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Mapping the din

The European Union has embarked on a unique project to map a long-overlooked but often-overheard form of pollution – noise. In 2003, regulation passed in the European Union mandated the member states to prepare by 30 June 2007 “noise pollution maps” of all major cities, roadways, railways, airports and industrial sites. The maps, it is hoped, will facilitate formulation of concrete plans to muffle the din.

Paris is the first of the cities that has stepped ahead in this direction. In a city government’s office room warmed by computers crunching numbers round the clock, engineers and cartographers have prepared the world’s first two-dimensional (2-D) and three-dimensional (3-D) maps of the clamour sickening the city and its inhabitants.

The computer simulations reveal the noise levels in the way x-rays reveal broken bones. The daytime maps represent road traffic in rainbow colours, ranging from green for silence to deep blue for chronic noise. The cartographic exercise has revealed that about seven per cent of Parisians regularly endure traffic noise above 71 decibels, and 46 per cent are exposed to noise levels of 61-70 decibels. Prolonged exposure to a cacophony of this order can cause high blood pressure and stress.

City planners can use the maps to find out what impact the installation of noise barriers would have – lowering of the din will be instantly denoted by a change in colour. This would help in zeroing in on the most cost-effective means to turn down the volume. The Paris officials have used “virtual microphones” to map the noise, as using real microphones would have been a hugely expensive and long-drawn out affair. Each of these “microphones” is a point in a computer model that simulates sound at a place under a set of given circumstances. A 2-D map has virtual microphones installed every two metres. The 3-D versions have microphones up the sides of buildings at three-metre intervals vertically and 10-metre intervals horizontally.

To prepare the noise maps, the French team fused the available data with digitised aerial photographs of Paris and a database of every postal address in the city. Calculations were then made on how sound propagates. For doing this, the engineers used the Mitra software, which models sound energy as rays and calculates how they interact with different sources. Mitra adds up the energy of all the sound rays that hit a microphone.

Calculating the decibels is difficult in Paris, because many of its buildings create canyons, where noise rebounds back and forth like a tennis ball. Which means, the engineers have to also take into account the noise that bounces – either off a building, or the ground, or both. It took these engineers eight computers and an entire year to produce one 3-D map of Paris. The meticulousness and the painstaking attention to detail have been rewarding – tests against real measurements show an error rate of +/- 1 decibel for the maps that have been already generated.

— CSE Feature Service