The Dreamliner 787 has been much hyped, for its theoretical reduction in fuel use and in noise. Boeing claims it is 60% quieter, a statement that needs to be understood in terms of how aircraft noise is measured. It does not mean 60% quieter, in the way a layperson would understand the statement. It means actually a reduction of perhaps 3 decibels, to anyone standing under the flight path – a difference that is barely noticeable, even to trained ears. These figures are also theoretical, along the lines of the car fuel consumption figures given by manufacturers, and very difficult to replicate in real life. If the planes are heavier, taking off with more fuel and luggage on board, or landing on full power, they are still very noisy. And if there are more flights overhead, that is actually what people are bothered by, rather than a 3dB difference.
Boeing ‘Dreamliner’ offers only marginal noise benefit
May 4th 2012 (Aviation Environment Federation)
The Boeing 787 ‘Dreamliner’ was flown in to Britain for a number of demonstration flights this week, with the company’s press release announcing that it has “larger dimmable windows, bigger bins” and environmental credentials to boot.
The Boeing website describes the plane as having been “designed for environmental performance”. On noise it says “ .. the noise footprint of the 787 is 60% smaller than those today of similarly sized planes.” But what will this actually mean for people on the ground? (See below)
A reduction in noise ‘footprint’ presumably means a reduction that in the area that would experience a particular level of noise, probably 85dB in this case. A 60% reduction in the footprint corresponds, by our calculation, to a reduction of about 3dB in noise level, which would produce a reduction in perceived noise of about 20%.
Of course, any reduction in noise is welcome. But this modest improvement from what will initially be only a very small number of aircraft (which, incidentally, were promised many years ago) will sadly do little to address the problem of noise around airports, with any noise savings likely to be quickly swallowed up by increases in air traffic.
Aviation Environment Federation. http://www.aef.org.uk
What does 60% quieter mean? How does the ordinary person on the ground understand it?
If that is a 60% drop in noise power then it means only a drop of around 3dB to the noise – ie. you may not notice much difference.
An average person cannot generally distinguish between sounds 2 – 3 decibels apart.
There was a quote from a Boeing spokesman in an interview on The One Show link that “… the noise signature is contained entirely within the airport boundary.”
What in fact he was probably trying to say was that all sound of 85dB (above the level of loud traffic by the side of the road) stays within the airport boundaries. Not quite the same thing.
And Boeing says: “In fact, the noise footprint of the 787 is 60% smaller than those today of similarly sized planes.”
The noise footprint presumably refers to the size of a noise contour for a given dB (level of decibels). A 3dB reduction in noise from aircraft typically reduces the area of a noise contour by a factor of about 2.5, ie to 1.00/2.5 = 40% of the original. That is, a
reduction of 60%. We can therefore surmise a reduction in noise of the Dreamliner 787 is about 3dB.
The way in which noise is measured means that an increase in noise of 10 dB means a doubling of loudness.
So if a reduction of 10dB halves perceived noise, then a reduction of 3dB reduces perceived noise by about 20%.
If the area of a given contour is reduced by 60%, then if we assume the shape of the contour is the same, we can calculate that the radius of the contour drops by 37%.
So if the 65dB contour reaches 1km from the runway end at the moment then it would stretch 632m from the end if 787′s alone were used for the same routes as the existing planes. But as the 787 is likely to have a higher take-off weight due to higher fuel
loading to fly further, this may be completely nullified.
- If these new planes allow longer haul flights, the planes will be taking more fuel and luggage (as the passengers will be away for longer) so may well make more noise again – and the total emissions might be increased
- It is likely the airports will use this apparent reduction in noise of the Dreamline to show there is no need to constrain their expansion (because they are using “green” planes).
There has been a spate of breathlessly enthusiastic articles in the press, where excited journalists have been given tours of the Dreamliner and demonstration flights – with whoops of delight about luggage compartments.
This article from the Telegraph has a few more basic facts.
And a bit of typical Daily Mail coverage at
Dreamliner touches down in the UK for first time: Airline bosses welcome new ‘green’ jet on first stop of round the world tour
More about the Dreamliner on Wikipedia http://en.wikipedia.org/wiki/Boeing_787_Dreamliner
ASA brands Boeing “untruthful” over claim the 787 Dreamliner will be 60% quieter
The Advertising Standards Authority (ASA) has upheld a complaint by a London resident over Boeing’s claims in a recent advertisement that the new 787 Dreamliner, which is due to go into service later this year, will be “60% quieter than ever before”. The ruling comes barely a week after British Airways announced that it had finalised an order for 24 Boeing 787 Dreamliners. (HACAN press release)