Diffractors: a fascinating alternative to noise screens?

A recent invention by the University of Twente in the Netherlands – diffractors – might be an interesting alternative for noise screens for shielding certain areas from traffic noise. Diffractors basically consist of a series of slits in the ground or mounted on a low shield which are parallel to the road. The sound waves passing over the slits cause standing sound waves in them with a maximum amplitude on the edge, where they interact with the sound wave passing over, causing the sound energy to bend upwards. As each slit resonates at a particular frequency, a series of slits is foreseen with varying depths, each resonating at a different frequency, in order to deal with the broad band traffic noise. Numerical simulations by the inventors and measurements ordered by them look very promising and suggest that a device of about 0,5 m high and 1 m wide would have the same effect as a 4 m high noise screen. An independent team has been formed by the Belgian Road Research Centre and the Flemish Agency for Traffic and Roads which has done “Controlled Pass By” measurements with a car and a lorry at a prototype installation (diffractors on a low screen) along a secondary road in Losser near Enschede in the East of the Netherlands. One measured at different distances from the road. This contribution summarizes the findings of these measurements and gives an answer if diffractors are a valid alternative for noise screens or not.