Graded index sonic crystal noise barriers: An introduction to design and optimization

Licentiatavhandling, 2012

Environmental noise pollution and its impact on human beings remains to be an important topic in acoustics. Conservative estimations of the World Health Organization have shown that traffic related noise alone causes a loss of approximately one million healthy life years, only within Western Europe. Broad selections of noise control devices to attenuate road traffic noise during propagation are available. Still, traditional noise barriers are most commonly used, but alternative solutions exist. A solution with great potential is to place periodic arrangements of cylinders in between the source and the receiver, often referred to as sonic crystal (SC) noise barriers. Generally, there are two important frequency regimes when using a SC, i.e. the region below and above the lowest band gap frequency. In the high-frequency limit sound may be attenuated due to pass-stop behaviour, which translates to whether the structure supports acoustic wave propagation through it or not. The frequency limit near and above the first band gap attracted considerable attention. However, studies on phenomena well below the lowest band gap frequency, where sound may be redirected or focussed into a certain direction, are less common. One aim of this thesis is to introduce, and numerically demonstrate, a SC noise barrier that gains in broadband noise reduction by upward refraction of sound propagating through the structure. Alternatively, one could see a device with such properties as a broadband acoustic lens, since it will focus incoming waves into a distinct direction, for a broad range of frequencies. An upward refracting medium, or broadband acoustic lens, is created by orienting cylinders parallel to the ground surface and increasing the cylinder radius as a function of height, i.e. a graded index sonic crystal (GRIN SC). The study is restricted to a two-dimensional semi-infinite space, bounded by a hard reflecting ground surface. A beneficial aspect of GRIN SC barriers, compared to SC barriers with vertically oriented scatterers, is that it can simultaneously benefit from stop-band and focussing phenomena. Optimization of the broadband noise reducing performance of GRIN SC barriers placed in a realistic but simplified outdoor road traffic situation is also presented. To do this, a method is introduced, which iteratively creates complex cylinder formations using a genetic algorithm. Calculated results show that (i): by properly selecting the number of scatterers per wavelength a GRIN SC noise barrier is well approximated by homogenization techniques in the low-frequency regime and (ii): a combination of upward refraction and stop-bands contribute to broadband noise reduction of GRIN SC structures.