Evidence on how the 57 Leq noise contours for Heathrow fail to fully reflect aircraft noise impacts
In a blog on the anomalies of how aircraft noise is currently measured, John Stewart writes of the odd situation where roads in London are regarded as quiet, ignoring the obvious impact of Heathrow flightpaths overhead. This arises in areas such as Clapham, which are well outside the 57 Leq contour, which it is wrongly alleged, is the limit at which aircraft noise is a problem, or annoys/upsets people. The number of complaints about aircraft noise that come from areas well beyond the 57 Leq contour are evidence that it is not a measure that reflects reality. A better system for measuring aircraft noise experienced is Lden (day, evening, night) with noise in evening and night given a higher weighting, to reflect the greater impact, and greater annoyance, it has on those overflown. The European Commission requires member states to use 55Lden when drawing up their noise maps. That is more realistic than 57 Leq. It is understood that Sir Howard Davies, Chairman of the Airports Commission, is looking seriously at a more realistic noise metric.
By John Stewart
They won’t know whether to laugh or cry this weekend, the residents of Sabine Roadin Battersea. Their street has been named one of the quietest in London. “Sorry you did say ‘quietest’ didn’t you?” They’ll be asking each other over breakfast as the next plane roars overhead.
Sabine Road, not far from Clapham Junction, is in an area where there have been countless complaints about aircraft noise the years. It’s on the flight path to Heathrow.
So what is going on?
The researchers, from the noise consultancy firm 24 Acoustics have fallen into the classic trap of using the official UK method of measuring aircraft noise.
“To determine the quietest streets, researchers used existing data to locate which ones were outside of the 57 decibel noise contours for airports, and had a night road traffic noise level of lower than 35 decibels”
This reliance on the 57 decibel noise contour has made a mockery of their results. It has meant that aircraft noise can be heard in just about all their top quietest streets. Streets in places like Fulham and Putney make it into the top 10. http://www.standard.co.uk/news/london/silent-nights-the-capitals-10-top-quietest-streets-revealed-8928506.html
The blame lies not with the researchers. I imagine that in good faith they accepted the official measurement of noise annoyance from aircraft. It was a mistake waiting to happen. For years HACAN, along with many other bodies, has argued the measurement is utterly misleading.
We wrote in our response to the Airport Commission’s consultation on noise:
“The current 57 db Leq contour – the official area which defines where community annoyance sets in – excludes places like Putney and Fulham in West London! Not the real world!”
The European Commission agrees with us. It requires member states to use a different metric – called 55Lden – when drawing up their noise maps. That is more realistic. It extends the noise boundaries to places like Vauxhall and Clapham. But even it does not cover all the places where people are annoyed. The ANASE Study, commissioned by the last Government but quietly buried when it found the findings were not to its taste, found that there is significance noise annoyance well beyond the 55Lden contour.
Of course the current 57 decibel cut-off point suits the aviation industry down to the ground because it minimises and underestimates the numbers affected by noise.
However, here are distinct signs the tide is turning. Sir Howard Davies, who heads up the Airports Commission, is known to be looking seriously at a more realistic metric.
In their responses to the Commission’s noise paper, MPs queued up to criticise the current cut-off point: https://www.gov.uk/government/publications/stakeholder-responses-to-airports-commission-discussion-papers
Mary Macleod MP: “There is widespread evidence that the existing measure of the threshold of annoyance is inaccurate and misleading.”
Zac Goldsmith MP: “The measurement of noise – and of noise annoyance/disturbance – needs revising. Currently it is misleading. Any noise measurement that does not reflect reality lacks credibility”.
Former Transport Secretary Justine Greening MP: “I believe this strongly shows that taking a traditional 57dB approach to assessing the level of noise annoyance from any new aviation strategy will exclude a large number of people who will be annoyed and affected but live outside of the 57dB noise contours.”
John Randall MP: “Clearly, a 57dB threshold is unhelpful if it excludes population areas that are experiencing significant annoyance from aviation noise”.
Murad Qureshi for the London Assembly Labour Group: “The committee has previously recommended the adoption of an Lden measure and the use of lower thresholds for identifying the areas most affected by aircraft noise”.
Boris Johnson, the Mayor of London: “The development of a new noise metric is strongly supported. It must fully represent sensitivity to and the impacts of aviation noise and how individual aircraft events are experienced during different times of day and night”.
Even the Government in its Aviation Policy Framework, published in March, recognised the current measurement was flawed:
“Average noise exposure contours are a well-established measure of annoyance and are important to show historic trends in total noise around airports. However, the Government recognises that people do not experience noise in an averaged manner and that the value of the LAeq indicator does not necessarily reflect all aspects of the perception of aircraft noise. For this reason we recommend that average noise contours should not be the only measure used when airports seek to explain how locations under flight paths are affected by aircraft noise.”
The Airports Commission has been charged with reassessing the way aircraft noise is measured. A change to a more realistic noise metric could be the most lasting decision it will make. It will ensure that future aviation policy decisions are based on sound measurements.
And it will save future researchers falling into the same trap that ensnared the benighted people from 24 Acoustics.
“Lden” (day-evening-night noise) is a measure of noise (see how it is defined on page 25 of link ) that attempts to give a different weight to noise in the evening period, and the night period, to noise in the day. It presumes a day of 12 hours, an evening of 4 hours, and a night of 8 hours – though those times can be varied. It counts noise in the evening to be greater than the same noise in the daytime, and night noise as even greater – so there are different weightings to each time of day. It thus attempts to average noise out, but giving particular weight to noise at times of day that particularly trouble people.
Lden : Day-evening-night equivalent level
A-weighted, Leq. noise level, measured over the 24 hour period, with a 10 dB penalty added to the levels between 23.00 and 07.00 hours and a 5 dB penalty added to the levels between 19.00 and 23.00 hours to reflect people’s extra sensitivity to noise during the night and the evening.
Leq – equivalent continuous noise level
Noise levels often fluctuate over a wide range with time. For example in the middle of the night the level might go down as low as 30dB(A) with occasional passing vehicles of 70dB(A) or more. Later comes the dawn chorus followed by the general noises of the day before relative peace returns in the late evening.
Alternatively it may be a factory with different noise emissions throughout the day or week, with deliveries, intermittent compressors, and lots of varying noisy processes on top of the routine production noise levels.
How do you measure these noise levels and come up with an overall value?
This is where the Leq noise or equivalent continuous noise level meter comes in. This meter faithfully follows all the fluctuations, stores them in it’s memory and at the end of the measurement calculates an ‘average energy’ or Leq value. When we say average, this is not a simple arithmetic average because we are measuring in decibels which are Logarithmic values. So our meter converts the dB values to ‘real numbers’, adds them all up then divides by the number of samples and finally converts this equivalent level back to decibels – dBs.
LAeq – It is common practice to measure noise levels using the A-weighting setting built into all sound level meters. In which case the term is properly known as LAeq and the results should say so – for example LAeq = 73 dB or Leq = 73 dBA
A good Leq sound level meter samples and ‘captures’ the noise levels 16 times a second which means over an hour it makes 16 x 60 x 60 = 57600 calculations, not difficult for a modern meter but quite an achievement a few years back.
Leq noise levels are Logarithmic (dB) values and cannot be added directly. A doubling of sound level results in a measured increase of 3 dB, four identical sources in a room would increase the noise level by 6 dB and so on. This works both ways, say 10 similar machines in a room produce 100 dBA then removing one machine completely will only reduce the overall noise level to 99.5 dBA, you would need to silence or remove 50% of the machines to achieve a 3 dB reduction. See also our dB page for more details on adding and subtracting decibels – Leq Calculation.
Leq is also used in the assessment of Noise Dose or Sound Exposure in the workplace and the 3 dB ‘doubling rule’ applies to time and/or level. For example an Leq level of 85 dBA over 8 hours is currently assessed as 100% dose in the UK. Using the doubling rule then 85 dBA(8 hour) = 82 dBA(16 hour) or 88 dB is only acceptable for 4 hours a day. Similarly if 85 dBA = 100% dose then 88 dBA = 200% dose. A note of caution – the Leq assumes an Exchange Rate of 3 dB and applies to all ISO and British Standard measurements. In some Countries, for example the USA OSHA Standards use a 5 dB exchange rate.