Sound calculating cuts noise

October 24, 1997

Equations silence motorways

Predicting noise levels from major developments such as motorways, checking the impact of sound barriers and measuring the flow of fuel at power stations are all a question of mathematics.

So say researchers at the Open University who are working on a variety of projects that involve solving differential equations.

The crucial factor in the equations is the effect of what mathematicians and acoustics experts call "boundary conditions" - the surrounding environment of the area under scrutiny. In the case of a motorway or airport, for instance, this would be the ground and atmospheric conditions, while for fuel pumped through power stations it would be the pipes.

The importance of such conditions is shown by the fact that a development producing a lot of low-frequency rumbling sounds will be noticeably less noisy if surrounded by ploughed fields rather than by grass.

Under some weather conditions at night, where the air temperature increases with height rather than decreases, sound is "bent", which means it can be heard further away from the source.

These are the kinds of factors with which an Open University research team, led by professor of acoustics Keith Attenborough, are dealing in projects modelling likely levels of noise that will be produced by developments such as Heathrow Airport's Terminal 5.

Professor Attenborough said: "In the case of outdoor noise the particular challenge we face is to find a model which is simple enough to be of practical use but does not ignore a lot of important factors. People use parallel computing models and run them for days to predict the weather. But what we have to work with is an approximation that is good enough for the job in hand."

Arriving at these useful approximations is all about solving some "basic equations" for sound levels in different environments, he said.

The same principle is applied to investigating small high-pressure environments such as those within pipes carrying coal particles and air in a coal-fired power station. Researchers from the OU and the University of Greenwich have won funding from industry and from the Engineering and Physical Sciences Research Council to develop ways of using sound to monitor the flow of the coal and air mix and the density of coal particles - information that is vital for the efficient running of a coal power station.

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