AN ANALYTICAL STUDY FOR THE PREDICTION OF THE HORN EFFECT ABOVE A GROUND SURFACE

Attenuation of urban traffic noise has become an important challenge in highly populated cities. It constitutes one of the main sources of environmental pollution that has to be reduced to ensure a sustainable acoustic environment. The noise nuisance in cities, when vehicles run smoothly on the road, is mainly due to the rolling noise caused by the interaction between the road pavement and the tyre surface. This noise is amplified due to the hornlike geometry formed between the tyre and the ground, generating the so-called horn effect. The noise levels radiated at the receiver position can then be substantially increased. In this work we aim at developing a theoretical model for the prediction of the horn noise effect by considering the sound diffracted by an absorbing sphere placed above an outdoor ground surface of finite impedance. The sound field at the receiver position is expressed as the sum of the contributions of the direct field radiated by the point source, the waves scattered by the sphere and the reflexion from the impedance ground. The incident sound field is represented as a series of associated Legendre polynomial functions and spherical Bessel functions. The predictions obtained from the analytical model are compared with numerical BEM results and with experimental measurements acquired by an on-board acquisition system for the characterization of the near-field levels on the proximity of the back vehicle tyre. This provides an assessment on the accuracy with which the theoretical model can predict the horn-noise effect.