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Noise Feature - Stansted battlelines drawn

The public inquiry about Stansted Airport's future development means that challenges are once again being raised about the validity of LAeq as a measure of aircraft noise nuisance. Lisa Russell and Tim Kaye report.

Modern aircraft are certainly quieter than their predecessors - but the relentless rise in travel means that there are far more of them.

So which is worse? Are more, quieter planes less annoying than fewer of a noisier generation? Questions have long been asked as to whether the official indicators based around noise contours derived using the averaging approach of the LAeq metric remain valid for assessing the impact. The current Stansted public inquiry is again giving noise consultants on both sides of the issue the opportunity to put their cases. There are arguments that LAeq has had its day as the key indicator and that something else is needed to better illustrate the actual disturbance of more frequent - albeit quieter - aircraft. Australia is cited as a leader in the field, thanks to work done following an outcry after the opening of Sydney's third runway.

LAeq is a method of averaging all the events producing sound energy over a period to produce a single figure which is the same as a constant unvarying level over the same period. Presenting evidence for the National Trust, Mike Stigwood of MAS Environmental discussed in detail the limitations with the use of the LAeq descriptor, including the change in sound that is perceptible. A 3dB figure is widely used. "BAA misrepresents the concept that a 3dB change in a sound is the minimum perceptible under normal conditions," said Stigwood. Unlike an increase in music noise for instance, "the concept does not apply to average equivalent sound energy levels (LAeq) that are significantly longer than an event."

The size of the 57dBA, LAeq,16hr contour and hence the number of people within it is also of prime significance to the inquiry. Bickerdike Allen & Partners consultant Jeffrey Charles is representing BAA London Stansted. Charles said that the current application does not involve any relaxation of the 43.6 sq km area of the contour. Stansted Airport currently handles some 23.7 million passengers per annum (mppa) and this is expected to rise to about 25 1 is million next year. Runway capacity is already available to cater for about 35 mppa but the rise is prevented by two planning conditions which were set in 2003. AMTI imposes a limit on air transport movements of 24 1,000 a year and MPPA 1 sets a 25 mppa limit. BAA's current Generation 1 (G 1) application is about raising these 6 constraints to 264,000 and 35 mppa. The public inquiry has arisen because Uttlesford District Council's refused the G 1 planning permission last year (NB Dec 2006 p2).

Bureau Veritas director of acoustics Stephen Turner has been engaged by Uttlesford District Council (UDC) to consider the proposals and to present evidence on its behalf. In his proof of evidence, Turner said that UDC had identified noise as an issue that has not been adequately addressed in the proposals. It believes that the current proposals will cause detriment to the amenity of occupiers in the vicinity, adversely affecting their quality of life. The council also received representations from further afield, expressing concern about the impact on locations valued for their beauty and tranquillity.

In essence, those outside the 57 dB contour are not considered to suffer significant annoyance - though this is also being debated. Turner estimates a 44% increase in the number of people likely to be "highly annoyed" by aircraft noise under G 1. However, he says that there is concern about the currency of this information. "The original survey work upon which it is based is 25 years old and there is some indication that the proportion of people highly annoyed is increasing at lower levels of exposure," said Turner. He referred to a current study being carried out by the Department for Transport, Attitudes to noise from aircraft sources in England (ANASE) which may shed light on the issue. "Nevertheless, the recent reaction to the noise impact from those affected by activities at Stansted Airport seems to support the view that the tolerance to aircraft noise is reducing at lower noise exposures," he said. Complaints are received from well outside the 57 dB contour.

However, BAA says that the noise impact has been properly assessed in accordance with the most recent government policy guidance, including the Air Transport White Paper (ATWP) - confirmed in the Air Transport Progress Report - and that the impacts have been shown to be relatively small. "The assessment of the effect of air noise arising from the G 1 proposal that has been undertaken and reported in the environmental statement is reliable and accurate," said Charles. "That assessment has been undertaken using the appropriate methodology, consistent with the approach taken by the Secretary of State for Transport in the ATWP, the guidance in PPG24, and the consistent practice of UDC and other local authorities. It is an approach that has been tested and upheld in a number of major public inquiries."

Points from Heathrow and Manchester inquiries are being cited for Stansted: equally, Stansted will no doubt be quoted as a precedent in years to come.

In the UK, aircraft noise assessment is based on the findings of the 1982 Aircraft Noise Index Study. From the results of the study, the government considers 57dB LAeq as the onset of significant community annoyance, with 63dB equated to moderate annoyance and 69dB to high annoyance.

The 57db contour uses the A weighting scale, which was developed for human response to comparatively quiet sounds, with B and C weighting for louder sounds. A specific scale introduced for aircraft noise dB(D) has since been dropped as it does not compare well with modem aircraft, explained Stigwood. Aircraft noise is commonly measured using A weighting. However the World Health Organisation recommends the use of dB(C) as more appropriate for noise such as aircraft which are a source of continuous low frequency noise.

An increase in LA eq of 3 dB represents a doubling in 'noise energy'. In simple terms the same LAeq value for a time period can result from smaller numbers of noisy events or larger numbers of quieter events. For Stansted, the LAeq is presented as an average over 16 hours. This could mean that the level every hour is approximately 57 dB LAeq, or that it varies so that some hours have less sound energy and others have more. Different permutations of event numbers and loudness can give identical results.

"The ideal situation is to have noise contours, but equally to take account of aircraft numbers with a quota count on the noise of each," says Martin Peachey who chairs the Stop Stansted Expansion (SSE) noise group. "To some extent, the path is already open - it's effectively what we do for night time restrictions." For the eight hour night period, Sound Exposure Levels are also considered to take account of the effect of individual flights. "People want to have a decent night's sleep. 1 think the situation in the day is starting to approach that. People are now saying that they can't be in the garden, or in the house with the windows open," he adds.
"We contend that the LAeq metric ingeneral and the 57 dBA LAeq contour in particular is an inadequate indicator of likely annoyance from aircraft noise. It is clear from several years of anecdotal evidence presented repeatedly to SSE that changes in the numbers of overflying aircraft are far more noticeable, important and intrusive than small changes in average noise levels of each aircraft. Yet the LAeq metric masks this effect," says Peachey.

The inspector has requested that, when the inquiry resumes in September, the parties should put in proposed conditions for use if he is minded to grant the appeal, says Peachey. "SSE's document will include recommendations such as better ways of measuring noise. We'll be putting in details of the quota count system which comes out of the Australian approach."

This identified that where an event's loudness is above a certain threshold -- a point of intrusiveness - it is the number of events that becomes significant. "It is commonly recognised that the duration of those events and the length of respite between each are important factors," said Stigwood.

The SSE team sees one of the drawbacks of LA, as a failure to represent the potential noise impacts in a manner that can be intuitively grasped by the lay person. As an ,averaging' metric, it does not adequately reflect the annoyance caused by intermittent, 'peaky' noise such as noise from aviation - as, for instance, contrasted with road noise - and gives an unrealistic impression of the effects at peak times, says Peachey.

In November 1994 the third runway opened at Sydney Airport. The resultant changes in noise exposure patterns triggered a public outcry from people who had not expected to be affected. Australia's Department of Transport & Regional Services (Dotars) was prompted to seek ways of avoiding such "surprise noise" as it seems to cause particular annoyance. People felt they had been misled by the use of contours which aggregate and average out the various noise components "Experience has shown that describing aircraft noise in terms of where aircraft fly, the times and numbers of overflights, the loudness of individual noise events, etc is likely to give a person a good feel for aircraft noise exposure patterns," found Dotars.

A discussion paper Expanding ways to describe and assess aircraft noise describes the alternative approaches. In essence, descriptors have been developed based on treating aircraft noise as a series of single events. The information supplied to the public includes respite charts, based on computing the number of whole clock hours when there are no movements on particular flight paths, reported as a percentage of the sum of the clock hours in the period. The Department has also begun to produce 11 noise above" contours, which combine information on single event noise levels with aircraft movement numbers.

Peachey says that Thaxted in Essex could expect nearly 300 noise events that would interrupt conversation. "This measurement relates to a 16 hour summer day and equates to one interruption every three to four minutes."

BAA says that noise difference contours between the 2004 baseline and the 35 mppa (G 1) cases show that no locations would see an increase in noise exposure of more than 2dB and that a population of 250 would experience increases of between 1 and 2dB. It points out that PPG24 considers that a change of 3dB is the minimum perceptible under normal circumstances. However others take issue. Peachey explains the 3dB change referred to by BAA could result from, say, either increasing the noise levels of all aircraft concerned by 3dB or by doubling the number of aircraft. "Doubling the number of Many think the Australian contour system is better than LAeq flights at an airport is of course significantly noticeable," he says.

This issue has been explored in the inquiry, including a session in which Charles was asked to calculate a series of scenarios relating to more planes with different noise levels, including a hypothetical quadrupling of the movements. The equal energy principle inherent in LAeq means that the value doesn't change if the number of planes is doubled but they are each 3dB quieter.

Under cross examination, Charles defended the use of LA,. There was no fickleness in using it, he said. "A huge amount of research has been done all around the world to see how we should evaluate environmental noise, and they looked at other parameters. They looked at why do we need to measure the noise at all? Why don't we just say number of events?" All manner of different things were tried, he said, and the UK initially used the noise and number index. "And then we said, hang on, that allows too much for the effect of aircraft movement numbers, so we changed it to LAeq which we did in 1990. And that's what we use, and that involves this trade off. ',

One of the exchanges between him and SSE's counsel Paul Stinchcombe debated this point:
PS: "And that trade-off conceals localised impacts?"
JC: "It will conceal localised impacts, it must do. It is a general planning tool."
PS: "It will conceal, will it not, 1 think we have agreed, an additional 500 movements at Stansted?"
JC: "It would take them into account."
PS: "It would conceal them."
JC: "It would not conceal them at all."
PS: "It would say they are not perceptible, because they are less than 3 dB."
JC: "Ah, how you interpret a number when you get it, would determine whether you consider it hides them or not; but the actual unit, the LAeq unit, fully takes them into account in accordance with what the research has found."

For a given aircraft movement, it is doubtful whether someone would perceive a small dBA reduction, believes Turner. "It seems a little more likely that they would notice the increase in movements which would not be off-set by the aircraft being noticeably quieter."

Peachey says that the aim is not to stop people flying, "but we would like to have as many people as you can get onto as few aircraft with the least noise possible". A hundred percent load factor on the larger aircraft would give the least number of movements he adds.
Follow ups

NOISE BULLETIN August/September 2007

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