EFFICIENCY OF A NOISE BARRIER ON THE GROUND 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. This paper examines the sound shielding efficiency of a noise barrier with an acoustically “soft” cylindrical edge. “Soft” indicates that the sound pressure at the surface is zero; however, it is difficult to produce a soft surface using traditional materials. The authors previously reported that the “Waterwheel cylinder,” which consists of acoustic tubes arranged radially, approximates a soft surface cylinder. In the present study, a noise barrier with a Waterwheel cylinder installed on the edge of the barrier is investigated. Results of numerical simulations indicated that the Waterwheel cylinder improves the sound shielding efficiency of a noise barrier. The improvement is strongly frequency dependent; it exceeds 10 dB in a certain frequency range of an octave, but the Waterwheel decreases the noise shielding efficiency in another frequency range. The frequency characteristics of the waterwheel's effects are related to its self cross-sectional shape. The Waterwheel improves the efficiency much better in the effective frequency range of an octave as compared with an absorbing cylinder. All numerical calculations were carried out assuming an unrealistic two-dimensional sound field, but results of scale model experiments indicate that the calculations predict very accurately the efficiency of noise barriers in a three-dimensional sound field.