How are aeroplanes getting quieter?

semi-anechoic chamber, University of Southampton

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Standing in the half-light and eerie silence of a semi-anechoic chamber - designed to absorb sound - in the bowels of the University of Southampton, I'm beginning to wonder whether I've stumbled upon a real-life Stargate.

At closer inspection, the silver semicircle at the back of the room contains a microphone rig rather than a wormhole leading to other planets, and the instrument pointing at it is an enormously high-pressured air jet, not a teleporter.

Computers in the adjoining office can create 3D soundscapes of the noise recorded inside the chamber.

The £500,000 facility is one of a suite of testing labs built at the university's Institute of Sound and Vibration Research (ISVR) in an effort to create quieter aircraft.

It was created when Rolls-Royce asked the team to explore a potential noise issue regarding one specific valve on a particular turbo fan engine, explains Prof Jeremy Astley, director of the ISVR.

The valve's job was to blast extremely hot air out of the the engine.

"In previous engines there were other noise sources so we hadn't noticed this one," he said.

"In noise control when you bring one source down others start protruding. We weren't sure how much noise this made so we designed the facility to mount it and test it."

Their conclusion was that the valve did indeed generate "a lot" of noise.

"But we also found it was easy to reduce - with perforated sheets."

The facility is now a permanent fixture for teaching and research purposes, the engine in question is quieter as a result - and for Prof Astley, it's an example of the benefits of academics working more closely with industry.

"You have the sure knowledge that what you are doing is not a waste of time," he said.

"If the work was not of interest, it would not be supported."

Rolling Stones Noise levels at a rock concert like the recent Rolling Stones gigs can reach 115 decibels.
Community care

While sound levels have fallen by about 30 decibels since the early commercial flights, modern jet engines still register up to 160 decibels - much louder than the rowdiest of rock concerts at 115 decibels - and are at their most audible from the ground during take-off and landing, as anyone who has ever lived near an airport will tell you.

In August last year, more than 1,800 complaints were received about the noise around Heathrow - an increase from just 400 the year before - but for the aviation industry, keeping quiet is not only a matter of maintaining community relations.

The noise level of an aircraft is also a major factor in securing commercially vital slots at major airports.

"At the London airports noisier aircraft get fewer slots," said the professor.

"The loudest ones are the oldest - the 747s. It was the best technology we had 35 years ago."

While fuel efficiency and carbon emission levels are also taken into account, he cites noise as the "single most significant environmental restraint" in the expansion of commercial air transport.

Stale mate

The problem is that traditional engine quietening solutions are approaching stalemate.

liner casing Liner casings have long been used on board aircraft to absorb sound.

Basic acoustic absorbers known as liners - small, light, honeycombed lattices containing millions of tiny air spaces which absorb sound, have been commonly used by commercial aircraft since the 1970s.

"You can always make an engine quieter with more liners," said Prof Astley.

"But then you're increasing the mass and the drag [of the aircraft] and that increases fuel burn. You have to have a very balanced approach [to environmental factors]."

Another tried and tested approach to noise control - making the engines bigger, as a bigger, slower engine and fans makes a lower and less offensive noise - is squeezing the room left for the liners, which paradoxically need to be thicker to cope with the lower sound levels.

And there's a further dilemma.

"The engines are now getting too big to fit under the planes," Prof Astley told the BBC.

Sound-reducing windows

Researchers in South Korea are working on an idea once thought unlikely: a window which lets air pass through it while dramatically reducing the level of noise.

Sang-Hoon Kima, from Mokpo National Maritime University, and Seong-Hyun Lee, from the Korea Institute of Machinery and Materials, designed an acrylic resonance chamber, a bit like double glazing, which weakens sound at certain frequencies.

The window can be "tuned" to lessen lower frequencies by drilling small holes in the acrylic.

The researchers say their design is simple enough that an ordinary carpenter can make it, and suggest the concept could even be used to allow owners of homes by the sea to hear the sound of the waves, but to screen out other less attractive sounds from outside.

"We have reached the point of no return without redesigning the airframes."

Back to the future

The answer could lie in the revival of a radical idea first mooted more than 30 years ago.

Open rotor engines, containing contra-rotating propellers, are, as their name suggests, not enclosed.

Ironically the idea was originally shelved in the 1980s because the engines were thought to be too noisy - but Prof Astley dismisses the first prototypes as "primitive" by today's standards.

Soundproof window

In closed engines, one factor which can minimise noise is the flow of air around the engine rather than through it - known as the bypass ratio.

With an open engine, that ratio would become "infinite", according to the professor.

Another bonus is that these engines could be up to 30% more fuel efficient than current models - although an aircraft redesign would still be inevitable.

"It would change both the appearance and the sound," said Prof Astley.

"The sound would be more tonal - it would change not just the level but also the quality of noise."

But it's not likely to happen overnight.

"We are 90% of the way there. Perhaps we'll start to see them on the next generation of single aisle aircraft - in about 10 years' time."

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