Calculating Traffic Noise Reduction at Long Distance using Diffracting Elements

A series of resonators alongside a road or railway can be used to diffract noise upwards to create a zone of noise reduction behind the elements. The company 4Silence developed the so-called Whisstone, a concrete block with 18 resonators of different depths designed to operate in a broad frequency range. Several measurements at short distance, up to 15 meter, have proven that a noise reduction up to 4 dB(A) can be achieved. To determine the noise reduction at long distances calculations are necessary, due to the uncertainties of ground and meteorology effects. In this paper a comparison between measurements and numerical results (FEM) at short distance is presented. The positive results of this comparison allow the numerical method to be applied at larger ranges, up to 600 meters, taking a downwind condition into account. To this end the FEM method is coupled to a Green's function parabolic equation method (GFPE). The calculations for long distances show noise reductions up to 5 dB in octave bands and 3 dB(A) for the broadband level. These results will be used to account for the diffracting elements in standard noise calculation models used in practice, such as the Dutch national standard model or Cnossos.