ACOUSTIC PROPAGATION OVER PERIODIC AND QUASI PERIODIC ROUGH SURFACES

ACOUSTIC PROPAGATION OVER PERIODIC AND QUASI
PERIODIC ROUGH SURFACES
DL Berry.
GANG - IST. University of Lisbon 8. Department of Physics. University of Evora. Portugal S Taherzadeh, A Stronach & K Attenborough
School of Engineering and lnnovation,The Open University. Milton Keynes. UK
1. INTRODUCTION
Transport noise is an ever present concern in urban areas affecting the quality of life for millions of people. The traditional noise barrier is not always a convenient method of noise control and can divide communities. As a result. other types of barrier have been sought. Considerable interest has been devoted to the use of arrays of vertical cylinders☝ especially as a result of the publication of the measured transmission spectra of a minimalist sculpture☁. A particular advantage in using such arrays instead of conventional noise barriers is that they are relatively transparent and could be formed from natural means such as trees. However. such arrays do still present a barrier to pedestrian movement. Another approach is to use arrays of cylinders arranged horizontally on grounds near the noise source. the advantage being that they would allow greater pedestrian access and would beless intrusive. Such arrays effectively introduce a roughness to the ground. which is known to have a signicant inuence on near-grazing sound propagation? Here we present a theoretical and experimental investigation of this inuence on sound propagation and its
potential for transport noise reduction.
Atheoretical treatment for analysing the sound propagation over such rough grounds considers the multiple scattering of cylindrical acoustic waves by an array of nite impedance semi-cylinders embedded in a smooth acoustically hard surface6 and is based on a treatment for the problem of plane wave scattering by nite arrays of hard cylinders in water7 and on a treatment for identical nite impedance cylinders☜. Results from this treatment demonstrate good agreement between predictions and measured relative sound pressure level spectra for a range of different array distributions and with boundary element calculations. It is this treatment that we adopt in this study.
In section 2. we present an overview of this multiple scattering treatment. In section 3, we present results of this theoretical treatment for different source-receiver geometries, forms and sizes of both cylinders and semi-cylinders together with results of some simple laboratory measurements. An important challenge for this technique is to determine the optimum cylinder geometry which reduces - noise over a large bandwidth without introducing low-frequency surface waves. In section 4, we review the potential for this technique as a way of reducing transport noise and propose some suggestions for further research.