Ground Effect due to Periodic Resonant Roughness

Invited paper

Ho-Chul Shin

The Open University

Tuesday 2 june, 2015, 17:00 - 17:20

0.8 Rome (118)

Traffic noise is an ever-increasing problem due to the gradual urbanisation and growing popularity of motorised means of travel. Land between roads and nearby noise-sensitive receivers can be used to support a series of low-rise roughness structures as an alternative to conventional earth berms and noise barriers. Through measurements in an anechoic chamber and numerical computations using a two-dimensional boundary element method, we have investigated the effect of periodic roughness elements above a smooth hard surface on sound propagation. First, an array of solid rectangular roughness structures was studied to investigate the effect of the periodicity-induced diffraction. The broad- band insertion loss with reference to smooth hard surface was found to be considerable for near-grazing incidence. However, at frequencies below the first roughness- induced destructive interference, negative insertion loss was also observed both experimentally and numerically due to the creation of surface waves by the roughness elements. To improve the insertion loss in this low-frequency range, we have investigated the use of hollow resonant elements with slit openings constructed from pairs of aluminium angles, and observed the desired effect while maintaining the insertion loss at higher frequencies. Also we have created doubly-resonant roughness elements by inserting pairs of smaller aluminium angles within the larger structures and have measured associated multiple resonances. At near-grazing incidence, the doubly-resonant roughness array is found to produce a 3 dB increase in insertion loss compared to the array of single- resonance structures.

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