Tomonao Okubo

EFFICIENCY OF A NOISE BARRIER WITH AN ACOUSTICALLY SOFT CYLINDRICAL EDGE FOR PRACTICAL USE

This paper examines the sound shielding efficiency of a noise barrier having an acoustically soft cylindrical edge, as it compares to that provided by the well-known absorptive cylindrical edge. It has been reported that the waterwheel-shaped cylinder (acoustic tubes in a radial arrangement) approximates a soft surface cylinder, and that the sound shielding efficiency of a noise barrier is improved by placing the cylinder on its edge. The efficiency of the waterwheel-shaped edge barrier is strongly frequency dependent, thus the improvement in overall sound pressure level is smaller than expected when the source is broadband noise. The present study investigates the use of varied tube depths to improve the efficiency of the waterwheel-shaped edge. It is shown that adding tubes of different depths can flatten the frequency dependence, and that such tubes are only needed in the upper half of the cylinder. These findings led to the design of a new edge device for controlling road traffic noise, whose numerical simulations suggest that it is twice as effective in overall sound pressure level as the original waterwheel with uniform-depth channels.

Prediction of road traffic noise using two‐dimensional numerical analysis

For acoustically complicated road structures such as semi‐underground roads and special areas in which a viaduct road and a flat road with noise barriers exist together, prediction of road traffic noise is complicated because of multiple reflections and diffractions that occur inside the road structures. For such road structures, an energy‐based engineering model cannot be applied and noise propagation should be addressed through introduction of wave theory. When the road structures have almost identical cross‐sectional shape along the road, two‐dimensional (2‐D) wave‐based numerical analyses are applicable. In the prediction model of road traffic noise, the ASJ‐RTN Model 2003, published by the Acoustical Society of Japan (ASJ), the application of 2D wave‐based numerical analysis was introduced as a prediction method for such complicated road structures. Comparisons between calculations by BEM and FDM and field measurements and experiments for three actual road structures were conducted. Consequently, cal...

Noise Propagation Simulation

This chapter shows examples of numerical analyses on various noise propagation problems including outdoor noise propagation, noise barriers, depressed roads, building facades, building windows, and floor impact sound. In each section, considering the features of each problem, methodology of applying the simulation techniques presented in Part I to the practical problem is introduced and the calculated results are illustrated. Some of the calculated results are compared with measured ones and the applicability and efficiency of the analysis method are discussed.

Efficiency of noise barriers with an acoustically soft cylindrical edge

It is well known that an absorptive obstacle installed on the edge of a noise barrier can improve sound shielding efficiency without increasing its height. In the present study, efficiency of the noise barrier with a ‘‘soft’’ cylindrical edge is investigated. Soft means a surface on which the sound pressure is zero, but it is difficult for traditional materials to realize this surface. It is shown, by numerical and experimental analysis in a two‐dimensional sound field, that the ‘‘Waterwheel cylinder’’ approximately realizes a soft surface. The Waterwheel consists of acoustic tubes arranged radially; thus the acoustical properties of the Waterwheel’s surface (i.e., the sound pressure at the open ends of the tubes) depend on the relation between the depths of the tubes and the wavelengths. Then sound shielding efficiency of a half‐plane and a barrier sitting on the ground with the Waterwheel edge has been investigated. The results show that the Waterwheel on the edge improves the noise shielding efficiency...