EUREKA MAY 1998 COVER FEATURE STORY
Tuning in to the sound of silence
A pneumatic actuator could deliver fast and powerful pulses that drown out the noise of jet engines. Tom Shelley reports
A prototype actuatator uses a fast-acting valve and compressed air to deliver up to 1,000W of sound power
Its purpose is to demonstrate a technology that can cancel the noise of jet engines on the ground or in the air.
Besides reducing the noise from aircraft, it could also be put to work controlling the airflow in jet engines or counteracting shock waves in other transport applictions.
The most crucial part is a valve that responds in 1.5ms. This opens up the possibility of better control of what happens inside jet engines, as well as offering new ways of transmitting energy and making faster responding actuators.
Wladyslaw Wygnanski and Cambridge Concept is a remarkable enterprise. He has so far won two DTI Smart Awards and the 1995 Flight International Award for the best innovation likely to help the environment. The June 14 edition of the magazine explains that he received the accolade for his scheme for cancelling out the sound produced by jet engine exhausts at source, combining his mechanical and electronic acoustic control skills in the same innovation.
At first glance, the idea might seem impossible: the engines of a DC10 airliner produce around 10kW of sound power. Conventional loudspeakers are typically only 3 to 4 per cent efficient, so producing 10kW of 'anti sound' is likely to require at least 250kW of electricity.
Wygnanski, however, devised a scheme using pneumatics, which he believed could work at 50 per cent efficiency. While we cannot reveal full details, we know that the initial scheme was sufficiently convincing to earn his company a place in a major European research project, 'Reduction of aircraft noise by Nacelle treatment and active control'. The consortium has a 36 month duration programme and is lead by Aerospatiale. Members include Snecma, Rolls-Royce and B down the list - Cambridge Concept.
Engineers know that jet engine noise has two main sources: the compressor fan blade passing frequency; and the turbulence generated by the boundary of the jet exhaust plume interacting with the atmosphere. Noise produced by the compressor blades is predictable and relatively easy to counteract. That produced at the rear of the engine is chaotic, but that does not mean it is random.
Some years ago, the Cambridge company then called Topexpress took part in a project to suppress gas turbine surges by counteracting before they had a chance to grow larger. This builds on the idea that if one knew which beat of which butterfly's wing was going to start the first eddy which would grow into a hurricane, all storms could be prevented. Weather control in this way is clearly impractical but we are told that the experiments with gas turbines showed that the technique could be applied to them.
Wygnanski's project has now reached the point of successfully making and testing a pilot scale anti noise generator, powered by some of the sort of air available from the compressor stage of a jet engine.
The crucial part is a valve. Wygnanski says it is largely the brainchild of the Whittle Laboratory in Cambridge B which controls the flow of air through an orifice with a response time of 1.5 milliseconds. The issuing air enters a cylindrical chamber, about 0.5m long and of similar diameter. Wygnanski describes this as a 'pneumatic condenser'. The sound emerges from a pattern of holes at the other end. Our photograph shows a test being set up at Cranfield University. Wygnanski is seated, while the young man standing behind him is Frederick Silvert, an MPhil student at the university being supported by Cambridge Concept. Once the system is powered up, they recommend standing at least 5m away and wearing ear protection!
The electronic side of active noise control is relatively easy. Modern electronics can work fast enough to detect a pulse of noise sound and generate a counter pulse before the original sound pulse has a chance to travel more than a short distance. Noise cancelling headphones, once only available for fighter pilots, can now be purchased in domestic audio shops. Another company in which Wygnanski is heavily involved produces acoustic processing units for controlling delay times between loudspeakers in theatres. They use 32-bit Analog Devices Sharc digital signal processors to perform complex manipulations in real time. Sharc processors are also much favoured for latest real time acoustic processing in ultrasonic military and civilian sonar systems.
Wygnanski is confident that he can demonstrating the active suppression of noise by the pilot scale actuator this September.
The next stage is the financing of a project to build a full scale active noise suppression system. The first implementation of this is likely to be a system for reducing sound levels in one of the maintenance bays at Heathrow Airport. For this reason, both the British Airports Authority and British Airways are now supporting the project. Mounting systems onto flying aircraft will come later.
Other potential applications discussed including controlling airflow inside jet engines, both to control surge and to direct it for maximum efficiency. It should be possible to use such a system to counteract the shock wave generated by high speed trains as they enter and leave tunnels. At present, the 186mph Eurostar trains have to slow appreciably when they enter and pass through the Channel Tunnel so they do not blow the train windows in (or out). It may also not be long before teenagers encounter Wygnanski sound generators as the ultimate Sub Woofers at pop concerts.
* The system offers a way of cancelling jet engine noise at source by creating a powerful 'anti sound' signal
*Prototype pilot scale sound generators use compressed air to produce up to 600W of sound at 26 to 42 per cent efficiency
*Pneumatic valves have been developed for the sound generators with a response time of 1.5ms
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