Reducing Aircraft NoiseWith the global expansion in air travel, aircraft noise has become a major public issue. For over 40 years, scientists and engineers at the Institute of Sound and Vibration Research (ISVR) at the University of Southampton have been working with the aircraft industry to reduce aircraft noise, by improving the design of engines. Today, many people are concerned about the environmental impact of flying, but it’s easy to forget the success of technological developments which have resulted in quieter planes across the world.

The history of ISVR:

ISVR was founded in 1963 with the aim of increasing our understanding of noise and vibration across a very wide range of applications, including studies as diverse as noise from trains and automobiles, the effect of vibrations on the human body, the restoration of the hearing of the profoundly deaf, the diagnosis of defects from the sound of the human heart, and sound propagation through the oceans. One application which has attracted particular attention at the ISVR since its foundation has been the development of new technology to mitigate the effect of noise from aircraft engines on humans. Much of this work has been undertaken in collaboration with Rolls-Royce plc, and the ISVR has become a leading international centre for aircraft noise research.

Rolls-Royce had supported a readership in noise at the ISVR since 1968, and, in 1999, took the decision to found a University Technology Centre (UTC) in Gas Turbine Noise to benefit from the ISVR’s capabilities. One of 28 Rolls-Royce UTCs around the world, the Southampton centre now supports more than 30 staff and postgraduate researchers, and has a turnover of £1.5m. Contributions from Rolls-Royce are supplemented with grants from other funding bodies, such as the Engineering and Physical Sciences Research Council (EPSRC), the Technology Strategy Board (TSB) and the European Commission.

Andrew Kempton, Chief Noise Specialist at Rolls-Royce noted: "Rolls-Royce has a long, proud history of working with universities, creating a win/win situation where the university is funded to work on important industrial challenges. The ISVR brings a breadth and depth of knowledge, an independence of thought and an aptitude for innovation that helps ensure the best technology is built into Rolls-Royce engines."

Demand for quieter aircraft:

The impact of aircraft noise on communities near airports became apparent in the 1960s and 1970s as turbojet and turbofan engines became widespread in civil aviation. A regulatory framework setting noise levels for new aircraft entering service was agreed internationally at that time, and these regulations have become progressively more stringent since then.

Increases in the volume of passenger traffic have, however, tended to increase noise in spite of significant reductions in the noise caused by individual aircraft. With the increase in scheduled services and in package holidays through the 1960s and 1970s, and the advent of low-cost airlines during the 80s and 90s - Easyjet sold its first online seat in 1998 - air travel has grown at an unprecedented level. Global passenger and freight air traffic was growing at 4.25% per annum until the recession in late 2007.

This volume of traffic, coupled with new routes and services from more airports located close to centres of population, has resulted in the need for increasingly tough pollution, CO2 and noise reduction targets.

In response to this demand, the work at the University of Southampton focuses on two key areas: reducing the amount of noise generated at source and trying to reduce the noise ‘effect’ before it reaches the ears of the public.

One of the most interesting challenges for those studying aircraft noise is that the public have grown less tolerant to noise over the last 25 years. Aircraft noise has been measured using EPNdB (Effective Perceived Noise in Decibels), which tends rely on engineering principles of measurement to assess the average noise levels heard by the listener. However, the annoyance people experience can be caused by any number of psychological factors, including the frequency of noises (perhaps caused by the number of overflights) and the variety of different noises heard.

Dr Ian Flindell, engineer and psychologist at ISVR comments: “The purpose of continuing research in this area is to find better ways of measuring and assessing aircraft noise so we can achieve a better balance between noise, the costs of noise control and the effects of noise on people, so society can continue to enjoy the social and economic benefits of aviation while minimising the environmental costs as far as possible.”

The technology behind noise reduction:

The most dominant sources of engine noise come from two parts of the engine: the blades of the fan at the front, and the jet exhausting at the rear. Much of the research undertaken at the UTC has been focused on acquiring a better understanding of both of these noise sources, and on developing new technologies to reduce their effect. The fan draws air into the engine and provides propulsive power, propelling the aircraft forward. In modern aircraft engines, only a small proportion of air actually goes through the core of the engine, the rest passes around it down the bypass duct. This ‘bypass ratio’ has steadily increased over the last 30 years. On the most recent large engines in the Rolls-Royce Trent range, some of which have fans that are over three metres in diameter, the bypass ratio exceeds 10:1

Professor Jeremy Astley, director of the Rolls-Royce UTC at the ISVR comments: “While the large bypass ratio of modern large engines has come about largely as a way to increase propulsive efficiency, it has had a hugely beneficial effect in reducing engine noise, and, in particular, jet noise. Further increases in the bypass ratio are, however, constrained by practical limitations, such as the sheer size and weight of the engine nacelle, and this poses new challenges for noise reduction.”

A successful method of reducing noise further, even in ultra-high bypass ratio engines, is to absorb sound created within the engine. Acoustically absorbent material or acoustic ‘liners’ can be placed on the interior surfaces - on, for example, the walls of the intakes and bypass ducts. Much work has been done at the ISVR on optimising such acoustic treatments, so the sound radiated from the engine is dramatically reduced to a tenth of that of an unlined engine.

What else has been achieved?

The Advisory Council for Aeronautics Research in Europe (ACARE) goal for EU research, established in 2000, was to develop the technology to reduce the EPNdB of new aircraft by 10dB (decibels) over 20 years. ACARE also sets targets for air quality, fuel consumption and carbon emissions.

Aircraft today are 20-30dB quieter than first generation of jet aircraft, such as the Boeing 707 and Comet. They now produce less than 1% of the sound of these early airliners, with less than a quarter of the annoyance.

The design of the Rolls-Royce Trent 900 engine - built into the new Airbus A380 ‘super jumbo’ airliner) benefited from years of research at UTC Southampton.

The new ‘Dreamliner’ (Boeing 787) is built with a light carbon fibre fuselage and is powered by another new quiet engine from Rolls Royce - the Trent 1000. But research will need to continue to drive down aircraft noise further, and research at the University of Southampton will continue to play an important part in meeting this goal.