Jet engine noise reduction system.
Jet engine noise reduction system.
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 refuelling before travelling 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.
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
The noise reduction system includes active noise control to suppress fan tone noise of an airplane flyover noise signature. The active noise control includes microphones with acoustic transducers upstream and downstream of the engine fan and fan exit guide vane stage to sense control system errors. Control signals are derived from the fan angular speed or blade passing frequency and the error signals sensed by the acoustic transducers. The control output signals actuate (modulate) air control valves on each side of the fan stage to direct conditioned (pressure and temperature regulated) high pressure primary air flow, thereby producing acoustic canceling of fan tone noise.
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.
