Aircraft noise

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Qantas Boeing 747-400 passes close to houses on the boundary of London Heathrow Airport, England

Small general aviation aircraft produce localized aircraft noise

Helicopter main and tail rotors produce aerodynamic noise

Aircraft noise is noise pollution produced by any aircraft or its components, during various phases of a flight: 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.^{[citation needed]}

Mechanisms of sound production

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:

• Aerodynamic noise
• Engine and other mechanical noise
• Noise from aircraft systems

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.^[1]

• 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).^[1]

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 Gas Turbine engines (Jet Engines) are responsible for much of the aircraft noise during takeoff and climb. However, with advances in noise reduction technologies - the airframe is typically more noisy during landing.^{[citation needed]}

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 of course 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 see a large reduction in Jet Noise.^{[citation needed]}

Noise from aircraft systems

Cockpit and cabin pressurisation 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 (or APU). An Auxiliary Power Unit is 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 specialised electronic equipment in some military aircraft.

Health effects of aircraft noise

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,^[2] in many developed nations the impact of noise is sufficient to impair hearing over the course of a lifetime.^[3][4] Elevated noise levels can create stress, increase workplace accident rates, and stimulate aggression and other anti-social behaviors.^[5]

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.^{[citation needed]}

In the 1980s the U.S. Congress authorized the FAA to devise programs to insulate homes near airports. While this does not address the external noise, the program has been effective for residential interiors. Some of the first airports at which the technology was applied were San Francisco International Airport and San Jose International Airport in California. A computer model is used which simulates the effects of aircraft noise upon building structures. Variations of aircraft type, flight patterns and local meteorology can be studied. Then the benefits of building retrofit strategies such as roof upgrading, window glazing improvement, fireplace baffling, caulking construction seams can be evaluated.^[6].

Night flying restrictions

At Heathrow, Gatwick and Stansted airports in the UK, and Frankfurt Airport in Germany, night flying restrictions apply to reduce noise exposure at night.^[7][8]

See also

• Aviation and the environment
• Electric airplane
• Farley v Skinner
• Hush kit
• Hearing impairment
• Helicopter noise reduction
• Jet noise
• Noise barrier
• Noise pollution
• Noise regulation
• Tinnitus
• XF-84H Thunderscreech, the loudest aircraft ever built.

References

• U.S. Noise Control Act of 1972 United States Code Citation: 42 U.S.C. 4901 to 4918
• S. Rosen and P. Olin, Hearing loss and coronary heart disease, Archives of Otolaryngology, 82:236 (1965)

External links

• Airport Noise Law United States
• European Aviation Safety Agency, Type Certificate Datasheets for Noise
• Federal Interagency Committee on Aviation Noise (FICAN)
• Aviation Environment Federation (AEF) United Kingdom
• Sound Initiative: A Coalition for Quieter Skies United States
• Airport Noise Report A weekly newsletter on litigation, regulations, and technological developments
• National Organization to Insure a Sound-controlled Environment(NOISE) United States
• Attitudes to Noise from Aviation Sources in England (ANASE) Study published by the United Kingdom Department for Transport