The relative contribution of these sources depends on the engine and airframe designs and the operating conditions. For example, during takeoff, when the engines are at full thrust, jet noise is the largest contributor to the noise signature of an aircraft. At approach, when the engine is throttled back, noise comes more from the airframe. Other sources, such as the fan, are significant contributors during both takeoff and landing.
Typical noise sources for a fixed-wing aircraft are shown in Figure 5-1. The noise received by an observer depends on the sources and propagation effects. The noise sources for a propeller-driven aircraft are shown in Figure 5-2.
The aeroacoustics community has made significant progress over the years in understanding and reducing aircraft noise. Figure 5-3 shows comparative contributions from different noise sources for 1960s and 1990s engines. The figure, which originally appeared in Rolls-Royce (2005a), shows that the development of the turbofan engine and reduction in noise from individual engine components resulted in smaller, more evenly matched noise contributions from engine sources (SBAC, 2009).
Over a period of 30 years, these improvements, coupled with advances in aircraft aerodynamics and weight technologies, have reduced aircraft noise by about 20 dB, which corresponds to a reduction in noise annoyance of about 75 percent (EU, 2007). The new Airbus A380, the largest commercial aircraft ever produced (average of 525 passengers), has takeoff and approach noise levels comparable to those of heavy road traffic, a lower noise level than in an underground train. The noise footprint of the A380 is about half that of older, large commercial aircraft (Rolls-Royce, 2005b).
Despite these impressive results, airport community noise continues to be a significant environmental problem, and research and development (R&D) continue in the United States and Europe to meet increasingly stringent noise requirements set by regulatory bodies, such as the Federal Aviation Administration (FAA), the International Civil Aviation Organization (ICAO), and individual airports (Rolls-Royce, 2005b). Over the years, the FAA and ICAO have required comparable reductions.
Both the United States and Europe have first-class aero-acoustics test facilities. Anechoic flight simulation facilities, the most useful for testing both jet noise and airframe noise, are available on both sides of the Atlantic on a rental basis. In the United States, high-quality anechoic chambers for model-scale testing are available at the National Aeronautics and Space Administration (NASA) Langley and Glenn Research Centers, as well as at Boeing, General Electric, United Technologies, and some U.S. universities, such as Georgia Institute of Technology, which inherited Lockheed Georgia’s aeroacoustics facilities. Rolls Royce in England has used the NASA Glenn jet noise acoustic chambers, and Boeing researchers have used facilities in England. NASA