Aircraft noise is defined as sound 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.
During take-off some aircraft may generate sound levels in excess of 100 decibels at ground level, with approach and landing creating lower levels. Since aircraft landing in inner-city airports are often lower than 60 meters (200 ft) above roof level, a sound level above 100 dBA can be realized.
Mechanisms of sound production
A moving aircraft including the jet engine
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 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
, which is typically broadband
. 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).
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.
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 reduction s in exhaust velocity will see a large reduction in Jet Noise.
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.
Lesser intensities of noise are produced for cruising velocities, mainly due to the altitudes of operation. This noise is more clearly heard in countryside settings where this noise can be intrusive even if much less in amplitude (say approximately 45 decibels). Landing aircraft descend on a three degree glide path towards an aiming point approximately 300 meters from the runway threshold. This places them at 60 meters (200 ft) above the ground at about 1200 meters (4,000 ft) from the aiming point or 900 meters (3,000 ft) from the start of the runway. This distance is usually outside the airport fence. Departing aircraft normally are over 150 meters (500 ft) above the ground before crossing the end of the runway.
Health effects of aircraft noise
The annoyance effects of aircraft noise are widely recognized; however, aircraft noise is also responsible for a significant amount of hearing loss as well as a contributor to a number of diseases. Only in the early 1970s did aircraft noise become a widespread topic of concern in the U.S. and federal regulations began to recognize the significance of abating these impacts in the vicinity of major commercial airports.
High levels of aircraft noise that may exist near major commercial airports are known to increase blood pressure
and contribute to hearing loss
. Some research indicates that it contributes to heart diseases
and other stress related diseases. Further research is being carried out to better understand these effects.
Research indicates that hearing loss is less a product of aging than a result of exposure to transportation related noise (Rosen, 1965). Any sound louder than normal conversation can damage the delicate hair cells in the cochlea, the structure in the inner ear that converts sound waves into auditory nerve signals. Initially damage to the cochlea may be temporary, but with repeated exposure, the damage becomes permanent and tinnitus may develop. More recently the Centers for Disease Control and Prevention's (CDC) National Center for Environmental Health (NCEH) conducted an analysis to determine the prevalence of hearing loss among children using data collected from 1988-1994 in the Third National Health and Nutrition Examination Survey. The analysis indicates that 14.9% of U.S. children have low or high frequency hearing loss of at least 16 dB hearing level in one or both ears.
From research of the National Institutes of Health, roughly 65 million Americans are exposed to sound levels that can interfere with their function at work or disrupt sleep, and 25 million are exposed to health risk (cardiovascular, immunological, etc.) from environmental noise.
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.
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.(Hogan, 1984).
Another idea to reduce aircraft noise on communities is floating airports which would be situated many miles out to sea. There are major drawbacks to this solution including expense, time and inconvenience to travelers in reaching such an airport. This includes the inability to integrate at-sea-airports with transport networks or proximity to business and cargo infrastructure.
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.
- C. Michael Hogan and Jorgen Ravnkilde, Design of acoustical insulation for existing residences in the vicinity of San Jose Municipal Airport, January 1 1984, FAA grant funded research, ISBN B0007B2OG0
- 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)