Small hills reduce aircraft noise

At Schiphol Airport a test done with small hills situated close to a runway shows a ten decibel reduction of noise pollution.

Noise pollution and Schiphol, it seems like a never-ending story. In 2003 the Polderbaan was opened, and this runway led to many of the residents of nearby Hoofddorp complaining about aircraft noise. The noise from airplanes taxiing and taking off is especially a nuisance. Schiphol and TNO have now presented a fascinating solution: several 2-meter high hills stretching for 2.5 kilometres. Research shows that this reduces the perception of aircraft noise by 50 percent.

“I think it’s a new, very simple and therefore excellent initiative. A lot of engineers devise very complicated solutions. The beauty of the hills is that it’s an ordinary approach to a difficult problem,” says professor of transport policy, Bert van Wee.
The low frequency sounds caused by airplanes is particularly a nuisance, emphasizes retired scientist, Rinus Boone, who was previously part of the Lab of Acoustical Imaging and Sound Control: “The small but steep hills block the low frequency sound, which scatters on the various hills. The scattering causes the sound to move in different directions - upwards or backwards towards the runway, for example. This is not the case with horizontal surfaces, such as asphalt or a grass field. Because of the small hills, some of the noise will not reach Hoofddorp. It’s a smart solution.”
Because the small hills have pointed tops, the sound is scattered. “Without pointed tops, the sound waves could still be directed at Hoofddorp,” Van Wee explains. “That’s why road traffic noise-reducing barriers have pointed tips as well.”
Although the small hills may reduce noise pollution, it is not only acoustic measurements that influence the experience of noise. Maarten Kroesen, a former PhD student of Van Wee, wrote a dissertation on sound pollution. He discovered that mistrust intensifies the impact of noise.

Van Wee gives an example: “Two people hear the same noise. One of them does not trust that everything is being done to limit the noise and the other person does. The first person will complain more about it and consider the noise more annoying. If Schiphol shows that they care about limiting aircraft noise, they might (re)gain trust from people who are currently complaining.”
The small hills are not the first design to limit the taxiing and taking off sound. Because so many complaints have been lodged regarding the Polderbaan, Schiphol created a competition some years ago, asking for innovative proposals to limit the noise. A floatable noise-reducing barrier won, but it was never actually used by the airport because of safety reasons.

At the end of April, the number of small hills at Schiphol will increase at the runways close to Aalsmeer and Badhoevedorp. Although the small hills seem to be a success, they do not reduce all aircraft noise, Boone stresses. “The sound of planes flying over will still be heard of course,” he says. “Unfortunately, the small hills can’t reduce that, so will only influence the taking off and taxiing noise. Consequently, the noise pollution will not end because of this measurement. Most of the aircraft noise will still be heard.”