Big noise to cut jet engine roar

Noise created by the increasing number of aircraft flying over residential areas could be permanently eliminated by a new acoustic control technique, NASA claims.

Noise created by the increasing number of aircraft flying over residential areas could be permanently eliminated by a new acoustic control technique, NASA claims.

Pressure on airports such as London’s Heathrow is growing as the number of passengers increases. Plans to build new runways or redevelop former military sites close to urban areas have met with fierce public opposition because of the anticipated disturbance it will cause.

But a new system developed by Dr Carl Gerhold, a senior researcher at NASA’s Langley Research Centre, is said to reduce or even cancel sound generated during take-off and landing.

‘The next generation of advanced turbo passenger jets will be propelled using an engine- driven fan, rather than providing power through the engine directly,’ said Gerhold.

‘This has advantages for fuel economy and will slightly improve noise control. but to a large extent engine noise will be replaced by fan noise.’

While the interior of the engine duct can be insulated to damp the amount of noise produced, this adds weight to the aircraft, affecting its fuel economy.

With the new system a number of speakers are placed around the engine’s fan duct. These measure the tone of the noise produced and fire back sound of the same tone, shape and power but out of phase with the oncoming waves. This cancels them out.Unlike previous acoustic control devices which have concentrated on reducing noise at points outside the fan unit, microphone sensors are placed at a number of locations around the fan duct.

According to the developers, because this reduces the sound at source it automatically reduces sound transmitted outside, whereas eliminating noise at one point may simply shift the problem to another area.

‘Based on the trend of legislation already in place, noise restrictions will only get more stringent,’ says Gerhold. ‘This system automatically aligns itself with noise, and so it can be turned off once you are away from a built-up area, saving energy, and switched back on when you come in to land. This will minimise the muffler system’s energy use.’

The technology can also be used to reduce the intake and exhaust noises produced by heating and air-conditioning fans, though these must be situated within a duct for the system to work.

At present the system is still in its development phase. But NASA’s developers say that the speed of advancement in sound technology means a unit should be available in the near future.

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