Noise Feature - Stansted battlelines drawn

Back ...general 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.

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