The Acoustic Research Laboratory at NASA's Langley Research Center houses several of the major acoustic research facilities. The primary research facility is the Quietflow Aerocoustics Facility that includes a 20 ft x 24 ft x 30 ft anechoic chamber with two open-jet wind tunnels. Facilities for testing human response to sonic booms, outdoor aircraft flyover noise, and aircraft interior noise are also in the laboratory.
Airports prepare noise exposure maps that show existing noise conditions at an airport. They are used to identify areas that may have too much noise.
The Noise Reduction Program on the hangar apron at NASA's Lewis Research Center (now Glenn Research Center) in Cleveland, Ohio. Noise from aircraft engines presents problems for wildlife and people, and NASA has undertaken various programs to reduce aircraft engine noise.
Noise Reduction Technology
Aircraft noise has always been a problem, and even though some piston-engine planes produced noise that many found annoying, it was the arrival of jet engines that increased the level of noise on many aircraft. The operation of the Boeing 707, first delivered in 1958, immediately presented problems to the airports where it landed and took off. Their noisy turbojet engines prompted complaints about "noise pollution" from surrounding communities. Airport authorities at London Heathrow and New York Idlewild (now Kennedy) airports instituted noise limits in the 1960s. They required long-range aircraft to fly at lighter weights (less fuel and passengers) so that they could climb faster and get farther from populated areas quickly. Some aircraft took off from Heathrow and landed at another airport in England for refueling before traveling across the Atlantic not because they lacked the range but because they were not allowed to take off at full weight and full engine power. This was obviously very inconvenient.
In 1966, after a number of lawsuits in the United States and a public outcry in Europe, the major commercial aviation authorities called an international conference on aircraft noise to establish rules for aircraft. After much politicking and delay, the U.S. Federal Aviation Administration (FAA) decided to implement its own rules in Federal Aviation Regulations (FAR) Part 36, in 1971. FAR Part 36 established limits on the maximum noise that could be produced at an airport at three points—two on either end of the runway beneath takeoff and landing paths and one at the middle and sides of the runway. It also established a sliding scale for allowable noise versus takeoff weight for large aircraft (in other words, bigger aircraft could be noisier).
The FAA regulations on aircraft noise were tightened several times after the initial rules were set. Naturally, this has prompted aircraft manufacturers to try to develop quieter aircraft. The first attempts to reduce aircraft noise preceded government regulation and were made in the 1950s. Although there was a lot of theoretical research by engine designers on the causes of aircraft noise and a number of proposed theoretical solutions, these theoretical solutions often did not solve the problem, and aircraft designers resorted to trial and error methods.
The biggest source of aircraft noise is the engines (although the air rushing over the airframe also creates noise). Designers of jet engines suspected that the major source of engine noise came from the region behind the engine where the high-velocity exhaust (or "efflux") mixes with the lower velocity surrounding air. Engine designers in Europe concentrated on changing the nozzle designs of engines, primarily by corrugating the outer edge of the exhaust nozzle. Rather than smooth and round, they made it warped or angled, often looking like a flower. This better mixed the high-velocity efflux with the air behind the engine. In the United States, designers used this approach and also another method involving venting the exhaust from several tubes. But both of these methods increased drag and reduced engine performance, and the multi-tube approach also increased weight, sometimes substantially. Some commercial aircraft in the 1960s were fitted with truly bizarre-looking multi-lobe and multi-tube nozzles to reduce noise.
Another solution that emerged in the 1960s was the "ejector-suppressor." This was essentially a large tube fitted aft of the engine around the exhaust nozzle that allowed air from outside the engine to mix with the exhaust, reducing the final efflux velocity. The inside of the ejector was also lined with a noise-absorbing material. Whereas most early noise suppressors reduced low-frequency noise, the lined ejectors reduced high-frequency noise as well. But work on fully lined ejector-suppressors was halted as high-bypass ratio subsonic engines became available for commercial aircraft.
The establishment of the FAA noise regulations in the early 1970s called for a 25 to 50 percent reduction in maximum noise generated by the existing long-range Boeing 707, Douglas DC-8 and Vickers VC-10 aircraft. Larger aircraft then entering service or in development, such as the Lockheed L-1011, Douglas DC-10 and Airbus A300 were designed with the more stringent requirements in mind. The Boeing 747, a monster commercial aircraft, did not meet the new regulations and required some modification.
The development of high-bypass turbofans that powered these newer aircraft was prompted by the need for greater thrust and fuel efficiency, but also resulted in a beneficial reduction in noise. In a high-bypass turbofan, the central turbine drives a large fan in front of it that passes a lot of air around the turbine (this is what is meant by high-bypass—a lot of air bypasses the turbine). Not only does the fan produce less noise per pound of thrust, but the cooler air mixing with the hot jet exhaust also insulates the engine, acting as a muffler (it "muffles" the noise). This reduction can be directly observed at a major international airport, particularly one that also includes flights of older turbojet-powered aircraft operated by poorer nations. For instance, a modern Airbus A340 jet is noticeably quieter than the rare 707. These older aircraft have to gain special permission to violate the noise regulations.
Many aircraft built during the 1970s and 1980s, such as the 737, have engines that do not appear perfectly round when seen from the front. This is because the bottom of the engine cowling (or covering) has to have clearance above the runway so that it does not scrape along the ground. When the 737 first appeared in the 1960s, it had fairly small turbojet engines that were suspended high above the ground. As the engines became larger with the introduction of turbofans, the fuselage and wing remained the same height above the ground, so the bottom of the engine cowling was flattened somewhat to still achieve the maximum air intake without scraping the ground.
Although it is easier to design quieter engines from scratch than to try and quiet an existing design, there is a substantial market for aircraft "hushkits" to reduce the noise on current aircraft. These hushkits include some of the same technological approaches first explored in the 1960s by aircraft designers. As the number of commercial airplanes flying increases, local communities around airports complain more and this leads to calls for even greater regulation of airplane noise. As a result, aircraft designers are constantly looking for ways to make their aircraft quieter.
During the late 1960s, several countries were trying to develop a supersonic transport, or SST, a plane that would fly faster than the speed of sound. There were numerous hurdles for the designers to overcome, including both engine noise and sonic booms. Ultimately, the United States abandoned its effort for various reasons, including concerns over excessive noise pollution. The British and French jointly developed the Concorde, but its use was restricted to only a few airports due to noise concerns. Noise remains a major challenge for any future large, fast aircraft.
--Dwayne A. Day
Sources and Further Reading:
Bond, David. "The Book on Noise Reaches Chapter 4." Aviation Week and Space Technology. January 1, 2001, 48-53.
Ott, James. "Europe Seeks New Noise Standards." Aviation Week and Space Technology, January 1, 2001, 53-54.
Smith, Michael, J.T. Aircraft Noise. Cambridge: Cambridge University Press, 1989.