Road traffic noise reduction by multiple scattering and absorption mechanisms

Doctoral thesis, 2015

Millions of people in Europe, and even more on a global scale, are affected by environ- mental noise. Traffic related noise is estimated to cause a yearly loss of approximately one million healthy life years in Western Europe, which is mostly related to road traffic noise. To reduce road traffic noise during propagation traditional noise barriers are mostly used. However, these structures are often visually opaque, bulky, and therefore typically not ap- plicable in city-centres or other urban areas. The main goal of this thesis is to study noise barriers that blend well in the urban landscape and are suitable to protect pedestrians, cyclists, and other outdoor receivers, from road traffic noise. A potential solution to meet those requirements is based on the idea to place an array of cylindrical scatterers between a source and a receiver. These structures, known as sonic crystals (SCs), reduce noise at a receiver due to a multiple scattering process and can be engineered to specific needs by modifying the cylinder formation or the scatterers. The acoustic properties of a SC can be divided into two main frequency intervals: (i) where wavelengths are much larger than the periodic length of the structure, and (ii) where wavelengths are in the order of the periodic length of the structure, or shorter. In this thesis, an array of cylinders parallel to the ground surface is studied, which due to the orientation can be designed to exhibit focussing properties in the long-wavelength re- gion, but simultaneously can utilise band-gap phenomena at shorter wavelengths. These structures, referred to as graded index sonic crystal (GRIN SC) noise barriers, have been optimised for an idealised two-dimensional outdoor configuration. To facilitate further improved SC barrier designs, scattering by an array of perforated cylinders with porous cores is studied in the framework of Multiple Scattering Theory (MST). Predictions and measured data show that a substantial noise reduction is possible in a wide frequency range. To simulate point source scattering by a cross-sectionally invariant array of rigid cylinders in a three-dimensional domain, the 2.5-dimensional Multiple Scattering Theory (2.5D MST) is introduced. Numerical predictions based on the 2.5D MST model show that the characteristic frequency response of a GRIN SC shifts up in frequency during a typical vehicle pass-by scenario, which will lower the global efficiency of the device. A SC noise barrier placed perpendicular to an impedance surface is also studied using 2.5D MST. The noise reducing performance of a SC may even improve for a vehicle pass- by scenario, and therefore seems to be a better candidate to mitigate surface transport noise. In addition, a conventional low-height barrier with porous covers is investigated, which is a simple, yet effective, alternative to the cylinder based structures.