Sound vision

Computer simulations to map noise pollution

 
Published: Sunday 07 June 2015

Sound vision

The daytime maps represent road traffic in rainbow colours, ranging from green for silence to deep blue for chronic noise. City planners can use them to simulate the effects of installing noise barriers. Changes in colour reveal instantly whether the measure has produced the desired effect or not. This helps in zeroing in on the most cost-effective means to turn down the volume.

The best way to map the noise is to place microphones every few metres throughout a city. But this takes years and costs a fortune. Therefore, the Paris officials have resorted to 'virtual microphones', each of which is a point in a computer model that simulates sound at a place under given circumstances. A 2-d map has virtual microphones installed every two metres. The 3-d versions add microphones up the sides of buildings at three-metre intervals vertically and every 10 metres horizontally.

Each microphone gauges the sound level on the basis of how sound from nearby noise sources should behave. But it's impossible to model all noise sources, such as neighbours arguing. Therefore, the engineers have focussed on the biggest culprit -- traffic noise, accounting for 90 per cent of urban noise pollution. Each virtual microphone calculates the number of decibels emanating from vehicles using data on the number of vehicles passing by, their speed and the type of road surface. The microphones also take into account local topography. "For instance, cars climbing a hill make much more racket than when they cruise down the other side," explains Yann Francoise, the engineer in charge of the Paris noise maps. His team has also considered whether the traffic is typically smooth or congested on a particular stretch of road.

Data on the traffic load has mainly come from the Paris government's daily records of the vehicles plying on the city's 1,700 kilometres of roads. Getting good traffic data is the biggest problem many places face as the 2007 deadline approaches. France is fortunate to have a tradition of centralised map-making dating back to Napoleon. There is detailed information on the country's topography and vegetation, as well as the size and location of buildings, roads, railways and waterways.

To prepare the noise maps, Francoise's team fused the available data with digitised aerial photographs of Paris, and a database of every postal address in the city. The engineers then calculated how sound propagates. For this they used the mitra software, which models sound energy as rays and calculates how they interact with different surfaces. mitra adds up the energy of all the sound rays that hit a microphone. Calculating the decibels in Paris is difficult because many of its buildings create canyons, where noise rebounds back and forth. So the engineers have to go one step further and take into account noise that bounces off a building, or the ground, or both. It takes the eight computers in Francoise's office, at the city's department for protection of the environment, an entire year to produce one 3-d map of Paris. But the meticulous efforts have been rewarding -- tests against real measurements show an error rate of 1 decibel for maps already generated.

What's more, the maps have already proved helpful. Calculations show that about seven per cent of Paris residents regularly endure traffic noise above 71 decibels. About 46 per cent bear between 61-70 decibels, which too can cause high blood pressure and stress on prolonged exposure. With the help of the cartography, made available on the internet, Paris dwellers can now demand healthier environs.

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