From Academic Kids
Aviation noise is a form of environmental noise. Noise can be defined as unwanted sound.
Sound will always be produced by aeroplanes. As an aircraft moves through the atmosphere, compression and rarefaction of the air will produce motion in the air molecules. Similarly, an aircraft jet engine or propeller will also cause movement in the molecules of the atmosphere. This movement propagates through the air as pressure waves. If these pressure waves are strong enough and within a certain frequency spectrum, a sensation of hearing is produced.
Sources of noise
Different aircraft types have different noise levels and frequencies. The contributions to the total noise level originate from 3 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 noise has a large aerodynamic contribution and is typically broad-band in its frequency range. Low-level, high-speed flight in military combat aircraft carries a large aerodynamic noise contribution. The level of aerodynamic noise varies with aircraft design. Thus, the shape of the nose, windshield or canopy of an aircraft can greatly affect the noise level. Much of the noise of a propeller-driven aircraft is of aerodynamic origin due to the flow of air around the propeller blades. The helicopter main and tail rotors also give rise to aerodynamic noise. This type of aerodynamic noise is mostly low-frequency and is related to the rotation speed of the propeller or rotor.
Engine and other mechanical noise
Much of the noise in propeller-driven aircraft comes from the propellers themselves and as such is aerodynamic noise. The highest levels of internal aircraft noise are usually found in line with the propeller blades. Helicopter noise has a unique spectral content, essentially being 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.
Noise from aircraft systems
Internal aircraft systems also form an important part of the noise in aircraft. Cockpit and cabin pressurisation and conditioning systems are often a major contributor in both civilian and military jet aircraft. Other internal aircraft systems can also contribute, such as specialised electronic equipment in some military aircraft.
External noise of civilian aircraft activity
Starts, approach and landing of aircraft may lead to a noise of more than 100 dB(A) at the ground. Since aircraft landing in inner-city airports are often lower than 200 ft (60 m) above roof level, they can cause a noise of more than 95 dB(A).
Landing aircraft descend on a 3 degree glide path which places them at 200 ft above rooftops at 4071 ft (¾ Mile 1.2 Kilometers) from the end of the runway. This distance is usually well inside the airport fence. Departing aircraft normally are over 500 ft above the ground before crossing the end of the runway. Most houses built within a mile of an airport were built long after the airport was established.
Aviation noise may cause problems in communities and has been claimed to cause health problems. It is said to cause heart diseases, immune deficiencies, neurodermatis, asthma and other stress related diseases. Although there are some studies that suggest connections with disease, further research needs to be carried out.
Since aviation noise became a major public issue in the 1960s and 1970s, governments have brought in legislative controls. Aircraft designers, manufacturers, and operators have developed quieter aircraft and more careful operating procedures. Modern high-bypass turbofan engines, for example, are significantly quieter than the turbojets and low-bypass turbofans of the 1960s.