60% quieter?

Last week, the Federal and NSW governments announced that work would begin on building a second airport in Sydney. Sydney’s current airport has a night-time curfew due to noise concerns over populated areas, but one of the reasons for building a new airport in the western suburbs is to allow 24 hour access. When the Prime Minister was asked about noise concerns:

60percentquieter

(http://blogs.crikey.com.au/planetalking/2014/04/15/badgerys-creek-a-modest-start-and-no-forced-flight-transfers/)

Now obviously someone had done a little research knowing that question would be asked. Had this been researched a little further, or the research done by someone with a knowledge of Physics, the answer might have been different. What exactly is 60% quieter?

Sound levels are normally measured in decibels. The decibel is not an absolute measure, but it is a comparison of an intensity (I2) relative to some kind of reference (I1). The difference in sound level is found by taking the logarithm of this ratio. If the second sound has 10 times the intensity, this is 10dB. If it’s 100 times the intensity, that is 20dB.

db_1

db_2

So here’s the problem. “60% quieter” means that the newer planes produce only 40% of the sound intensity‚Ķ. and that corresponds to a 4dB drop: quiet a small drop in reality. Most people can’t differentiate a sound level difference of less than 3dB anyway. So the while the newer model planes are producing less sound energy, people on the ground are unlikely to notice any difference.

More information: http://www.airportwatch.org.uk/?p=1134

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