Efficiency of a noise barrier with an acoustically soft cylindrical edge

Abstract

It is well-known that an absorptive obstacle installed on the edge of a noise barrier improves sound shielding efficiency without increasing the height of the barrier. In the present study, the efficiency of a noise barrier with an acoustically 'soft' cylindrical edge is investigated. 'Soft' indicates that the sound pressure at the surface is zero. It is difficult, however, for traditional materials to achieve a soft surface. It is shown that a 'Waterwheel cylinder' approximates a soft surface by both theoretical and experimental analysis in a two-dimensional sound field. The Waterwheel consists of acoustic tubes arranged radially; therefore, the sound pressure at the open ends of the tubes (i.e., the surface sound pressure of the Waterwheel) depends on the relation between the depths of the tubes and the sound wavelength. The sound shielding efficiency of a half-plane with the Waterwheel cylinder was measured using a scale model. The results show an improvement in the noise shielding efficiency by more than 10 dB in a certain frequency range but, unfortunately, the Waterwheel decreases the noise shielding efficiency of a barrier in another frequency range. The noise shielding efficiency is strongly frequency dependent, as is the surface sound pressure of the Waterwheel.