines, is the absence of the use of thrust reversers with more than idling power. The thrust reverser can be turned on when landing immediately after landing the aircraft. Due to the deflection of the engine jet, the thrust of the engines is forward, so the aircraft is decelerated. In civil aviation, however, airplanes are generally only allowed to approach runways at airports where a safe landing can be guaranteed without the use of reverse thrust. Thus, the full thrust reversal is increasingly dispensed with, as it is connected by the short-term startup of the turbines to high performance with significant noise emissions.

Turboprops and helicopters
In turboprops, the emitted sound is largely due to the propellers on the engines. By changing the blade geometry propellers could be made more effective, which is why the speeds at which the propellers are operated can be reduced. The speed reduction provides a reduction in aircraft noise and allows the engines to operate at lower power, again reducing noise. A similar effect applies to helicopters: by changing the blade geometry of the rotor, the helicopter can be operated at a lower speed in the blade tips, which could reduce emissions.

Approach procedure
The burden of the airport residents is significantly dependent on the choice of approach method of airplanes, since depending on the chosen method, a different number of people with different levels of sound pressure levels is charged. In addition to the standard method of approach (Standard Approach), in which the final configuration of the aircraft for landing (ie extended flaps and extended landing gear) is reached quite early, various other methods are now being tested and explored. In some cases considerable relief for the residents of the airport can be observed.

An important alternative approach procedure is the Low Power / Low Drag Approach (LP / LD), which is developed at Frankfurt Airport, with the landing flaps and especially the landing gear are extended much later – the LP / LD is the chassis only five nautical miles (NM) before reaching the runway extended, in contrast, the standard approach procedure already twelve NM before.

Another method is the Continuous Descent Approach, whereby horizontal flight phases during descent are to be largely avoided. This allows the engines to idle, while the standard approach procedure requires higher engine power due to intermediate horizontal phases. The Continuous Descent Approachmay therefore lead to noise pollution, in particular in the range of 55 to 18 km in front of the runway. Disadvantage of Gleitanflugverfahrens is that it is more difficult to realize with increasing traffic, because at cruising aircraft, a horizontal flight is inevitable, and thus at busy times at many airports not or only partially – for example, at night or at low traffic times – can be used, The largest airports using the procedure are the Frankfurt and Cologne / Bonn airports; In addition, the procedure will be tested at other airports. In the final phase of the landing approach has the plane in the beacon of the Instrument Landing System set and thus maintain a fixed rate of descent, which is why there, from about 18 km in front of the runway, no noise reduction by Gleitanflugverfahren is more feasible.

An older method, which follows a similar principle as the Continuous Descent Approach, is the approach in two segments (two segment approach), wherein in the first segment initially a steep approach angle is selected and this is then reduced in the guide beam to the specified value. The reduction of aircraft noise pollution occurs in particular by areas overflown at a higher altitude; Disadvantages are, due to the higher sink rate, safety concerns and less comfort for the passengers.

Approach angle
By default, airplanes sink at a 3 ° lead angle, which is the ICAO standard. If this angle is increased, so sink the aircraft so in the final approach with a higher rate of descent, the place where the final approach is initiated, moved accordingly closer to the runway. As a result, a certain area around the runway is overflown by the aircraft at a higher altitude, thereby reducing the noise pollution. Approach angles other than 3 degrees are possible only in the all-weather flight mode CAT I. In the case of the all-weather flight operations CAT II and III, according to ICAO PANS-OPS (Doc 8168) a mandatory 3-degree approach angle must be observed.

Departure procedure
Also in the context of the departure can be reduced by choosing the departure procedure, the noise emission. First, the engines must run at high power at the start in order to reach a sufficient speed for safe start and a stall to avoid. However, once a safe altitude and a sufficiently high airspeed for a stable flight condition is achieved, the power of the engines can be shut down.

The noise abatement method, which was developed in the USA in 1978, plans to lower the take-off thrust from 1000 feet (300 meters) above the ground, thus continuing the descent with a smaller angle of climb. When reaching an airspeed of 250 knots (460 km / h), the rate of climb is increased again. First and foremost, this method allows for a high kerosene saving, but the low altitude of only 300 meters above the ground results in continued high noise levels for the inhabitants of the overflown area.

A departure procedure developed by the International Air Transport Association (IATA) recommends climbing to 1500 feet (450 meters) with maximum engine power, then shutting down the engine power and raising it again at an altitude of 3000 feet (900 meters). This departure procedure relieves the airport residents, but leads to increased fuel consumption. Therefore, a total of 14 various profiles have been developed for different aircraft models to take into account the characteristics of the aircraft as possible.

Flight routes
In principle, when determining flight routes, attempts are made to avoid flying over metropolitan areas and to design the flight routes in such a way that skimmed areas are preferably flown over. This raises the question as to what extent the advantage of a larger community (common good) to the detriment of the inhabitants in the sparsely populated areas is justifiable. The choice of the standardized flight route in the context of airspace planning as well as short-term deviations from this flight route, usually by the air traffic controllerdepend on many and sometimes complex factors. The avoidance of aircraft noise plays an important role, but is fundamentally subordinate to flight safety.

Introduction of noise protection zones
Noise protection zones are areas around an airport, which are subject to special regulations and requirements for the purpose of noise protection. In Germany, they are set up on the basis of the FluLärmG; the calculation of the design of the noise protection zones as well as the individual issued conditions are carried out by mathematical models. A brief description of the noise protection zones defined by the German FluLärmG and the situation in other countries can be found in the section on the legal situation.

Noise protection buildings
There are many ways to build noise protection buildings and thereby protect the airport residents from aircraft noise. Some noise protection buildings are used directly at the airport, so the necessary test runs of the engines are carried out at larger airports in noise protection halls, which significantly reduce the sound emitted into the environment by sound insulation. Even soundproofing walls can dampen the noise emitted by an airport – but this applies only to a very limited extent to the noise of airplanes taking off and landing, as these are located very quickly above the noise barriers and the aircraft noise thus affects the airport residents unhindered.

An important measure of residents near the airport is the use of soundproofing ventilation systems and soundproof windows, which reduce the noise reaching the interior of the apartment through increased tightness and the use of special, differently thick window panes. Soundproof windows are divided into six classes, with the highest class capable of absorbing more than 50 dB (A) of sound.

Night flight ban
Another measure, which serves in particular to protect the night’s sleep of the population, is the issue of night flight bans. However, night bans generally do not prevent, as the name implies, all night-time flights, but rather restrict the take-offs and landings of aircraft at airports at night. In the German FluLärmG a night flight ban is not provided, but there are at all German airports to the airport Frankfurt-HahnLimited operating permits for take-offs and landings during the night. The period of validity of the night flight bans is regulated individually for each airport as well as the exact implementation. For example, despite the ban on night flights, night-time take-offs and landings are permitted for certain purposes of flights such as postal flights or rescue flights or aircraft models of certain noise categories at most airports.

Noise mitigation programs
In the United States, since aviation noise became a public issue in the late 1960s, governments have enacted legislative controls. Aircraft designers, manufacturers, and operators have developed quieter aircraft and better operating procedures. Modern high-bypass turbofan engines, for example, are quieter than the turbojets and low-bypass turbofans of the 1960s. First, FAA Aircraft Certification achieved noise reductions classified as “Stage 3” aircraft; which has been upgraded to “Stage 4” noise certification resulting in quieter aircraft. This has resulted in lower noise exposures in spite of increased traffic growth and popularity.

Satellite-based navigation systems
A series of trials were undertaken at London’s Heathrow Airport, between December 2013 and November 2014, as part of the UK’s “Future Airspace Strategy”, and the Europe-wide “Single European Sky” modernisation project. The trials demonstrated that using satellite-based navigation systems it was possible to offer noise relief to more surrounding communities, although this led to a significant unexpected rise in noise complaints (61,650) due to the concentrated flight paths. The study found that steeper angles for take-off and landing led to fewer people experiencing aircraft noise and that noise relief could be shared by using more precise flight paths, allowing control of the noise footprint of departing aircraft. Noise relief could be enhanced by switching flight paths, for example by using one flight path in the morning and another in the afternoon.

Technological advances

Engine design
Modern High bypass turbofans are not only more fuel efficient, but also much quieter than older turbojet and low-bypass turbofan engines. On newer engines noise-reducing chevrons further reduce the engine’s noise, while on older engines the user of hush kits are used to help mitigate their excessive noise.

Engine location
The ability to reduce noise may be limited if engines remain below aircraft’s wings. NASA expects a cumulative 20–30 dB below Stage 4 limits by 2026–2031, but keeping aircraft noise within airport boundaries requires at least a 40–50 dB reduction. Landing gear, wing slats and wing flaps also produce noise and may have to be shielded from the ground with new configurations. NASA found over-wing and mid-fuselage nacelles could reduce noise by 30–40 dB, even 40–50 dB for hybrid wing body which may be essential for open rotors.

By 2020, helicopter technologies now in development, plus new procedures could reduce noise levels by 10 dB and noise footprints by 50%, but more advances are needed to preserve or expand heliports. Package delivery UAS will need to characterize their noise, establish limits and reduce their impact.

Source from Wikipedia